White on pumpkin leaves


The Incomparable Pumpkin Leaf

What Do Pumpkin Leaves Look Like?

When your pumpkin plant germinates, the first two leaves break through the soil. These are called sprouts and are small round leaves. Although these are not considered “true” pumpkin leaves, they are the first indication that your pumpkin plant is viable.

The first actual pumpkin leaves emerge at the tip of the pumpkin sprout stem, in between the two small leaves. You can immediately see the difference in the two sets of leaves; the second set is dark green with jagged edges.

How Big Do Pumpkin Leaves Grow?

Pumpkin leaves can vary in size, depending on which variety of pumpkin you plant. However, generally speaking, the leaves are very large. Some measure the size of your two hands together, while others grow as large as a soccer ball.

Although the pumpkin fruits grow off of the vines rather than the leaves, pumpkin leaves do serve essential purposes for the plant as a whole.

  • The provide ground cover to keep the soil around the plant damp.
  • The huge leaves offer some natural weed control.
  • They shelter the young fruits from some harmful pests.

Other Uses for Pumpkin Leaves

Oddly enough, the massive pumpkin leaves have many other uses that most gardeners do not know. For example, some cultures use the foliage and flowers as a food source! Since they are similar to other green leafy vegetables regarding nutrition, you can use them as a substitute for vegetables like spinach or beet greens.

Additionally, you can use the pumpkin leaves for medicinal purposes. Although there are few scientific studies about the therapeutic value of pumpkin leaves, people who rely on homeopathic remedies use them frequently for a variety of ailments.

Tip: Be aware that some pumpkin leaves are bristly; take care when eating them!

An All-Purpose Plant

Because of the many uses of the pumpkin flesh, seeds, and leaves, this is one plant that you do not want to leave out of your garden plot.

Is This Plant A Pumpkin?

Ask the Expert: Pumpkin?

Young Pumpkin Plant

Last year I threw old pumpkins that were not cut open in my back yard. I think they were just the small kind you use to cook with and they are hard to cut. Anyways now this year there is a big mysterious plant growing close to where the old pumpkins were. In the background you cant see it very well but the pumpkin is still there and has seed there still too.

Pumpkin With Bloom Pods

Also on the other picture it looks like more leaves or maybe those yellow flowers are growing that grow on pumpkin plants. I was just wondering if this is a pumpkin plant and if so what can i do to keep it growing to grow pumpkins for this year? Amanda

Plant Expert Reply:

Yes, it seems to be a pumpkin plant. However, your pumpkins may be slightly different than the orginal plant depending on the cross pollination that occurred last year.

Your plant looks healthy and should produce pumpkins without any problems. Basically water and fertilizer it the plant. When you water try not to wet the foliage. Instead apply water at the ground level. You can use a water soluble fertilize or a granular fertilizer. On pumpkins I like to use a granular slow-release fertilizer. I have been using milogranite on my squash, cucumbers and the like. So you might try it.

You should see pumpkins start to form as soon as the blooms open up.

How to grow your own pumpkins

Fall is the time of year when we pull out the sweaters and boots, add a jacket to our attire and immerse ourselves in all things pumpkin. From creamer to donuts to home decor, pumpkins represent autumn from when the first leaf falls to long after the Thanksgiving dishes have been dried and put away. Of course, there is also the age-old practice of carving pumpkins for Halloween. While every supermarket has mounds of pumpkins ready for purchase, with a little planning you can grow and harvest your own pumpkins for everything from jack-o’-lanterns to pumpkin bread.

Plan ahead

By the time October hits, all you can really do is plan for next year’s garden (which is a great idea!). Seeds should go into the ground between the end of May and mid-July, depending on where you live. Be sure you don’t plant too early in the season. Although the plants will thrive and produce fruit happily throughout the late summer and early fall, you may find yourself with rotten fruit before the pumpkin-carving party if they ripen months beforehand.

Provide space

Pumpkin plants ramble. In fact, they will take over and may cause problems if confined, so give them a dedicated area to thrive. This is not a plant that will be successful on an apartment balcony. Allow them ample room to bush out without running into other garden crops, outbuildings or fencing. For planning purposes, set aside around 9-10 feet in each direction for each mound of plants (around 100 square feet).

Related: How to cook a whole pumpkin (seeds, guts and all)

Mound it up

Rounded mounds of soil provide the drainage and depth pumpkins need to thrive. Pumpkin seeds and vines are finicky, so you don’t want to handle or transplant them once they are in soil. To avoid disturbing them, make sure your mounds are established before planting. Set them at least 5-6 feet apart from each other.

Let it shine

When choosing the location for your pumpkin mounds, select a space that receives a lot of sunlight. Pumpkin seeds don’t do well in cold soil or dirt that is too wet. They prefer a warm environment, so choose your selection with that in mind.

Choose preferred varieties

There are many varieties of pumpkins, some that look more like squash in shape and color. In fact, many people use the labels squash and pumpkin interchangeably. There are also a variety of sizes, from small decorative options to giant, 100-pound versions. Choose your seeds well to match the space you have available.

Related: How to cook and enjoy 10 types of squash other than pumpkin

Make them share

Pumpkins grow well in clusters. To find the strongest plants, plant five or six seeds per mound. Seeds should be pressed into the soil about one inch deep and lightly covered. Once they are well-established, thin to the healthiest two to three plants per mound. Each plant will produce multiple pumpkins.

You can see the potential when the plants bloom flowers. Soon, each of those flowers will have a pumpkin behind it beginning to form. Be mindful though — only female flowers produce fruit. The male flowers bloom briefly, giving bees an opportunity to find the flowering vines. Then, they drop off the plant. Female flowers, however, will show the bulb of the green emerging fruit behind them.

Keep the weeds out

Weeds can choke out the productivity of your pumpkin plants, so keep them at bay by frequently checking for new growth and removing them early on. A hoe works well for this task to avoid the back and knee strain from getting on the ground. Try not to dig too deep, which could interfere with the roots of the pumpkin plants. Avoid harmful weed killers anywhere near your plants (and preferably your entire yard). Applying mulch to pumpkin plants will help keep the weeds away and hold the moisture in.

Stick to a watering schedule

Pumpkins are fairly forgiving of a little neglect when it comes to water, as long as they have a chance to get established with reliable drinks. Give them a drink at least once each week, saturating the soil around the base of each plant while avoiding leaves and fruit wherever possible. In the beginning though, avoid flooding the seed and seedlings as they become established. Instead, give them shallow drinks. Schedule an extra watering if the weather is extreme during the early summer growing season.

Growing care

You won’t have to dote on your growing pumpkins too frequently. Given the right location, soil and temperature, they are pretty self-sufficient. If you are planning to use your pumpkins for carving, you may want to gently rotate them occasionally. This will help avoid pumpkins with a flat side and help them grow into a more uniform shape; however, the vines are persnickety, so use caution or the vine may be damaged. Tip: Set each pumpkin on a piece of cardboard and gently rotate it every few weeks for even heat and light.


Your pumpkins will likely be ready to harvest during the last two weeks of September. They are ready when the stem is firm and the pumpkin turns from green to deep orange. Cut the stem carefully as most have sharp prickles. Use gloves and a sharp blade. Leave around 3-4 inches of stem attached to the pumpkin. You can leave the pumpkin attached to the vine, or cut it and leave it outside. However, if freezing weather is coming, cut your pumpkins and store them in a cool, dry location. Use as soon as possible for decor or your favorite recipe.

Images via James Wheeler, Waldo Jaquith, Austin Kirk and K. Sayer

Since the dawn of civilization, the practice of growing and cultivating plants has played a major role in people’s lives. Throughout history, thousands of different plants have been used to feed and heal all nations in all parts of the world.

But growing plants has never been easy. Facing so many parasites and other enemies of plants, growers always had to stay alert in order to prevent the fruits of their labour from being contaminated and destroyed.

After all, their lives and the lives of their loved ones depended on the health of their crops, so they had to give their best to keep their plants in good condition.

Even today, with all the pesticides, herbicides and technologies we have at hand, plants fall victim to various diseases and parasites. Among the dangers that always lurk is powdery mildew – a silent killer of many plants.

What is powdery mildew?

Powdery mildew, also known as white mold, is a fungal disease that can affect a number of plants.

It is caused by various species of Erysiphales fungi, the most common reported cause being Podosphaera xanthii (also known as Sphaerotheca fuliginea). Different species will infest different plants with similar symptoms appearing on all of them.


In most cases, spores spread via the air inside and around areas where the plants are growing. However, they can occasionally be spread by coming into contact with tools, animals or humans brushing against the plants.

This fungus reproduces incredibly quickly (both sexually and asexually), so if you don’t recognize it on time, it can destroy your entire crop. Luckily, anyone who regularly checks their plants can easily notice a contamination and take steps to prevent it from spreading.

In an agricultural setting, powdery mildew can be controlled using different chemical methods, genetic resistance and careful farming methods. We will go over some of these methods later in the article.

What does powdery mildew look like?

Powdery mildew is fairly easy to identify as its symptoms are quite distinctive:

  • On infected plants, you will see white, powder-like circular patches (hence the name powdery mildew) on the leaves and stems.
  • Usually, the lower leaves will be the most affected, but spots can also appear on any part of the plant that’s above the ground.
  • Powdery mildew generally covers the upper side of the leaves, but it can also grow on the undersides.
  • Young foliage is most frequently affected. Leaves turn yellow and dry out.
  • The spores will get larger and thicker as the disease spreads, often forming a solid mat over the entire infected surface.
  • Late in the growing season, the plant’s leaves, buds and growing tips will all become disfigured.
  • In case of severe infections, you can expect stunted and distorted foliage, leaf drop and even fatal consequences on some plants.

Source: Flickr

What causes powdery mildew?

Unlike the majority of fungal diseases that require moisture and humidity, powdery mildew needs warm and dry conditions to thrive (although humidity around the plant itself has to be relatively high). The fungus will not spread in cool, rainy areas.


With all this in mind, we can easily conclude that this type of mildew is able to spread in many different conditions.

The main causes of powdery mildew are:

  1. High humidity. Because this fungus needs only small amounts of moisture to thrive, you can expect it to appear in any humid environment. This is particularly problematic for young cannabis plants since they need 40-60% relative humidity to grow.
  2. Poor airflow. If you have air circulating properly throughout your greenhouse or grow room, you have nothing to fear. However, when high humidity is combined with limited airflow, powdery mildew has all necessary conditions to thrive. So, if there is bad ventilation or no airflow in the area where you grow your plants, the spores will take advantage of the lack of fresh air to land on your plants and reproduce.
  3. Leaf-to-leaf contact. When the leaves of your plants touch each other, moisture forms between them. To prevent this, you can prune some of the leaves or space plants farther apart from each other so that their leaves never touch.

Is powdery mildew harmful?

Although white powdery mildew does not pose a direct threat to humans, it can be indirectly harmful.

It won’t hurt you if you touch it, but if you are allergic to mold and you pick or consume a plant infected with mildew, you can have a serious reaction to it.

Your plants are the ones that are most affected by powdery mildew. As we already said, this fungus can infest a variety of plants, from grass and flowers to vegetables, fruits, grains and even cannabis.

This type of mildew can prevent your plants from blooming, and can even kill them. If an infected plant does manage to produce fruit, it will most likely be smaller and it will have a dull and inferior taste.


Is powdery mildew systemic?

This is one of the most common questions we get. A number of people who grow plants, particularly cannabis growers, have been debating whether powdery mildew is a systemic pathogen or not.

But first, let’s explain what “systemic” actually means.

Systemic pathogens spread through a plant’s vascular system, while superficial pests spread across the surface of the plant.

Even though the debate is fierce, the most experienced growers claim that powdery mildew is not a systemic fungal pathogen. It does not reproduce in the plant cell—only on the surfaces of the plant, such as the stem and leaves.

Nonetheless, this does not completely eliminate the threat this fungus poses to your plants. As soon as you notice it, you should take appropriate steps to control and prevent it.

Where does powdery mildew grow?

Powdery mildew can grow in any place where it finds favourable conditions. This can be a field, garden, greenhouse or grow room.

Besides being able to grow in a wide variety of environments, it can affect a number of different plants, such as:

  • cucumber
  • zucchini
  • tomato
  • rosemary
  • peonies
  • barley
  • cannabis
  • roses
  • grass

Below, we will list certain plants that are particularly affected by this fungus along with the tips on how to prevent it from getting the best of your crop.

Powdery mildew on cannabis

Have you noticed white spots on your marijuana leaves recently? If yes, you may want to visit your grow facility and ask about powdery mildew.

What is white powdery mildew that grows on cannabis?

As we mentioned earlier in this article, white powdery mildew is a fungus that grows on many plants, including cannabis. As a matter of fact, this contaminant is among the most destructive fungal pathogens for cannabis.

The organisms responsible for the growth of powdery mildew on cannabis are Leveillula taurica and Podosphaera macularis.

How does powdery mildew spread on cannabis?

In general, mildew attacks the youngest plants first. It spreads across the entire plant, ruining the stems, leaves and buds of your plant.

If you notice a damp smell coming from your buds, your marijuana has likely been affected.

It is best to act before the fungus invades the bud; otherwise, the reproduction process cannot be reversed.

When the mildew reaches advanced stages of growth, it will make your cannabis plants go yellow first. The plants will then become brown and, eventually, they will simply die.

This could mean the end of your dreams of becoming a successful marijuana grower. That is why you need to make sure you are constantly inspecting your plants and paying attention to any unusual changes.

What causes powdery mildew on cannabis?

This fungus often affects cannabis plants since grow rooms have to be very humid. In addition, grow rooms sometimes have bad ventilation, which reduces proper airflow and generates perfect conditions for mildew growth.

Also, if your cannabis cultivation facility is overcrowded with marijuana plants, leaves will touch other leaves, which will help powdery mildew to spread, making it even harder for you to eradicate the problem.

Is powdery mildew harmful to smoke?

When you smoke moldy cannabis, you are sucking in thousands or millions of mold spores. Does this sound safe?

Of course smoking mildew-contaminated marijuana is harmful. If you do this, you will inhale a number of dangerous toxins, which can seriously damage your immune system. And this is only one of the reasons to get rid of powdery mildew on cannabis.

How to get rid of powdery mildew on cannabis (DIY solutions)

The best way to make sure this contaminant does not take a toll on your plants is to protect your plants right from the start.

If you want to know how to get rid of powdery mildew, here are some tips:

  • Make sure that plants have plenty of space in the room where they are growing. This will prevent the mildew from spreading from leaf to leaf.
  • Improve ventilation in your grow room. This will give your plants enough fresh air and stop the contaminant from spreading. However, it is best to leave this to professionals.
  • When watering your plants, make sure that they get at least five hours of light after that, be it natural or artificial. If you want it to be natural light, you have to water the plants shortly after noon.
  • Prune the fan leaves that do not receive enough direct light because of shade.
  • If you’re growing marijuana indoors, you can use a UVC light to detect any missed spores from spreading any further.

Moreover, you can spray different kinds of DIY solutions on your plants. However, this will work only if your plants are just slightly contaminated with mildew.

Here are some examples of DIY mixtures you can prepare:

  • milk
  • baking soda
  • apple cider
  • hydrogen peroxide

We will explain how to apply each of these DIY solutions later in the article.
*Please note that we don’t give any guarantee that these DIY solutions will help you and we disclose ourselves from any damage impacted by using them.

How to remove powdery mildew on cannabis (the right way)

If you fail to prevent mildew from growing on your marijuana or if you are unable to remove it once it has already started growing, you risk losing your entire crop, which means all your work has been in vain.

For this reason, it’s best to let professionals take care of your mildew problem.

For optimal results, you should leave inspection, mold removal, grow room ventilation setup and other complicated procedures to experts.

To get your grow area inspected and your mildew removed by professionals, call Mold Busters.

Powdery mildew on roses

If you are a professional gardener or just like having roses in your garden, you know that they are delicate flowers, especially in early growth phases.

In order for your flowers to be healthy, you need to keep them safe from various toxic parasites such as powdery mildew.

What is white powdery mildew that grows on roses?

The organism that is responsible for powdery mildew on roses is called Sphaerotheca pannosa var. rosae.

Source: Flickr

It forms a white powder over the surface of rose leaves, sometimes even spreading to the stem and new rose buds.

It causes disfiguration of the rose leaves and blooms, preventing the buds from opening.

What causes powdery mildew on roses?

This fungus is commonly grows on roses when warm, dry days are followed by cool, humid nights. If you are not careful, it won’t take long for it to spread to your entire garden.

How to treat powdery mildew on roses (DIY solutions)

Similarly to cannabis, you can use a few DIY mixtures to protect your roses:

  • milk
  • baking soda
  • fungicide

*Please note that we don’t give any guarantee that these DIY solutions will help you and we disclose ourselves from any damage impacted by using them.

How to remove powdery mildew on roses (the right way)

Again, trying to apply DIY solutions will only protect your roses up to a certain point.

To ensure that your roses are truly safe and protected, it is necessary to consult experts, especially if you grow your flowers in a greenhouse, where the air is not as fresh as outdoors.

To make your gardening a bed of roses easier and safer, contact Mold Busters today.

Powdery mildew on buds after harvest

No cannabis grower wants to see powdery mildew on buds after harvest.

In fact, the harvesting process itself can spread the spores to uninfected plants.

Source: Flickr

And once mildew is on the buds at harvest time, there is not much you can do to fix the problem.

What causes powdery mildew on buds?

The same fungal pathogens that cause powdery mildew on the stem and leaves of cannabis plants are responsible for powdery mildew on the buds, namely Leveillula taurica and Podosphaera macularis.

The fungus will first grow on young leaves of mature plants or on younger plants. After that, it will spread to buds.

How to remove powdery mildew from buds (DIY solutions)

Having powdery mildew on marijuana buds is a real catastrophe because there is very little you can do at this point.

There are only a few ways to remove mildew from the buds.

  • You can try to remove the infected plants from the grow area by bagging them up.
  • You can cut out the infected leaves and buds in a plastic bag (if the plants are not severely infested).

However, if the fungus has spread to other plants, cutting and removing the infected buds and leaves is practically all you can do.

How to remove powdery mildew on cannabis buds (the right way)

Once plants get infested, it won’t take long for mildew ruins the entire crop. That is why you need to prevent it before the infestation takes hold.

For this, you need a team of experts to inspect your grow facility for proper air quality and other conditions. Only then will you know what exact steps you should take to solve the issue effectively.

Powdery mildew on grass

Even though this may sound impossible, even grass can be affected by powdery mildew.

Source: Flickr

As you can see, this fungus is everywhere. In fact, it may be hiding in your lawn as you read this article.

What is white powdery mildew that grows on grass?

The name of the fungus pathogen that affects grass is Blumeria graminis (also known as Erysiphe graminis).

It is responsible for the powdery mildew that appears on various grasses and cereals.

What causes powdery mildew on grass?

Powdery mildew will appear on grass that grows in poor conditions.

Some of the causes can be:

  • drought
  • compacted soil
  • overwatering
  • improper mowing
  • weather conditions

How to treat powdery mildew on grass (DIY solutions)

To remove powdery mildew from your lawn, you can take the following steps:

  • spraying fungicides
  • watering early during the day so that the grass has enough time to dry before nightfall
  • opening up the area to allow better air movement and more sunlight
  • reducing the amount of nitrogen fertilizer that you use in shaded areas
  • allowing the grass in shady areas to grow a little taller than the rest of the lawn

*Please note that we don’t give any guarantee that these DIY solutions will help you and we disclose ourselves from any damage impacted by using them.

How to remove powdery mildew on grass (the right way)

Can you say with certainty which of the causes is responsible for the mildew growth on your grass?

Furthermore, are you sure that you can remove this fungus effectively?

To save your time and energy, and to be sure that the mildew is removed properly from your grass, you will need professional mold remediation technicians like Mold Busters who have over 10 years of experience identifying and treating mold problems of all sorts.

How to control and prevent powdery mildew growth

So far, we have listed a number of ways to prevent and control powdery mildew from forming on your plants such as ensuring adequate ventilation, giving your plants enough space to grow and pruning.

But there are also a few mixtures that you can use to control this fungus. As promised, we will now list a few DIY mixtures that can help you treat powdery mildew on different plants.
*Once again please note that we don’t give any guarantee that these DIY solutions will help you and we disclose ourselves from any damage impacted by using them.

Baking soda (for cannabis and roses)

To create a baking soda mixture, combine:

  • 1 tablespoon of baking soda
  • ½ spoon of liquid soap
  • 1 gallon of water

Water the leaves of your plants a couple of days before applying the mixture, and do not apply it in full sunlight. Try on a small area first to see how the plant will respond.

Milk (for cannabis and roses)

Use a 40:60 milk-to-water dilution and spray biweekly.

The salts and amino acids found in milk seem to kill the fungus and help the plant’s immune system.

Apple cider vinegar (for cannabis)

Another DIY solution to consider is apple cider vinegar.

Mix 4 tablespoons of organic apple cider vinegar with 1 gallon of water.

Mist the mixture onto your plants every three days to destroy the mildew and prevent it from growing back.

Hydrogen peroxide (for cannabis)

Pour the following ingredients into a one-gallon spray bottle:

  • 1 quart of standard 3% hydrogen peroxide
  • 3 quarts of water

Shake the spray bottle and spray the mixture onto your plants, focusing on areas infested with powdery mildew. Repeat this process every day until the mildew disappears.

Fungicide (for roses and grass)

There are a number of off-the-shelf fungicides that can treat powdery mildew effectively. One of them is chlorothalonil, which is strong and effective but leaves a white milky film on the leaf surface.

You can try any of these solutions today and see how your plants react. However, always keep in mind that DIY solutions can only partly prevent powdery mildew growth.

If you want to know what causes powdery mildew in your garden or grow room and to find the best way to solve this issue, you have only one option—ask experts for help.

The Mold Busters team has 15 years of experience in mold testing and removal, grow op remediation, odor control and other mold-related procedures.

When you ask us for help, we will conduct a detailed inspection and implement the best solution for your situation.

Contact Mold Busters today to stop powdery mildew from further damaging your plants.


Powdery mildew grows on many different plants and causes serious problems in gardens, fields and grow facilities. It usually thrives in small spaces with high humidity and insufficient airflow.

There are various solutions you can try today to remove this persistent fungus, from leaving more space between plants to applying some of the DIY anti-fungal mixtures.

When you use these methods, however, you can never be certain that you have removed the mildew completely.

The only option that guarantees success is a professional mold inspection and mold removal conducted by a team of experts. When you call us, you will know everything about the mildew on your plants—from what caused it to how to prevent it completely.

Get rid of powdery mildew for good and call Mold Busters today.

Powdery Mildew


Powdery mildew symptoms on rose leaves.
Jody Fetzer, New York Botanical Garden, www.forestryimages.org

Powdery mildew is the name given to a group of diseases caused by several closely related fungi. Their common symptom is a grayish-white, powdery mat visible on the surface of leaves, stems, and flower petals. There are many hosts; and although this disease is not considered fatal, plant damage can occur when the infestation is severe.

Disease Cycle

In spring, as daytime temperatures rise above 60 °F, the fungi responsible for powdery mildew begin to produce spores (conidia) which are dispersed into the air. Infections occur when they contact a suitable host and environmental conditions are favorable. Initial symptoms are small, circular, powdery, white spots which expand and eventually join as infections progress. Infections spread as spores produced in these white patches move by wind and splashing rain to other locations on the plant or nearby plants.

Close up view of powdery mildew mycelium mat and spore producing bodies.
Elizabeth Bush, Virginia Polytechnic Institute and State University, www.forestryimages.org

The fungus survives the winter attached to plant parts and plant debris such as fallen leaves. As weather warms in spring, the process begins again.

Favorable Conditions

Humidity is an important factor related to the onset and spread of powdery mildew. Unlike most fungi, these do not require free water to germinate; only high levels of relative humidity. High relative humidity favors spore formation, and low relative humidity favors spore dispersal, which explains why powdery mildew tends to be a problem when the days are cool and the nights are humid. Temperature is also a factor. Although powdery mildew can occur all season long, it is less common during the heat of the summer.


Powdery mildew on zinnia leaf.
R.K. Jones, North Carolina State University, www.forestryimages.org

Powdery mildew is caused by several different species of fungi, and they each have a limited host range. In other words, observing powdery mildew on oak leaves should not be cause for concern for nearby zinnias. Plants that commonly become infected with various powdery mildews include; azalea, crabapple, dogwood, phlox, euonymus, lilac, snapdragon, dahlia, zinnia, crape myrtle, rose, pyracantha, rhododendron, spirea, wisteria, delphinium, oak, English ivy, photinia, blueberry, pecan, cucumber, and squash.


As powdery mildew fungi grow over the plant surface, they develop structures that are inserted into plant cells enabling them to extract nutrients necessary for growth and spore production. This results in a general decline in growth and vigor of the host, as well as the common visible symptoms.

Abnormal growth, such as leaf curling, twisting, and discoloration, may be noticed before the white signs of the fungus are visible. On dogwood, for example, leaves may take on a yellowish or reddish cast in summer or may develop reddish blotches or dead, scorched patches. The white powdery growth is not always apparent.

Advanced powdery mildew on pumpkin with leaf distortion, yellowing and browning.
Howard F. Schwartz, Colorado State University, www.forestryimages.org

When visible, the powdery fungal growth can usually be found on the upper surface of the leaves, and tends to begin on lower leaves. As the disease progresses, leaves become dwarfed, curled and generally distorted. In severe cases, leaves will turn yellow or even dried and brown.

Powdery mildew fungi will also infect flowers, causing them to develop abnormally or fail to open. On azaleas and rhododendrons, small areas of dead tissue are often seen.

Powdery mildew creates other effects that are not readily visible. For example, a severely infected plant may have a reduced level of winter hardiness. Trees have also been observed to leaf out later in the spring after being infected the previous season.

Cultural Controls

Powdery mildew damage on rose flower buds.
Clemson University – USDA Cooperative Extension Slide Series, www.forestryimages.org

As with all diseases, optimum plant health is the first line of defense. This begins with selection of healthy plants that are planted properly and in the proper location, giving attention to requirements for light, soil, and moisture. Space them so they are allowed to grow without being crowded and water thoroughly during establishment, and later during dry periods. Avoid overhead irrigation which raises the level of relative humidity within the plant canopy.

If powdery mildew is noticed on a few leaves, simply removing them will help with control. At the end of the growing season, prune out infected stems and remove fallen leaves which can serve as a source of further infection. Suckers are common on crape myrtle, dogwood and other plants. These should be pruned as they develop because they are especially susceptible and the disease will spread from them upwards to other plant parts.

Fertilize to optimize plant health, but avoid over-fertilization with nitrogen as it stimulates young, succulent growth which is more susceptible to infection.

Plants with a severe infection should be monitored closely the following spring so that if infections reoccur, they can be treated early.

When possible, select plants that show resistance to the disease (see Table 1).

Chemical Control

Ornamental Plants: For fungicides to be effective, they must be applied as soon as symptoms are noticed. Product labels will provide information on how often to spray. When ranges are given, use the shorter interval during cool, damp weather. Be sure to cover both the upper and lower surfaces of the leaves.

Table 2 lists fungicides labeled for ornamental plants. Myclobutanil, propiconazole, and thiophanate-methyl have systemic properties and can be sprayed less often than chlorothalonil, sulfur or copper-based fungicides. When powdery mildew persists and sprays are repeated, it is recommended to rotate (alternate) fungicides to decrease the chance of fungi developing resistance.

When deciduous plants are infected, consider the season. Generally, foliar diseases occurring in late summer do little damage. The leaves have already produced food for the plant and are going to fall off soon anyway. Just be sure to rake and dispose of them as they fall.

As with any pesticide, read the label and heed all precautions. Sulfur, for example, can damage plants if applied when temperature and humidity are high.

Vegetable Plants: For information on vegetable crop disease controls and tolerant varieties, see HGIC 2206, Cucumber, Squash, Melon and Other Cucurbit Diseases, HGIC 2200, Bean and Southern Pea Diseases, and HGIC 2202, Cabbage, Broccoli and Other Cole Crop Diseases.

Table 1. Plants with Resistance to Powdery Mildew.

Species Cultivars
Kousa dogwood, Cornus kousa ‘Milky Way’, ‘Milky Way Select’, ‘National’
Cornus florida x kousa hybrids ‘Aurora’, ‘Constellation’, ‘Celestial’, ‘Stellar Pink’
Flowering dogwood, Cornus florida ‘Cherokee Brave’, ‘Springtime’, ‘Pygmy’, ‘Jean’s Appalachian Snow’, ‘Karen’s Appalachian Blush’, ‘Kay’s Appalachian Mist’
Crepe Myrtle: The Lagerstroemia indica x faurieri hybrids ‘Apalachee’, ‘Fantasy’, ‘Hopi’, ‘Miami’, ‘Osage’, ‘Tonto’, ‘Tuscarora’, ‘Tuskegee’, ‘Wichita’, ‘Acoma’, ‘Sioux’, ‘Natchez’
Phlox ‘David’, ‘Delta Snow’, ‘Natascha’, ‘Robert Poore’
Zinnia Pulcino and African varieties, Zinnia angustifolia, Profusion Cherry, Profusion Orange
Hybrid Tea Rose ‘Duet’, ‘Eiffel Tower’, ‘Grand Slam’, ‘Mister Lincoln’, ‘Tiffany’, ‘Jamaica’, ‘Matterhorn’
Floribunda Rose ‘Golden Slipper’
Grandiflora Rose ‘Camelot’, ‘Queen Elizabeth’, ‘John S. Armstrong’, ‘Pink Parfait’
Rugosa Rose ‘Rugosa Alba’, ‘Blanc Double de Coubert’, ‘Fru Dagmar Hastrup’, ‘Topez Jewel’, ‘Alba’, ‘Alba Semi-Plena’
Monarda ‘Marshall’s Delight’, ‘Cambridge Scarlet’

Table 2. Fungicides for Powdery Mildew Control on Ornamental Plants.

Active Ingredient Examples of Brand Names & Products
Myclobutanil Spectracide Immunox Multi-Purpose Fungicide Concentrate
Ferti-lome F-Stop lawn & Garden Fungicide Concentrate
Monterey Fungi-Max
Sulfur1 Safer Brand Garden Fungicide Concentrate; & RTU
Hi-Yield Wettable Dusting Sulfur
Southern Ag Wettable or Dusting Sulfur
Bonide Sulfur Plant Fungicide
Propiconazole Ferti-lome Liquid Systemic Fungicide II Concentrate; & RTS
Bonide Infuse Systemic Disease Control Concentrate; & RTS
Banner Maxx Fungicide
Thiophanate-methyl Cleary’s 3336-WP Turf & Ornamental Fungicide
Southern Ag Thiomyl Systemic Fungicide
Chlorothalonil Ortho Max Garden Disease Control
Garden Tech Daconil Fungicide Concentrate
Hi-Yield Vegetable, Flower, Fruit & Ornamental Fungicide
Southern Ag Liquid Ornamental & Vegetable Fungicide
Tiger Brand Daconil
Ferti-lome Broad Spectrum Landscape & Garden Fungicide Conc.
Bonide Fung-onil Concentrate
Horticultural Oil2 Ferti-lome Horticultural Oil Spray Concentrate
Monterey Horticultural Oil Concentrate
Southern Ag ParaFine Horticultural Oil
Bonide All Seasons Spray Oil Concentrate
Summit Year Round Spray Oil Concentrate
Neem Oil Extract Southern Ag Triple Action Neem Oil Concentrate
Ferti-lome Rose Flower & Vegetable Spray Concentrate
Garden Safe Fungicide 3 Concentrate
Garden Safe Neem Oil Extract Concentrate
Bonide Neem Oil Concentrate
Monterey 70% Neem oil Fungicide/Insecticide/Miticide Conc.; & RTS
Copper-based Fungicides Bonide Liquid Copper Concentrate
Camelot O Fungicide/ Bactericide Concentrate
Monterey Liqui-Cop Fungicide Concentrate
Natural Guard Copper Soap Fungicide Concentrate; & RTU
Southern Ag Liquid Copper Fungicide
Tebuconazole Bayer Advanced Disease Control for Roses, Flowers & Shrubs Conc.
Potassium Bicarbonate Monterey Bi-Carb Old Fashioned Fungicide
Milstop Broad Spectrum Foliar Fungicide
Bacillus subtilis AgraQuest Serenade Garden Disease Control Concentrate
Bioworks Cease Biofungicide
Note: These active ingredients are listed in approximate order from most efficacious (best control) to least, but this also depends upon the plant and species of powdery mildew fungus. Be sure to check the product label for which plants can be sprayed with that product. For many vegetable crops, sulfur, copper-based products, chlorothalonil, horticultural oil, potassium bicarbonate and Bacillus subtilis can be used for powdery mildew control.
1 Do not apply sulfur if temperature is greater than 90 ºF or to drought stressed plants. Do not use sulfur in combination with, or within 2 weeks before or after the use of horticultural oil treatments. Sulfur will also control mites.
2 Do not apply horticultural oil if temperature is greater than 90 ºF. Horticultural oil may injure Japanese, armur and red maples, cryptomeria, junipers, cedars, redbud, smoke tree and hickories. Add 3 tablespoons of horticultural oil to a gallon of water with 3 tablespoons of baking soda for better powdery mildew control.
RTS = Ready-To-Spray (hose-end sprayer). RTU = Small, pre-mixed bottle.

Caution: Pollinating insects, such as honey bees and bumblebees, can be adversely affected by the use of pesticides. Avoid the use of spray pesticides (both insecticides and fungicides), as well as soil-applied, systemic insecticides unless absolutely necessary. If spraying is required, always spray late in the evening to reduce the direct impact on pollinating insects. Always try less toxic alternative sprays first for the control of insect pests and diseases. For example, sprays with insecticidal soap, horticultural oil, neem oil extract, spinosad, Bacillus thuringiensis (B.t.), or botanical oils can help control many small insect pests and mites that affect garden and landscape plants. Neem oil extract or botanical oil sprays may also reduce plant damage by repelling many insect pests. Practice cultural techniques to prevent or reduce the incidence of plant diseases, including pre-plant soil improvement, proper plant spacing, crop rotation, applying mulch, applying lime and fertilizer based on soil test results, and avoiding over-head irrigation and frequent watering of established plants. Additionally, there are less toxic spray fungicides that contain sulfur or copper soap, and biological control sprays for plant diseases that contain Bacillus subtilis. However, it is very important to always read and follow the label directions on each product. For more information, contact the Clemson Extension Home & Garden Information Center.

Oidium and mildew

The terms oidium and mildew refer to a group of phytopathogenic fungi that cause diseases in plants and have similar symptoms. Oidium is also known as “powdery mildew” and mildew as “downy mildew”.

Fungi of this kind appear on the back of the leaves (abaxially). These fungi can usually be recognized because some whitish, gray or pink-hued marks appear on the leaves as if they have had ash sprinkled on them. Initially they look like traces of salt left on the leaves after the water has evaporated or like remains of powder, and they can therefore go unnoticed by the grower. However, as the disease advances, the leaves can end up being completely covered in this white layer and it can even colonize other plant parts, with subsequent losses in crop size and quality.

Although the symptoms of these fungi are apparently quite similar, there are differences between them that will help you to tell which particular fungus is attacking your plants. The best treatment for this type of fungi is prevention; once they set in and develop they are very difficult to eradicate, sometimes even with chemical fungicides. Knowing which fungus you are fighting against will allow you to be certain of picking the best treatment and the one with the fewest side-effects, as well as knowing the best time to apply.

If you want to use biological products to stop the fungus you need to bear in mind that the effect is not very long-lasting; so unless you get the timing right, all you’ll be doing is wasting time and money. Also, a product that is effective against one type of Oidium may not work against another similar-looking mildew fungus.

For effective treatment, you need to know what conditions the fungus prefers to develop in and what other factors —such as weather conditions— affect it and in what way. That is why it is so important to determine which specific fungus is attacking your crops, since the optimum conditions for each one are different. For example, some fungi cannot germinate on wet leaves, while others actually need water to spread on the plant.

One of the differences in the symptoms that will help you identify the fungus responsible is the side of the leaf on which the marks appear. Symptoms on the upper side of the leaves are the most common cases. According to McPartland et al., the fungi responsible in tomatoes are:

Sphaerotheca macularis

This is an oidium that attacks a broad range of hosts, so it is very likely to have a reservoir near your grow area. It is a serious problem of strawberry and hop plants.

This fungus spends the winter in the form of asci (an ascus is a kind of bag, fruiting structure, contain- ing a type of spore called ascospores) or mycelium on other wild or cultivated host plants (in parks and gardens, on verandas and terraces, etc.). Infected indoor and greenhouse crops also act as a good reservoir for the disease. This mycelium, which is harbored on other plants, releases conidia (a type of spore). These conidia, mostly carried by the wind, are what end up on the surface of your plant leaves; if the conditions are right, they then germinate and develop, forming new conidia, starting the cycle all over again and causing the fungus to spread to the whole plant.

The higher the concentration of conidia in the atmosphere, the greater your plant’s chances of suffering a serious attack by this fungus, and any preventative measures you take should therefore be targeted at creating conditions and applying treatments that prevent the conidia from taking hold on your plants.

This fungus develops on the surface (it does not penetrate deeper layers of the leaf). Another way of identifying it is by rubbing your finger across the leaf: the powder leaves a mark on your finger. That might fool you into thinking that it is easy to eradicate with fungicides. However, if you use biological products with a low systemic effect, the parts you haven’t reached with the fungicide (or where it has not penetrated far enough), will continue to contain small parts of the fungus, which can grow back quickly under favorable conditions. That is why you must keep up the treatment even when you can no longer detect any oidium.

Environmental factors

According to the studies we have consulted, the environmental factors involved in the establishment and development of the fungus on different hosts are as follows:

  • 1. Average daily temperature:
    Temperatures of over 15°C / 59°F favor the development of the fungus and the spread of the conidia. In general, production of conidia is reduced at temperatures below 15-20°C / 59-68°F or above 26°C / 79°F. According to studies conducted on hops, exposure of around two hours to temperatures over 32°C / 90°F reduces the incidence of the disease.
  • 2. Relative humidity:
    The optimum range for germination of the conidia is between 75% and 98% humidity. At relatively low humidity rates, the fungus reacts by releasing a greater number of spores. This dispersion is also favored by sudden fluctuations in humidity. The more the ambient humidity falls and the more abruptly it does so, the greater the number of spores released into the air. Although the environmental humidity is low, due to plant transpiration the leaf surface may be quite damp, facilitating germination of the conidia. In such cases, a digital temperature and humidity gauge with maximums and minimums is a must, since it will give you a precise idea of when these high-risk conditions arise.
  • 3. Rain:
    The rain washes away any spores floating in the air and the likelihood of infection on a rainy day is therefore low. A layer of water on the leaves can also prevent the spores from germinating and the conidia from developing and spreading. The conidia need light to ripen, so the spores are scattered by day, roughly between 1 pm and pm. The most critical time of day tends to come between 5 pm and 9 pm. Studies with hops show that spores that germinate in this time range are more likely to cause greater harm.

Pink rot

This is the fungus Trichothecium roseum. Its symptoms are similar to those described above, but usually occur on the upper side of the leaf. What distinguishes this fungus from S. macularis is that during certain phases of its development it can take on a pink hue. However, in another of its stages, it has the same white or grayish color as S. macularis, and is therefore easily mistaken for it. According to McPartland et al., another difference with S. macularis is that this fungus is usually limited to the leaves, whereas pink rot can colonize even the stems.

Unlike the previous case this fungus is a saprophyte (it also develops on dead matter) which means that there is always a reservoir of this fungus nearby. As a result, the fungus begins to develop on the plants helped by sticky remains of pests such as white fly, plant lice, wood lice, etc., or on remains of dust or pollen that may have been deposited on the leaves. Once it has developed and built up its strength on these remains it is in a better position to infect the living tissue. There are practically no references on the damage caused by this fungus or on its biology as a pathogen.

Leveillula taurica

This oidium differs from the others mainly in two aspects: while sphaerotheca macularis and Trichothecium roseum only penetrate the surface layers of the plants, L. taurica colonizes areas further in. The result is that unlike the other oidia, when you run your finger over the typical “powder”, it doesn’t come off altogether or leaves a mark on the leaf. in addition, whereas the conidiophores (the parts of the mycelium in which the reproductive spores –or conidia– are located) of the fungi s. macularis and T. roseum grow on the surface in the mycelium that is produced, the conidiophores of L. taurica emerge through the stomata of the plant, which are located mainly on the back of the leaves.

Symptoms vary greatly, depending on the species on which they live. under the microscope, L. taurica can be distinguished from s. macularis by the morphology of the conidia and conidiophores, and by the emergence of the conidiophores through the stomata. Remember that the stomata are mostly located on the back, so you might notice a whitish felt running round the leaf. There is very little information available on the factors that affect this species in hop plants. Although this phytopathogenic affects around 700 different species, the principal studies carried out have been on hop plants.

These studies have observed that the conidia germinate at temperatures of between 50°F and 95°F (10- 35°C), with the optimal temperature being 68°F (20°C). More extreme conditions (6 hours at 104°F (40°C) significantly reduce the viability of the spores (meaning that in very warm areas there is a smaller chance of infection in summer). It has previously been noted that the fungus needs climatic variations to develop its full cycle and this is also the case here. While the optimal temperature for germination is 68°F (20°C), no new spores form in infected plants at higher temperatures; however the optimum temperature for growth of the mycelium is between 59°F (15 – 25°C).

As for relative humidity levels, the most favorable conditions for germination are a daytime figure of between 85% and 95% and also very humid nights. Although high levels of humidity favor germination, they also hinder development of the mycelium. It is easy to see that the best way of ensuring that plants don’t become infected is by taking preventative treatment when the average temperature is between 50°F and 95°F (10-35°C) and when relative humidity remains high both day and night (e.g. during long periods of rain or constant daytime showers, etc.).

If your indoor plant has already become infected (whether the symptoms are visible or not), you should make sure to keep the temperature and humidity constant without sudden variations. By keeping the temperature as high as possible (without impairing development of the plant) you will help prevent the formation of new spores and hinder growth of the mycelium, thus slowing or stopping the disease from spreading. You should also regularly use natural fungicide treatments. This will prevent the new spores that would spread the disease from forming or make them less viable. At the same time, it will hinder growth of the fungus, thus helping the natural fungicide act more effectively.

Toxicity of the Oidium

Turning to the intrinsic toxicity of the oidium fungi that attack the hop, L. taurica and S. macularis do not produce toxins that might be considered dangerous for human health. However, don’t forget about false powdery mildew or pink Oidium, which even advanced growers find difficult to distinguish from the real thing.

Scientific studies have been carried out to try to establish the differences between the two fungi on hop so they can be safely identified. This pink oidium, Trichothecium roseum, produces a number of micotoxins which are highly toxic in mammals. Studies carried out in 1969 shows that extracts of this fungus at different levels of concentration were capable of killing mice, rabbits and even 19-day-old pigs, as well as causing other injuries.

Given how dangerous this fungus is, you should obviously do everything in your power to prevent the development of Oidium on your plants rather than taking the risk of consuming contaminated plants.

Protectant (contact) fungicides are an important component of the fungicide program recommended for managing powdery mildew in cucurbits. They play a critical role in delayingdevelopment of resistance to systemic fungicides. The fungus causing cucurbit powdery mildew has an established track record worldwide of being able to develop resistance to systemic fungicides. Most systemic fungicides are at risk for resistance development because they have single-site mode of action. Thus modification of one gene in the pathogen may be enough to enable the pathogen to resist the action of the fungicide. Most protectant fungicides are not at risk for resistance development because they have multi-site mode of action. Their role in resistance management is to control both sensitive and resistant strains thereby reducing the overall size of the pathogen population subjected to the resistance-selecting action of the systemic fungicides. Systemic fungicides are essential for managing powdery mildew on the underside of leaves (Figure 1) to avoid premature death of leaves (Figure 2). Therefore, managing resistance to prolong the utility of systemic fungicides is very important. It is important to recognize that the goal of resistance management is to delay development of resistance rather than to manage resistant strains; therefore, resistance management should always be a component of powdery mildew management. For more information, see the on-line articles on resistance.

The integrated program currently recommended for managing powdery mildew includes nonchemical control measures plus a fungicide program with both systemic and protectant fungicides. Nonchemical control measures include selecting resistant varieties and separating successive plantings. These are important for managing fungicide resistance because they reduce the overall size of the pathogen population and because fewer applications of systemic fungicides may be needed for control, thereby reducing selection pressure for resistance. Further reduction can be achieved by using systemic fungicides only when needed and not curatively; this can be accomplished by scouting to ensure applications are started very early in powdery mildew development. Additionally, protectant fungicides alone may be sufficient for the last applications late in the growing season. The fungicide program should consist of an alternation among systemic fungicides with different modes of action (eg a QoI, aka strobilurin, fungicide, Quadris or Flint, alternated with the DMI fungicide Nova) and mixing these with protectant fungicides. When new systemic fungicides are registered, they should be added to the program or used in place of other systemics whose efficacy has been reduced due to resistance.

Click on image for magnification

Figure 1. Because powdery mildew colonies typically are larger with denser sporulation on lower leaf surfaces than on upper surfaces, control on lower surfaces (left section of this leaf which is folded over) is very important.

Click on image for magnification

Figure 2. Leaves on pumpkin plants sprayed weekly with the protectant fungicide chlorothalonil provided excellent control of powdery mildew only on upper leaf surfaces.
Leaf is partially folded over to show that the lower surface is completely covered with powdery mildew, in contrast with the upper surface. Leaf is senescing prematurely due to severity of powdery mildew infection on the lower surface.

Several protectant fungicides are now registered for use on cucurbits to manage powdery mildew in the United States (Table 1). Chlorothalonil has been the primary protectant fungicide used for powdery mildew. Copper fungicides and sulfur have also been used. Several new products have recently been registered by EPA; others are being developed. Most of these new products have active ingredients considered to be biocompatible; several of these have been approved by OMRI for organic production. The active ingredients include mineral oil (products include JMS Stylet-oil, SunSpray Ultra-Fine, and Spray oil), potassium bicarbonate (Armicarb and Kaligreen), and monopotassium phosphate (Nutrol). Serenade is a biofungicide containing the bacterium Bacillus subtilis. Milsana, which contains extract of giant knotweed, has not been registered yet. Additionally, fertilizers such as Prudent Plus are being developed that promote protection from disease by stimulating the natural defense systems present in plants. There are formulations of JMS Stylet-oil, Kaligreen, and Serenade approved for organic production. These products provide an alternative to organic growers who prefer not to use copper or sulfur because they are elements and thus do not degrade.

Efficacy is another important consideration, in addition to toxicity, when selecting a fungicide. The more efficacious the product, the greater its contribution to resistance management. Several protectant fungicides have been compared in field experiments conducted in Riverhead, New York (Table 2). Quadris applied in alternation with Nova plus Bravo was used as a grower standard in these experiments. It is the last treatment listed in Table 2. It is important to look at performance of fungicides used alone to assess their contribution within an integrated program. Products that were significantly less effective than Bravo when evaluated alone were equally effective when used with Quadris and Nova. Sulfur formulated as Microthiol Disperss, mineral oil formulated as JMS Stylet-oil, and chlorothalonil formulated as Bravo Ultrex were the most effective protectant fungicides, providing 93-98% control on upper leaf surfaces in 2001 (Table 2). Although these aren’t systemic, they did provide 37-55% control on lower (under) surfaces of leaves, which was at least partly due to reducing inoculum by controlling mildew well on the upper surface; better control was achieved with systemic fungicides. Sulfur can be phytotoxic on melons. The next most effective material was copper hydroxide formulated as Kocide 2000, providing 84% and 18.5% control, respectively. The biofungicide Serenade (Figure 3) and the health-promoting fertilizer Prudent Plus were less effective, providing 36% and 40% control, respectively, on upper leaf surfaces.

Click on image for magnification

Figure 3. Pumpkins in the foreground were treated weekly with Serenade while those in the background were treated on the same schedule with JMS Stylet-oil in 2002.

Treatments with high powdery mildew severity had more defoliation late in the growing season and lower yield than treatments that provided better control of powdery mildew. For example, Serenade was the only treatment in 2001 that did not have less defoliation than nontreated plants on 6 September while the amount of defoliation with the more effective products, JMS Stylet-oil, Microthiol Disperss, and Bravo, was not significantly different from the standard program with Quadris and Nova. Pumpkins treated with Microthiol Disperss or the grower standard program of Quadris alt. Bravo plus Nova had the most fruit with good solid handles, an important measure of quality in Halloween pumpkins. These treatments were significantly better than the other treatments tested in 2001.

Protectant fungicides that are less effective than Bravo when applied to a susceptible variety can be more useful for suppressing powdery mildew on resistant varieties. Typically resistant varieties do not provide complete control of powdery mildew and an integrated program including a few timely fungicide applications is recommended to improve control and thwart selection of new races of the pathogen able to overcome the resistant variety. A ‘biocompatible’ fungicide program (consisting of Kocide and Kaligreen applied in alternation on a weekly schedule beginning after mildew detection) provided 82%, 98% and 99% control on upper leaf surfaces of susceptible melon variety Superstar, variety Eclipse with race 1 resistance, and variety Apollo with resistance to races 1 and 2, respectively. This program provided 19%, 47%, and 86% on lower leaf surfaces, respectively.

The various protectant fungicides differ considerably in price and in their range of activity (Table 1). Microthiol Disperss is the least expensive product tested. Armicarb and Serenade are the most expensive. Bravo and Kocide have the broadest spectrum of activity (Table 1).

For more information on powdery mildew and its management, see the other on-line articles and the fact sheet on powdery mildew.

Occurrence of Strobilurin resistance and impact on managing powdery mildew of cucurbits(June 2003)

Powdery Mildew of Cucurbits Fact Sheet (1997)

Posted by Rosemary Gordon|April 1, 2016

Powdery mildew, as shown on the pumpkin leaf here, can reduce the marketability of crops it affects.
Photo credit: Beth Krueger Gugino, Penn State

Powdery mildew can be a problem, especially for pumpkins. Resistant varieties are available, but it’s important to note that none are immune. It’s also critical to make a correct diagnosis when the disease shows up and to know which crop protectants to use.

I caught up with Beth Gugino, an Associate Professor in the Vegetable Pathology Department of Plant Pathology and Environmental Micro-biology College of Agricultural Sciences at Penn State, to find out more on keeping this pathogen out of your pumpkin fields.


Q1 Powdery mildew typically shows up first as white powdery spots on the leaves’ undersides. Why is it so important to scout the entire plant for powdery mildew?
Gugino: The microclimate within the plant canopy tends to be more favorable for disease development, so it is important to not only look at the upper surface of the leaves, but also the lower surface as well as the petioles and crown of the plant. Often powdery mildew will first be seen on the older leaves. Powdery mildew is favored by dense foliage and lower light intensity. Infection is favored by the higher relative humidity that can be found within the crop canopy. However, unlike many other pathogens, infection also can occur at a relative humidity less than 50%.

Q2 Why is accurate diagnosis so critical?
Gugino: It is very important to be able to distinguish between the symptoms of powdery mildew and downy mildew because the fungicides that are effective for powdery mildew are not effective for managing downy mildew and vice versa. They are very different pathogens despite both having mildew as part of their common name.

Q3 When do symptoms typically begin to show up? What environmental conditions are best for the disease?
Gugino: Powdery mildew typically develops around the onset of fruiting. Drier conditions favor pathogen colonization, sporulation, and spread, so more frequent rain events tend to slow down powdery mildew development during the season. Powdery mildew also is favored by temperatures from 68°F to 81°F. Disease development essentially stops once temperatures get more than 100°F.

Q4 What fungicides do you recommend? Should growers use preventive products, too?
Gugino: Fungicides, when combined with host resistance, can be very effective for managing powdery mildew. Single site mode of action products specific for powdery mildew tend to be most effective. Many of these also have translaminar or locally systemic activity meaning when they are applied to the upper surface of the leaf, it then will move through the leaf tissue and protect the underside of the leaf. This is in comparison with protectant type fungicides, such as chlorothalonil, which will only protect the plant surface to which the product was applied so adequate coverage is essential.

It is important to include protectants in with a powdery mildew fungicide program for resistance management so the pathogen population is exposed to more than one fungicide active ingredient at a time.
I have been conducting pumpkin powdery mildew product efficacy trials at the Russell E. Larson Research and Education Center in Centre County, PA, annually since 2009. There are a lot of products registered for managing powdery mildew, so it is very important to pay attention to their Fungicide Resistance Action Committee (FRAC) codes when deciding on a fungicide program.

I suggest initiating a program with one of the more effective products such as Torino (Gowan Co., FRAC code U6), Vivando (BASF Ag Products, FRAC code U8), or Quintec (Dow AgroSciences, FRAC code 13) and then rotating them with products that have different modes of action such as Fontelis (DuPont Crop Protection, FRAC code 7) or Pristine (BASF Ag Products, FRAC code 11 + 7) or a FRAC code 3 fungicide like Procure (Arysta LifeScience North America) or tebuconazole, Inspire Super (Syngenta, FRAC code 3 + 9), and Aprovia Top (Syngenta, FRAC code 7 + 3).

It is important to always read the fungicide label because not all cucurbits are on all labels and the label is the federal law.

Powdery mildew is one of the easier diseases to manage organically on cucurbits and there are a number of options including copper, sulfur, oils like Eco E-rase (jojoba oil), JMS Stylet oil (JMS Flower Farms Inc., paraffinic oil), Trilogy (Certis USA, neem oil), and Organocide (sesame oil), as well as potassium bicarbonate-based products such as Kaligreen (Arysta LifeScience North America) and MilStop (BioWorks) to name a few.

Q5 What are the dangers of an early outbreak of powdery mildew that is left unmanaged?
Gugino: Powdery mildew is primarily a foliar disease that — if left unmanaged — can reduce the photosynthetic capacity of the plant thus reducing both fruit yield and quality. It also can lead to defoliation and sunscald on immature fruit. The disease can severely reduce the quality of the handles on ornamental pumpkins and other cucurbits grown for the fall harvest season.

Q6 Is powdery mildew on pumpkin as big of an issue as it was 15 years ago? How have resistant varieties helped keep this disease at bay?
Gugino: Powdery mildew continues to occur annually, typically moving across production regions following the succession of cucurbit production along the East Coast during the season. Overwintering inoculum is not a common source of the pathogen so disease management strategies such as crop rotation are less important for a disease like powdery mildew. Resistant varieties are important because they delay the onset of the disease, which in turn reduces the amount of potential inoculum available to spread to other cucurbit fields.

Q7 There are varieties highly resistant and some that are intermediately resistant or tolerant. Can you explain the difference?
Gugino: The terminology used to describe powdery mildew resistance varies. The important point to keep in mind is that none of the varieties are immune. The selection and planting of resistant varieties will typically delay the onset of the disease and will reduce overall disease severity.

Often, varieties that are designated as highly resistant obtained a copy of the powdery mildew resistance allele from each parent through conventional breeding methods compared to those designated as intermediately resistant or tolerant, which only received one copy from one parent. The resistance is most effective when it is from both parents (homozygous resistance) compared to one parent (heterozygous resistance).

Q8 What advice do you have on avoiding fungicide resistance?
Gugino: Fungicides are an effective tool for managing powdery mildew; however, there is considerable concern over the development of fungicide resistance. For resistance management, it is best to start applying the most effective products when you first start to see symptoms (one lesion on 45 to 50 leaves) and then switch to a protectant spray program later in the season. In the long run, this will reduce the selection pressure for powdery mildew spores that are resistant to the fungicide because fewer spores are exposed to the active ingredient when disease severity is low.

When applying powdery mildew-specific fungicides, each application should be tank mixed with a protectant fungicide again to manage for fungicide resistance. However, due to increasing concerns about pollinator health and the use of fungicides such as cholorothalonil, when possible, time fungicide applications when fewer pollinators are foraging and visiting flowers.

One resource developed for growers in the Mid-Atlantic region is the “Fungicide Resistance Management Guidelines for Vegetable Crops in the Mid-Atlantic Region” (http://is.gd/PSU_Guides).
This guide complements the “Mid-Atlantic Commercial Vegetable Production Recommendations” summary and provides a chart for each crop, which details information about FRAC codes and fungicide resistance management for recommended products and diseases. Our growers have found this to be a tremendous resource for making fungicide selection decisions.

Rosemary Gordon is editor of American Vegetable Grower, a Meister Media Worldwide publication. See all author stories here.

Integrated Pest Management

Fact Sheets > Vegetables > Crop Specific Articles > Pumpkin

2014 Pumpkin Fungicide Program

By Jude Boucher, UConn Extension Educator, Commercial Vegetable Crops

Pumpkin disease management begins with cultural and preventative controls such as proper site selection, field preparation and the use of resistant varieties. Some of the most effective alternative control measures include planting in well-drained soils on open sites with good air circulation, away from early cucurbit crops such as summer squash that may spread diseases, using powdery mildew-resistant varieties, and using deep zone tillage to improve soil drainage and protect fruit from rots.

Fungicide Program: Recent experiments at UConn showed that unsprayed pumpkins yielded less than half as much as plants with an effective fungicide program. There are four common and important diseases* you can help manage with a good fungicide program: powdery mildew, Plectosporium blight (Plecto), black rot (BR) and downy mildew. Powdery mildew (PM) tends to be the most common and the most important in many years, so we design the spray schedule around it. The basic idea is to slow disease spread with the fungicides by limiting spore formation, new leaf infections, and stem and fruit problems, so that you maximize your yields and net profits.

To determine when to make the first application for PM, scout 50 lower leaves each week after the plants run, and spray when you find the first small, round, colony of white powdery mildew spores (usually on the underside of the leaf). Then, continue to make applications at 10-day intervals until mid-September.

Systemic fungicides, which move through the plant or leaves, usually provide the most effective control of PM because it is impossible for any sprayer to provide good coverage where the infection first occurs: on the underside of the lower, older leaves, in a waist-high pumpkin patch. As part of your PM resistance management strategy, use each systemic family/group only one time each season. Unlike protectant fungicides that have several modes-of-action to stop infection, systemics tend to have a single mode-of-action which is easily skirted by the billions of spores encountered by the product. Add a protectant fungicide to the tank mix during each application to help slow resistance, and to provide control of Plecto and BR on fruit.

Currently, there are four effective systemic groups to choose from for PM control: fungicide resistance groups 3, U6, 7 and 13. So, if you choose one of the most effective products from each resistance group, your spray program in 2014 should look something like this: Torino (U6) + Bravo (i.e. chlorothalonil), Quintec (13) + Bravo, Procure (3) + Bravo, Pristine (7 & 11) + Bravo, and then switch to a mix of two protectants for the remainder of the season (sulfur + Bravo). This program uses the most effective systemic groups early in the disease cycle to restrict spread, and the less effective later. You could also skip the Pristine application because it is the most expensive, or mix the Pristine with a protectant such as sulfur or Dithane, to rest the Bravo, while still providing good protection against PM, Plecto and BR. Group 11 products are the most effective against Plecto if it becomes the major concern in your field.

Since this fungicide program is ineffective against downy mildew (DM), you usually need to add another mobile fungicide to your spray mix late in the season when (or if) this disease shows up. DM first appears as widely scattered, yellow spots or ¼-1/2 inch, yellow or brown squared-off spots on the leaves. If more than one DM application is needed, alternate between two effective products to slow resistance. Some of the more effective downy mildew products include: Ranman, Presidio, Previcur Flex, Revus, Tanos, Curzate, and in some years Dithane or a phosphorus acid-type material such as ProPhyt. You can also mix sulfur (protectant) and Tanos (systemic) to control all four diseases. Organic growers can mix sulfur for PM control with other broad-spectrum products (i.e. Double-Nickel) to minimize fruit rots and defoliation from DM.

*Recent research shows that you can also reduce problems with Phytophthora blight using preventative soil and foliar applications of certain fungicides (see ‘Chemical/Microbial Control of Phytophthora Blight’ – this issue). However, effective chemical control of all five pumpkin diseases may be cost prohibitive.

Powdery Mildew Infection in the Pumpkin Patch

Powdery Mildew infection is one of the most obvious: a plant’s leaves and stems – pumpkins and many others – are suddenly covered with what appears to be a white powder. It spreads within days and if not treated can pose a significant threat to susceptible plants.

First week of August-healthy, green foliage and a melon growing at the edge

The summer of 2012 was quite an adventure for gardeners in Pennsylvania and elsewhere, starting with our near-drought conditions. We weren’t officially in drought as much of the country was, but tell that to my thirsty vegetable garden. Plants barely held on if at all, and my garden hose got more of a workout than it’s seen in six years. But all in all I was doing okay, and there were no serious drought or disease issues, with the exception of some blossom end rot on a few tomatoes. Low yields however were the norm, due to lack of water and lots of heat.

Over the past few seasons, I’ve been planting melons at the ends of the raised garden beds where I grow sweet corn. The long vines and lush foliage of melons, members of the Cucurbitaceae family of plants, provide a natural mulch for the corn, which has an extremely shallow root system, prone to drying out quickly. This year I planted pumpkins at one end of the corn bed and melons at the other. I never grew pumpkins before, because let’s face it, there isn’t much use for a dozen pumpkins in one household unless you’re really into Halloween or fall decorating.

From drought conditions to downpours and high humidity

The near-drought arrived in mid-June. With the exception of one massive thunderstorm early in July, barely a drop of rain fell for six weeks. One and 1/4 inches of rain fell on July 20th, accompanied by high winds which almost blew the corn right out of the raised bed, but that was the extent of it. The sweet corn started getting “leggy”, that is thin-looking, and the ears on the stalks looked puny – a desperate cry for water. The melons and the pumpkins were surviving but not thriving, regardless of how many inches of water I dumped on them and the corn. Their normally lush green foliage was beginning to look crisp and brown at the edges. And did I mention our multiple heatwaves? With extended bouts of above-normal heat, many plants become dormant in order to conserve energy and stay alive. New foliage stops appearing, growth stops, no new fruit appears, and the roots dig deeper for water to keep the crown alive. This was the case as July ended.

First week of August – beautiful, healthy green pumpkin foliage. There are a few pumpkins growing in there, shaded by the leaves

Rain finally arrived the first week of August – just passing thunderstorms at first, but my neighborhood seemed to catch the bulk of them. Moisture was minimal at best, but enough to let me breathe a little easier. Then this past week, significant heavy rain, lower temperatures (mid-80’s) and high humidity. A fungal paradise.

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Powdery Mildew appeared almost overnight

When you look you must also see. I’m in the habit of at least making a brief inspection of my vegetable and flower gardens every day, depending on my schedule. The first week of August the melons were doing very well: rain had returned the green to the foliage and more fruit buds were appearing on the vine. Same with the pumpkins – the gigantic leaves were spreading places I never thought they’d go – like out of the garden bed, across the path and into the bean patch. The sweet corn had also bounced back – the stalks and ears thickened.

But two days ago, I noticed a few white, speckled spots on the pumpkin leaves. Fungus isn’t rare around here by any means, considering the humid summers we endure, but I’ve never been troubled with Powdery Mildew. So I didn’t give the blemishes much thought. In years past, the same blemishes appeared but never led to a wide infection. But within 48 hours, those few spots had turned into sheets of white fungus on most of the pumpkin foliage. The melons were less affected, due to more airflow around the plants and more sunlight on that side of the garden bed.

Second week of August – In only a few days, powdery mildew infected a majority of pumpkin and melon foliage.

Do your research on any plant infection

I did a little research and found this information from the University Of Vermont Extension: “Once you can easily see signs of powdery mildew… it is too late to apply any treatments for control.” Damn.

“Organically-approved fungicides… work by preventing infection of healthy tissue, so starting treatment early is key to their effectiveness… established powdery mildew colonies not only do not disappear when treated, they continue producing hundreds of spores. These give the spots their powdery appearance.” Missed my window of opportunity.

“Favorable conditions for the disease include dense plant growth, (guilty) low light intensity, (guilty – low light at the bottom of the dense corn patch) and high relative humidity.” (I hate this weather)

“Not only can powdery mildew reduce yields because of decreased fruit size or number, but the premature senescence (aging) of infected leaves can also result in … fruit becoming sunburned or ripening prematurely. Pumpkins with powdery mildew infection often have poor rind color and shriveled handles, and speckling or other imperfections on their rinds.” Case closed.

Leaves prematurely age and die from powdery mildew infection

How to combat Powdery Mildew infection

I removed and bagged (do not compost!) as many infected leaves and stems of the pumpkin and melon plants as possible. I paid attention to how much I removed, because depriving them of too much foliage lessens their ability to conduct photosynthesis and ripen the fruit. I also removed any corn stalks that weren’t producing ears and those from which I’d already harvested ears. This should increase air flow around the plants, which will help to keep the foliage dry.

I also mixed a batch of baking powder solution to fight powdery mildew, poured it into a pump sprayer and went to work on all of the foliage -pumpkins and melons alike. While the baking powder solution won’t kill the powdery mildew already on the plants, it will coat the foliage, stems, and fruit not yet infected and will hopefully keep the infection in check. Much will depend on the weather and humidity levels.

I’ll know soon if my efforts worked and will update this post with my progress. Hopefully, I’ll be using at least one homegrown Jack O’ Lantern this Halloween.

Update, August 24:

Alas, my efforts failed. I used the baking powder solution above, a milk spray solution, and a diluted hydrogen peroxide solution of various strengths over the last two weeks, but the pumpkins are a loss. Once I discovered the powdery mildew, it was too far advanced to control with organic solutions. It appeared to be checked at first, but when more rain fell, the fungus quickly spread to new foliage. I now have two small pumpkins in the garden, attached to vines that are withering.

Closeup of powdery mildew on pumpkin foliage

All of the information I researched made clear that the solutions I sprayed on the infected plants are to be used for the prevention of fungus before symptoms appear. I was holding out hope that I could control it and save the pumpkins and melons. Once powdery mildew or any fungus attacks your garden to this extent, it’s very difficult to kill or control and the best solution is to remove and dispose of all infected plants to keep it from further infecting your garden. Powdery Mildew can also overwinter, so plant resistant varieties in that area the following season.

Questions and Answers

Writing Designs on Pumpkins

Q. I have a pumpkin plant. I want to carve my son’s name on a pumpkin when it’s small so that the pumpkin (and his name along with it) will grow large. What size should the pumpkin be? What’s the best tool to use? At what depth should I carve his name?

A. It will depend on the variety of the pumpkin and its eventual expected size. You can begin writing on small varieties as soon as they are the size of a tennis ball. Bigger varieties may go up to a basketball in size before you need to do the monogramming. It doesn’t take much – just enough poking with any sharp pointed instrument to break the skin of the fruit. A closely-knit series of poked dots (rather than cursive or calligraphy) is the most effective technique.

Source: National Gardening Association

Growing Large Pumpkins

Q.How can I grow pumpkins that weigh more than 100 pounds?

A. Use one of the jumbo varieties. Plant in early June, and allow 150 square feet per hill. Thin out the best one or two plants. High fertility, proper insect control and shallow cultivation are essential. Remove the first two or three female flowers after the plants start to bloom so that the plants grow larger with more leaf surface before setting fruit. Allow a single fruit to develop and pick off all female flowers that develop after this fruit has set on the plant. Do not allow the vine to root down at the joints near this developing fruit because these varieties develop so quickly and so large that they may actually break from the vine as they expand on a vine anchored to the ground.

Tips for Growing Giant Pumpkins

  • Soil prep, most important factor, after testing and adjusting soil use large quantity of partially decomposed compost.

  • Start seeds in pots early to provide for longest growing season for your zone. Move seedling to warm outside soil and temperatures 65°F or provide mini-greenhouse.

  • Fertilize, first with higher phosphorus for roots, later with balanced fertilizer.

  • Water, fortifying with liquid fertilizer, growing pumpkins requires gallons of water.

  • After pumpkins start growing, limit vines to one or two. Be ready to adjust vine positions as pumpkins grow larger

  • Make sure pumpkins get as much sun as possible.

Giant Pumpkins

Q. Is there an organization or club for people interested in growing giant (800 lb) pumpkins or a seed exchange for the giant pumpkins?

A. There’s a whole society of people who share their growing tips for giant pumpkins.

Browse, enjoy and best of luck with your project!

Mini Pumpkins

Q. We would like to try growing mini pumpkins this year and train the vines to grow up a tomato plant wire cone. Is this a good idea? Any advice on seed types? What can we do to keep the mold away?

A. We’ve grown Jack-Be-Little pumpkins on a wire tunnel so children can crawl under the vines. The vines stay up on the top and the mini-pumpkins hang down through the wires. It’s great if you’re a little one! Mini-pumpkins require the same culture as regular-sized pumpkins: lots of sunshine, plenty of water and enough elbow room to provide good air circulation. If you’re having trouble with mildew and mold on the leaves, your plants need more sunshine and better air circulation around them. Try not to get the leaves wet when you water, or try to water earlier in the day so the leaves don’t remain wet overnight. Jack-Be-Little and Baby Boo are both great little pumpkin varieties.

Pruning Pumpkin Vines

Q.Will pruning my pumpkin vines back a little harm the plants and result in less pumpkins?

A. Pumpkin vines always grow bigger than we expect! When they escape from the vegetable garden, my husband mows the tips off when he mows the lawn (This reminds me to reroute the vines back where they belong.) It does result in fewer pumpkins because there are fewer flowers on the plant to form pumpkins, but apart from that, in my experience nipping the vine tips with the lawn mower has never set them back noticeably. Cutting them back hard however, would reduce the foliage enough to cut down on photosynthesis and that might make a big difference in the health of the plant and the number of pumpkins it can support. Good luck with your pumpkins!

Powdery Mildew on Pumpkins

Q. My husband and I grow and sell pumpkins but have had trouble with powdery mildew on the vines. What can be done to prevent and treat this problem?

A. Powdery mildew is a fungal disease. Warm temperatures and dew favor its development. You can help your plants avoid infection by removing some of the vines to increase air circulation among the plants and by directing water around the base of the plants rather than sprinkling water overhead on the fruits and leaves. Early in the season you can cut or pinch off a few of the vines without harming the plant. Wait until some fruits form and then pinch off the ends of the vines. This will increase air circulation and direct the plant’s energy into developing fewer, but larger, more flavorful fruit.

Pumpkin Mulch

Q. I’ve noticed some gardeners (and a few farmers) who have covered garden areas densely with pumpkins and let them rot over the winter. What is the purpose of this? Any danger of them seeding in the spring?

A. It’s never a good idea to leave plant debris in the garden over the winter. The practice seems like a lazy-man’s approach to composting. Rotting pumpkins will supply the soil with some nutrients, but will also provide a place for overwintering insects and disease pathogens. Plus, the seeds from the pumpkins will certainly sprout in the spring. This may not be a problem in a commercial field because the debris will be tilled into the soil before the field is planted again. I expect disease problems and errant sprouting seeds are dealt with chemically. But for the home gardener, the stuff will have to be picked up and thrown in a compost pile before the site can be used again. I’d rather handle the plants and pumpkins before they turn into a slimy mess! For the healthiest garden site, compost your end-of-season plant debris and add the compost to the soil before planting in the spring.

Pumpkins Think It’s Fall

Q. Help, my pumpkins think it’s fall. They are turning orange and the vines are dying back. What triggers this? Is it lack of sun? They grew in the same place last year. They don’t seem to have a disease.

A. There are a couple of reasons I can think of for why your pumpkins would slow down and stop growing. The first is lack of water; pumpkins are water hogs because about 90% of the pumpkin is actually water. Another is lack of nutrients; since they grew in the same place last year they may have depleted the soil in that spot. Third is that some disease has actually attacked; pumpkins are subject to a number of foliar problems which cause the leaves to shrivel; finally, perhaps you are growing an extra-early-maturing variety. (The normal range is 90 to 120 days with the approximate timing listed on the seed package or label.)

In any case, leave them on the vine as long as possible and do your best to cure them as well as you can to try to increase their storage time; depending on the variety and curing and storage conditions, some can be held for up to a year.

Source: National Gardening Association

Flowers Do Not Form Fruits

Q. The first flowers that appeared on my pumpkin plants did not form fruits. Why not?

A. This condition is natural for cucurbits (such as cucumber, gourd, muskmelon, pumpkin, squash and watermelon). The first flowers are almost always male. The pollen on these first male flowers attracts bees and alerts them to the location of the blooming vines. By the time the first female blossoms open, the bees’ route is well established and the male flowers’ pollen is transferred to the female flowers by the bees. Male flowers bloom for one day, then drop off the plants. The male flowers may predominate under certain conditions, especially early in the season, or under certain kinds of stress. The small fruits, visible at the bases of the female flowers, identify them. There is no swelling on the bases of the male flower stems.

Grandma’s Pies

Q. My grandmother made pies with a green-striped, long-necked pumpkin. Is this variety still available?

A. Yes. The variety is Green-Striped Cushaw. Because it has a unique texture, some cooks prefer it for custards and pies.


Q. Will pumpkins, squash and gourds cross-pollinate and produce freak fruit if I interplant several kinds in my garden?

A. Pumpkins, squash and gourds are members of the vine crops called “cucurbits.” The name is derived from their botanical genus classification of Cucurbita (often abbreviated C.). There are four main species of Cucurbita usually included in the pumpkin, squash and gourd grouping. The varieties within a botanical species (which may be referred to as pumpkins, squash or gourds) can cross-pollinate. Varieties from different species do not. For example, zucchini crosses with Howden’s Field pumpkin, acorn or spaghetti squash, small decorative gourds, or Jack-Be-Little miniature pumpkins because they are all members of the same botanical species (C. pepo). However, cross-pollination does not affect the taste, shape or color of the current season’s fruit. Crosses show up only if seeds from these fruits are saved and grown the following year. Butternut squash, Small Sugar pumpkin, White Cushaw pumpkin and Big Max pumpkin could all be grown in the same area without crossing because each variety comes from a different species. Because bees carry pollen for distances of a mile or more, in suburban areas where many gardens are in close proximity, fruits must be bagged and pollinated by hand if pure seed of non-hybrid varieties is desired.

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