Powdery mildew on rosemary

August reliably brings two things to the garden: hot days and powdery mildew. This common fungal disease looks like a silvery powder and it coats the leaves, stems, and fruit of many popular plants. In the vegetable garden it tends to infect members of the squash family, including melons and cucumbers, beans and peppers. Powdery mildew often attacks plants in late summer, when the weather is hot and dry. The spores wait out the winter season in the soil and on plant matter and disperse through the wind.

The disease typically begins as a few spots on a plant’s leaves, but in serious cases it can completely cover the foliage, stems, and fruit. Luckily powdery mildew doesn’t usually kill plants, but it weakens the plants and can dramatically reduce yields and flavor. To prevent the disease, start by choosing varieties that resist powdery mildew, such as Calypso pickling cucumbers, Orient Express slicing cucumbers, Ambrosia cantaloupe, Jack-Be-Little pumpkins, Royal Ace acorn squash, Bush Delicata squash, Wildcat zucchini, and Slenderette beans. For more disease-resistant varieties, check out Cornell University’s Vegetable MD Online.

Powdery mildew is more likely to infect plants that are already stressed out, so be sure to keep your plants well watered and give them plenty of space and sun, as poor air circulation and low light intensity encourages the disease. If you notice a few powdery mildew spots on the leaves of your plants, immediately remove those leaves and dispose of them in the trash. At the first sign of infection you can also try spraying the plants with a baking soda solution, which raises the pH of the leaf surface and creates a hostile environment for the spores. Simply mix 1 tablespoon of baking soda into a gallon of water until dissolved. Spray the solution on the leaves of susceptible plants, being sure to completely coat the surface of each leaf.

Willi Galloway is the author ofGrow Cook Eat: A Food Lover’s Guide to Vegetable Gardening, and she writes about organic vegetable gardening and seasonal cooking on her blog,DigginFood.

Physical Description

Tomato leaf with powdery mildew (UC Davis – IPM)

Powdery mildew is a common fungal disease that effects many types of plants, and is fairly easy to manage. It causes poor growth and lower yields, but seldom kills the plant. There are many different species of powdery mildew that cause the disease on many different species of plants. See list of species and their hosts below in “Plants Affected”. Unlike many other fungal pathogens, it is able to flourish under warm, dry conditions, when competition (other fungi and bacteria) are not doing well, and doesn’t require wet conditions to germinate.

Powdery mildew on Lavender Bergamot (Monarda fistulosa)

The white, powdery growth that you see on the leaf surface that starts in small, round spots, and can eventually spread to cover the leaf surface, stem and flower buds. Flower buds will fail to open. The powdery white coating you see is actually thin layers of mycelium that produce spores. The spores make up the bulk of the white stuff you see. The spores can be seen with a microscope or hand lens as chains of clear, white balls.

Species & Taxonomy

  • Kingdom:
  • Phylum:
  • Class:
  • Order:
  • Family:
  • Genus Species: see below, under “Plants Affected”

Lifecycle

The powdery mildew fungi requires living plant tissue on which to grow; as it is a parasitic fungi. Perennial plants allow it to survive through the winter and then infect the annual plants and vegetables. It also survives the winter on weeds. Some species of the fungi produce special spores that remain viable until winter is over. These powdery mildew species are the ones that infect peas, lettuce, parsley, cucumbers, etc. Areas where there is no real frost have an especially hard time with powdery mildew.

A spore of powdery mildew, dispersed by the wind or carried by an insect, lands on a leaf. The spore germinates then grows mycelium. The mycelium of the fungi then grows in a thin layer on the surface of the leaf, shoot, or flower. The mycelium then produces chains spores which are dispersed by wind to new hosts.

All powdery mildew, to different extents, will germinate without water. Free moisture (water droplets, water sprays, rain) actually kills spores and stops mycelial growth. Moderate temperatures (60-80 deg F) and part shade are perfect for an explosion of powdery mildew, since the mycelium and spores are both sensitive to extreme heat and direct sunlight; however, powdery mildew is one of my biggest issues in my garden, which is normally 90 – 110 degrees in the summer (with very low humidity) and full of blazing sunlight.

Plants Affected

  • Erysiphe cichoracearum affects composites:
    • endive
    • lettuce
    • parsley
    • sunflower
    • chrysanthemum
    • dahlia
    • zinnia
    • cucurbits:
      • cucumbers
      • melons
      • squash
    • potato
    • begonia
    • phlox
  • Erysiphe cruciferarum affects cole crops:
    • broccoli
    • brussels sprouts
    • cabbage
    • cauliflower
    • collards
    • kale
    • kohlrabi
  • Erysiphe pisi affects peas
  • Leveillula taurica affects:
    • artichoke
    • eggplant
    • pepper
    • tomato
  • Podosphaera leucotricha affects:
    • apple
    • quince
    • rarely pear
    • rarely almond
  • Podosphaera oxyacanthae affects leaves and shoots of:
    • cherry
    • peach
    • plum
  • Sphaerotheca macularis:
    • one strain affects strawberry
    • another strain affects raspberry, blackberry and other caneberries
  • Sphaerotheca fuliginea affects:
    • cucurbits:
      • cucumbers
      • melons
      • squash
    • beans
    • okra
    • black-eyed peas
  • Sphaerotheca pannosa affects leaves and fruit of:
    • apricot
    • peach
    • plum
    • rose
  • Uncinula necator affects:
    • grapes
    • ivy

Plants Unaffected

Geographical Range

Powdery mildew is found everywhere in the United States and Canada.

Signs & Symptoms

Whole Plant

  • lackluster growth; powdery film covering stems & leaves
  • often look like they’ve been dusted with flour

Leaves

  • most easily detected and recognizable symptom: white or light-colored, powdery spots, often covering large areas, on top or on the underside of leaves and/or shoots
  • Leaves will eventually turn yellow, die, and fall off
  • On tomatoes, eggplants, peppers and artichokes, there is no powdery growth; just irregular yellow patches
  • in peppers, the white powdery growth only shows up on the underside of the leaves, then the coating turns brown, and yellow spots appear on the upper side of the leaves…and then the leaves fall off the plant (VGPS)
  • on vegetable crops, powdery mildew will first be noticed by yellow spots on top of the leaves, then they will get powdery-looking spots
  • leaves are most susceptible 2-3 1/2 weeks of unfolding

Shoots

  • light-colored, powdery areas of spore growth

Flowers/Buds

  • sometimes the white, powdery spore growth can be seen on flowers
  • infected buds are flattened or shriveled

Fruits

  • young fruit gets cracked-looking scars in a shattered glass-type of formation (russeting)
  • may also show powdery growth
  • fruit may ripen slowly or not at all
  • powdery mildew seems to suck all of the flavor out of the fruits.
  • Especially harmful on grapes and fruit trees.

How to Positively Identify

The white or light-colored, powdery growth on the leaf surface is pretty indicative of powdery mildew. It’s harder to identify on tomatoes, eggplants, peppers, and artichokes, since it just shows as yellow patches. These yellow patches are often confused with other problems, like nutritional deficiencies or other fungal diseases.

Treatment

Prevention & Control

Choosing resistant varieties is the only sure way to avoid powdery mildew, or just not choosing the really susceptible varieties. Although, I don’t think I’ve ever seen a seed catalog describe a cultivar as “exceptionally susceptible to powdery mildew”.

Cultural Controls:

Temperature & Humidity:

  • irregular watering and heat leads to more powdery mildew

Mulching & Cultivation Practices:

  • don’t plant susceptible crops in shady areas
  • provide enough water
  • avoid excess fertilizer
  • overhead sprinklers, in this case, are desirable. You can also hand-spray frequently to reduce incidences
  • watch new growth closely for powdery mildew; pruning off shoots that have it can save you from an epidemic
  • keep circulation up by clipping bottom leaves and not planting too densely
  • pick off infected leaves
  • garden cleanup: the spores can overwinter on plant debris and spread by wind, so remove your plant cuttings and compost (hot composting is good to kill the spores)

Natural Enemies & Biological Controls

  • AQ-10: a biofungicide, is a solution of a fungi (Ampelomyces quisqualis) that kills powdery mildew. It is available as a preventive foliar spray, and research shows that it is good for when your disease levels do not exceed 3% on leaves and clusters. This fungi has also shown that it works (in vitro) against some other fungal diseases: Botrytis cinerea, Alternaria solani, Colletotrichum coccodes, and Cladosporium cucumerinum.
  • Serenade Garden Disease Control: another biofungicide foliar spray, which uses the bacteria Bacillus subtilis (QST 713, a patented strain) against a broad spectrum of plant diseases. These include scab, powdery mildew, sour rot, downy mildew, and early leaf spot, early blight, late blight, bacterial spot, and walnut blight diseases. It directly attaches and colonizes to fungal pathogens and outcompetes other pathogens. The spray contains bacterial spores of B subtilis and also a protein that the bacteria makes during fermentation. This protein is a natural fungicide that actively assists the bacterium in killing the fungal pathogens. It is approved by NOP and OMRI for use in organic gardening. Spray preventively, before the disease is seen and during, to combat it.
  • Actively Aerated Compost Tea:
    • full of biological activity, it provides lots of competition for the mildew, potentially also contains bacteria or fungi that kill the powdery mildew fungi
    • also fertilizes the plant
    • this is best made at home, so it’s fresh
    • fungally dominant teas are most effective against powdery mildew

Barriers

  • Trap crops are not a valid method for controlling powdery mildew

Sprays & Dusts

Not in any particular order:

  • Water
    • Spraying water on your plants should be your first line of defense. If you spray your plants down periodically throughout the week, you can avoid seeing powdery mildew altogether. If you live in a humid area, you may not wish to do this unless you actually have powdery mildew, since it additional free moisture can cause or exacerbate other problems. Water kills the powdery mildew spores and prevents the mycelium from growing.
  • Biofungicides
    • there are biological controls that are available in a spray-on application. See section above on “Natural Enemies & Biological Controls”.
  • Milk
    • 1 part milk mixed with 9 parts water slows the spread of powdery mildew
    • spray twice per week, but keep refrigerated
    • discovered by Wagner Bettiol; he found that this was more effective against powdery mildew than toxic fungicides
    • also found to be an effective preventative for tomato mosaic
  • Aspirin
    • dissolve two uncoated aspirin tablets (325 mg) into 1 quart of water and spray onto infected areas
  • Baking Soda and potassium bicarbonate:
    • sprays of potassium bicarbonate are supposedly more effective than sodium bicarbonate (baking soda).
    • It will also help prevent and reduce other fungal problems.
    • Good choice for controlling fungal diseases on cucurbits (squash, cucumber, melons) and tomatoes
    • start spraying 2 weeks before you expect symptoms to show up, spraying every 7 days until conditions are not favorable for the powdery mildew; increase frequency if it’s humid
    • at home formula:
      • 1 teaspoon baking soda (one source recommended 1 tsp, another source recommended 2 tsp; test your plant with the lower concentration first)
      • 1 drop liquid soap (another source recommends 2 teaspoons)
      • 2 quarts water
      • 1 tablespoon of mineral oil (optional; it makes it stick to the leaves and kill some insects)
      • spray full-strength via a pump spray bottle or a hudson sprayer
    • caution: some foliage can get burned if the concentration of baking soda is too high; just pick off leaves if this happens
    • 1% solution works best on grapes & ivy (Uncinula necator)
  • Jojoba Oil
    • E-RASE is a natural fungicide containing jojoba oil
    • toxic to fish
    • doesn’t work as well on hairy-leafed plants
  • Garlic
    • Swedish studies showed 5% solution of garlic extract protected plants from Erysiphe cichoracearum (cucurbits, lettuce, parsley, etc)
    • another formula is to mix 1 gallon water with 1.25 cups garlic powder; spray on to plants early in the day
  • Soap Sprays
    • only soap sprays with sulfur added are effective against powdery mildew; review labels before buying
  • Garlic & Diatomaceous Earth Dust
    • mix equal parts garlic powder and diatomaceous earth (DE) and use a flour sifter to apply to plants with powdery mildew
    • don’t use sifter for cooking after this
  • Cinnamon
    • combine 1/4 teaspoon ground cinnamon with 1 quart water in a spray bottle; spray both sides of leaves thoroughly
  • Neem Oil:
    • also known as azadirachtin, however if a product only lists azadirachtin as its active ingredient, then it will not act as a fungicide. Must be the full component of neem oil to work against powdery mildew
    • mostly preventive; spray before it shows up, but one study showed that 1% solution controlled 95-100% of the powdery mildew on hydrangeas, phlox, and lilacs when repeated every 7-14 days
    • broad spectrum repellant, growth regulator, and insecticide
    • also has a bit of a systemic effect
    • can harm beneficial insects
    • works fastest in hot weather
  • Teas
    • Comfrey Tea
    • Chamomile Tea
      • antibacterial & fungicidal
      • use as a foliar spray
      • also helps prevent damping off
  • Chemical Fungicides (sulfur, copper, and other synthetic fungicides) should be your very last ditch effort. The problem with using these is that they kill all fungi indiscriminately. There are many fungi in your soil and on the leaves of your plants that are living symbiotically with your plants. Killing these beneficial fungi that are protecting – and feeding – your plant makes way for a host of other diseases, insects, nematodes and even nutritional problems. Powdery mildew can also become tolerant (resistant) to fungicides, especially systemics.
    • Sulfur
      • considered to be an organic fungicide
      • moderately toxic
      • can slightly decreases the pH of your soil – although this can be a bonus, if you live in arid regions
      • Sulfur can injure the plant, burning the leaves, if used when it’s over 90 degrees F (although another source stated the threshold as 80 degrees F).
      • Sulfur injures some melons no matter what the temperature is.
      • Timing of when you spray during the season is important
      • doesn’t kill the spores, but it does prevent germination on the plant surface
      • Sulfur mixed with lime can kill recently germinated spores
      • harms beneficial insects
    • Copper
      • considered organic
      • toxic to humans and animals, very toxic to fish, but it doesn’t harm the beneficial insects
      • often combined with sulfate in bordeaux mix, this will inhibit germination and spore production
    • Bordeaux Mix:
      • combines copper sulfate and hydrated lime
      • fungicidal & insecticidal

      Sources:

Can Sick Plants Make People Sick?


Symptoms of viral infection of zucchini.

Every year we receive inquiries from gardeners and the general public about apples covered in black powdery spots, wondering if those fruits are safe to eat. When presenting at extension programs or events, we show some of the dramatic symptoms viruses can cause in pumpkins, zucchinis, and other cucurbit fruit, and receive the question, “Could I catch the virus that made that plant sick?”

In most cases, the answer is no. The fungi, bacteria, viruses, and nematodes that cause disease in plants are very different from those that cause disease in humans and other animals. However, some plant pathogens may be able to infect humans as well as plants, and those that do tend to be “opportunistic pathogens,” especially on a segment of the population at risk. For example, people with suppressed or compromised immune systems, taking certain medications or suffering from medical conditions or other causes that may cause the human immune system to be weak (immunosuppressed).

Eating or touching infected plants or their parts would not likely infect us with the same pathogen that is making the plant sick. Though, consider that produce from infected plants often has a flavor or texture very different from healthy fruit, so eating it may not be desirable anyway. Unless the disease is merely a superficial spot (such as sooty blotch and flyspeck on an apple), it may be best to avoid diseased produce. Canning of symptomatic produce is not recommended. There are chances the acidity of the final product may change, resulting in spoilage or increased risk of undesirable conditions that may encourage microorganisms potentially harmful to humans to thrive in this new environment.

Some examples of microorganisms that are reported causing problems in humans and plants include some bacteria, fungi but also their products (toxins, etc). An example of this is the bacterium Pseudomonas aeruginosa, which can cause a weak, soft rot of plants such as lettuce. In people with compromised immune systems, this bacterium is known to infect the urinary tract, lungs, blood, and burns and other wounds. It is especially common in hospitalized patients whose immune systems are compromised by severe burns, cancer, AIDS, or cystic fibrosis. For most of us (and for most healthy plants), P. aeruginosa is not a concern.


Sooty mold and flyspeck on apple fruit.

Some fungi that live on decaying plants can cause disease in humans. One example is Sporothrix schenckii, a fungus that frequently lives on dead rose thorns. This fungus can cause sporotrichosis, also called “rose-picker’s disease”, if it gets into a person’s skin (such as through a scratch) and into the lymph system, or if a person inhales its spores. Symptoms of this disease in humans can include problems with the lungs, eyes, central nervous system, bones and joints. For more interesting information on this fungus, including a gory picture of infected skin, please see (if you can handle the hard images of diseased skin) http://botit.botany.wisc.edu/toms_fungi/feb2003.html

Additionally, some plant pathogenic fungi produce compounds that can be toxic to people, although the pathogen itself does not infect people. For example, some fungi that cause ear rots on corn, such as Fusarium, produce “mycotoxins” (toxins produced by fungi). The mycotoxins produced by Fusarium include fumonisins, zearalenone, and the aptly-named vomitoxin. Effects of mycotoxins in livestock that are fed contaminated grain can include development and reproductive problems, vomiting, general lethargy, and death, depending on the particular mycotoxin present and the level of contamination. Aspergillus flavus is a common contaminant of grain and peanuts, and it produces mycotoxins called aflatoxins. At very high levels (acute exposure), aflatoxins can cause vomiting, pain, convulsions, and death. At lower levels of longer duration (chronic exposure), they can lead to cancer. Mycotoxins are generally an issue only on grain, not on common garden produce, and grain for human consumption is well monitored for their presence.

In general, pathogens that infect plants do not specialize in infecting people. You are not likely to catch a disease from working with diseased plants in your garden, but it is a potential risk (depending on the infection), and consideration should be taken. Garden produce from a sick plant is generally safe to eat, although it may not be desirable. Avoid eating moldy or rotten produce, though, as some fungi and bacteria can produce toxic compounds.

Originally prepared by Christine Engelbrecht, updated by Lina Rodriguez Salamanca

White Powder On Rosemary: Getting Rid Of Powdery Mildew On Rosemary

A lot of people enjoy having small kitchen window sill plants like rosemary. However, although they are easy to grow, they aren’t without faults. Often, you’ll find there are problems with growing rosemary, one of them being a common fungus.

Powdery Mildew on Rosemary

Perhaps, you’ve noticed a white powder on your rosemary plants in your kitchen. If so, you aren’t alone. The white powder is actually powdery mildew on rosemary, a common plant ailment. It is caused by many different fungi that are closely related.

This is one of the most common problems with growing rosemary plants, and all indoor plants actually. Each indoor plant has a white powdery mildew that is specific to that particular plant. Rosemary is no different.

Powdery mildew won’t kill the rosemary plant, but it will weaken it. This is one of the easiest plant diseases to diagnose. Powdery mildew appears as a white powder which coats the leaves of the plant. The powder is actually thousands of little spores and can spread to other plants if severe enough.

How to Get Rid of Powdery Mildew on Rosemary

Powdery mildew can be partially

removed if you rub the leaves of your rosemary plant carefully. If you don’t try to remove some of it, the white powder on rosemary can result in leaf drop. The powdery mildew on rosemary can rob the plants of the nutrients they need to grow.

Powdery mildew can definitely make the plant look a little ragged, but it shouldn’t kill it. Pick up any infected leaves that have fallen off the plant. Also, take infected plants out of high humidity rooms, like the bathroom or kitchen. Rosemary prefers drier conditions.

Finally, spraying the rosemary with a fungicide, such as neem oil, will help to kill the fungus. You might want to try spraying water on it first every few days to knock the mildew off before resorting to fungicide.

You may need to repeat this every few days for it to be effective, but be careful not to over water the plant itself or you will end up with root rot, another of the common problems for rosemary plants or other indoor houseplants.

Preventing Powdery Mildew on Rosemary

One of the best ways to treat powdery mildew is to prevent it in the first place. Even if you still have an outbreak, with a few precautions beforehand, the fungus will not have as good a stronghold, making its treatment even easier.

  • When it comes to the prevention of powdery mildew, the use of bicarbonates seems promising, at least for many people.
  • Since powdery mildew fungus thrives in moist, humid conditions, ensure that your plant has plenty of light and well-draining soil. Only water the plant as needed to avoid overly saturated soil and keep the water off the foliage.
  • Keep your rosemary plants well ventilated too, meaning do not overcrowd them with other plants. This only creates a moist environment for the fungus to thrive in.
  • Oftentimes, powdery mildew attacks new growth, so avoiding excessive use of nitrogen fertilizers should help limit this growth.
  • Purchasing plants that are resistant to the disease, whenever available, is a good idea too.

Now that you know what the white powder on rosemary is and how to treat or prevent it, you can go back to enjoying your rosemary plant indoors or in the garden.

In herb gardening, rosemary is considered one of the easiest plants to grow. This herb is native to the Mediterranean region, so it can withstand harsh conditions, such as drought.

Generally speaking, rosemary requires minimal maintenance and attention, but this doesn’t mean that it comes free of problems.

Rosemary, like any other herb, can encounter several life-threatening issues as it goes through its life cycle. It might be a disease or a problem with the environment, and you’re here to be your rosemary’s inspector, guardian, and savior.

For me, it’s ultimately important that you sharpen your knife and learn all about rosemary’s problems before growing it.

In this article, I will investigate the problems rosemary could face so that you’ll be ready to confidently grow, protect, and treat your rosemary plant.

Rosemary and Illness:

Rosemary is immune to most diseases and has a relatively low probability of getting sick. In poor conditions, however, rosemary becomes more prone to being infected by pathogens that the plant is naturally less resistant to.

Diseases that gardeners face the most with rosemary are:

– Powdery Mildew

Powdery mildew is one of the most known diseases among herb gardeners. It affects a great range of plants, including rosemary.

The most prevalent and apparent symptom of this disease is the formation of white ash-like powdery spots on the leaves of the infected plant.

It is caused by some species of fungi that invade the plant gradually and start forming their spores on leaves. These whitish spores are capable of infecting surrounding plants by traveling to them through the air.

In general, powdery mildew isn’t fatal for plants, but it will weaken your rosemary and reduce the amount of harvest.

  • Prevention:

Powdery mildew usually spreads when the soil is overly moist or if there isn’t enough sunlight reaching your plant. Another reason that could contribute to the appearance of powdery mildew is bad air circulation and ventilation.

To boost your rosemary’s defense against the disease, you need to make sure that all of the above factors aren’t present. Keep your plant growing in the best conditions.

Overcrowding of plants and branches can be another reason why your rosemary might contract powdery mildew.

Spacing your plants a few inches apart is vital to avoid creating the perfect medium for the fungi to duplicate. Also, regular pruning of your rosemary, which prevents branches from overlapping, helps your rosemary stay immune to the disease.

  • Treatment:

Treatment of powdery mildew includes mixing one tbsp of baking soda and a few drops of biodegradable liquid soap with a gallon of water and then using the mix to spray the leaves weekly until the disease disappears.

Neem oil is also an effective natural fungicide that can help you get rid of powdery mildew. It is used the same way as the soda and soap mix.

If the disease persists after trying these natural methods (which is unlikely to happen), you can purchase some organic or synthetic fungicides.

IMPORTANT! It’s better to use neem oil only if you’re planning on consuming your rosemary.

– Root Rot

When the leaves of a plant start to wilt, most people think that either there’s a problem with the leaves or that the plant is thirsty, but what they don’t know is that root rot can also be the reason behind this issue.

Root rot is usually caused by excessive watering that enables mold to spread all over the roots of the plant. Rosemary is highly prone to this disease since it isn’t adapted to moist environments.

In most cases, root rot is fatal. Unfortunately, when the roots of your rosemary decay, you can’t do much to revive them.

  • Prevention:

In this case, prevention is key. It’s very important to monitor the amount of water you’re providing your rosemary.

Remember that rosemary is a drought-tolerant herb, so even if you’re growing the herb in a container or the weather is getting hotter, rosemary doesn’t need to be watered excessively.

Use a high-quality, fast-draining soil that doesn’t trap a lot of water and allows the roots to breathe properly.

Also avoid applying a lot of amendments and organic matter to the soil because they retain a lot of water, and in most cases, rosemary needs a minimum amount of fertilizer to grow.

  • Treatment:

Root rot needs immediate treatment. If your rosemary’s leaves are gradually wilting and turning yellow for no apparent reason, you should check the roots.

Gently remove the plant from the soil and wash the roots for a few minutes with a lot of water.

Plants with root rot will have some of their diseased roots fall off when you touch them. Such roots may look black and feel mushy.

Use scissors to cut off all the diseased roots. Afterward, use some fungicides to spray the healthy roots to prevent the disease from spreading.

Bring a new container or clean the used one with a bleach solution. Fill the container with fresh, high-quality potting soil mix and repot your rosemary in it.

Remove the yellow and wilted leaves from your rosemary and cut back the plant to stimulate new growth.

– Verticillium wilt

From its name, Verticillium wilt indicates that it is a wilt disease. It is also caused by a fungal infection caused by fungi from the genus Verticillium. Overall, it is not that common in rosemary.

The most common symptom of this disease is wilting of the leaves and stems. It affects the older foliage, and it usually spreads from the bottom to the top of the plant. Sometimes, one part of the plant can be affected, while other parts will remain healthy for some time.

This disease can easily kill rosemary seedlings, but it needs some time to take over an adult plant.

  • Prevention:

The most effective way of preventing this disease is by using disease-free soil. This can be ensured by choosing premium-quality soil.

Choosing a resistant cultivar is also another way of prevention. This can be hard to achieve, though, especially when the workers at your local nursery don’t answer your needs.

  • Treatment:

Unfortunately, it is almost impossible to treat this disease once it infects your rosemary.

The best way to deal with Verticillium wilt is by getting rid of all the diseased plants and starting all over again by using new soil, new containers, and new rosemary plants.

Pests:

Rosemary is known to be a pest-resistant plant, but some insects can still land on this herb to live and reproduce. Even most of these, however, will leave rosemary alone.

– Aphids

Aphids are very small and usually appear as little black or pale dots on the plant. They grow fast in quantity and might be a big problem for weakened rosemary.

The problem with aphids is that they suck the food rosemary is producing for itself, depriving it of energy.

The best way to deal with aphids on rosemary is to spray the plant with a strong water jet every other day. We don’t use insecticides here in case you want to use rosemary in the kitchen.

– Mealybugs

Mealybugs are also small insects that suck on rosemary’s sap. Females are bigger and wingless, while males are smaller but have wings.

Mealybugs can be worrying and annoying, especially if you’re growing rosemary indoors.

You can prevent them by using neem oil, which is also an effective insecticide in this case.

Other Rosemary Problems:

– Overwatering

Being a drought-tolerant Mediterranean plant, rosemary doesn’t need much water.

Gardeners, however, don’t pay attention to this when they water their herbs. They usually water all of their potted plants on a schedule.

Rosemary doesn’t like water as mint does, for example, so you need to take that into consideration when you water your herbs.

Overwatered rosemary may show signs of wilting or slow-growing. Sometimes, leaves will turn yellow or black.

To make sure you’re not overwatering your rosemary, wait until the soil is very dry to water the plant. At the same time, make sure the container you’re growing rosemary in is draining water properly.

– Heavy Soil

Heavy soil, such as clay soil, is a major problem for rosemary. Not only does this kind of soil trap a lot of water, but it also prevents the roots from breathing. This is almost fatal for the plant, as the roots start to gradually die due to a lack of oxygen.

To avoid such a problem, always use a premium, well-drained soil for your rosemary.

– Lack of Light

A rosemary plant that gets little light and not enough warmth will grow stretched and elongated branches. It will also become weakened and more susceptible to diseases.

This problem is especially common in rosemary plants that are growing indoors and don’t get enough light during the day.

Rosemary grows best with at least 6 hours of direct sunlight exposure per day, so in case you’re growing it indoors you need to find a sunny location for it.

If that’s impossible, you can use a full-spectrum LED light to help your rosemary get the right amount of light per day.

– Lack of Space

Rosemary plants that are growing in tight containers will start feeling claustrophobic at some point.

As your rosemary grows more branches and foliage above the ground, it also develops more root systems below ground.

Rosemary will stop growing as it should when its roots are unable to extend anymore. This will make it more weakened and fragile.

Make sure you repot your rosemary every year or so to guarantee that the roots don’t bump against the walls of the container.

– Pruning

Pruning your rosemary is necessary to stimulate the growth of newer and more flavorful foliage and to prevent it from turning woody and susceptible to disease.

Rosemary is an evergreen perennial herb, which means it can stay alive throughout the year if the climate is suitable.

One of the mistakes gardeners make is that they prune rosemary just before winter begins. This makes the plant too unstable and weak to survive during cold weather.

If you’re growing your rosemary under unregulated conditions, avoid cutting it back when the weather starts to get colder.

Have fun growing your rosemary, and don’t forget to share all your thoughts and questions in the comments below!

Powdery mildew on rosemary. (Photo from Flickr by Scot Nelson)

Wally in Toronto, Ontario, asks:

When I bring my rosemary plants into the house for the winter, they get a powdery mildew that kills new growth. How do I treat this?

Beckie’s reply:

Good air circulation is important to prevent or at least control powdery mildew. Don’t crowd your rosemaries with other houseplants and keep them away from humid locations. Consider directing a small electric fan on your plants a few hours a day to increase air circulation. Remove infected leaves as they appear and spray plants with a mixture of one tablespoon (15 mL) baking soda, one-half teaspoon (2 mL) liquid soap and one gallon (4L) water.

Unlike most herbs, rosemary doesn’t like completely dry conditions. Water when the top of the soil is dry, but avoid waterlogging the soil. There’s no need to fertilize in winter, but begin feeding every other week in early spring with a balanced formula, such as 20-20-20.

More answers to common gardening questions

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Learn About Rosemary

Common Disease Problems

Botrytis Blight: This causes the older leaves and the center of the plant to rot. It can start with a yellowish brown irregular spots on the leaves or water soaked spots on the stems. The fungus turns a fuzzy gray and emits a cloud of spores when touched. Burpee Recommends: Remove infected plants and plant debris to avoid the spread of the disease and make sure plants have good air circulation. Keep organic mulches away from the plants as spores can live in the organic matter. Use pea gravel as mulches they will help decrease humidity around the plants.

Damping Off: This is one of the most common problems when starting plants from seed. The seedling emerges and appears healthy; then it suddenly wilts and dies for no obvious reason. Damping off is caused by a fungus that is active when there is abundant moisture and soils and air temperatures are above 68 degrees F. Typically, this indicates that the soil is too wet or contains high amounts of nitrogen fertilizer. Burpee Recommends: Keep seedlings moist but do not overwater; avoid over-fertilizing your seedlings; thin out seedlings to avoid overcrowding; make sure the plants are getting good air circulation; if you plant in containers, thoroughly wash them in soapy water and rinse in a ten per cent bleach solution after use.

Downy Mildew: This fungus causes whitish gray patches on the undersides and eventually both sides of the leaves. Burpee Recommends: Rotate crops with plants in a different family. Avoid overhead watering. Provide adequate air circulation, do not overcrowd plants. Do not work around plants when they are wet.

Powdery Mildew: This is a fungus disease that causes a white powdery look on the foliage and is often a problem when growing rosemary indoors. This disease weakens plants as it inhibits their ability to make carbohydrates for themselves using sunlight. Burpee Recommends: You can remove infected plant areas, increase air circulation, and try to reduce the humidity in the room. Check with your Cooperative Extension Service for fungicide recommendations.

Root and Crown Rot: A number of diseases can cause root and stem rots in rosemary. Symptoms include yellowing of the leaves, drying of the leaves and leaf tips, and whole branches may become brown and die. Burpee Recommends: Make sure the soil is very well drained and that the plant does not sit in water. Replace the soil with fresh potting mix for potted plants, after thoroughly cleaning the pot with a 10% bleach solution. Check with your Cooperative Extension Service for fungicide recommendations.

Common Pest and Cultural Problems

Aphids: Greenish, red, black or peach colored sucking insects can spread disease as they feed on the undersides of leaves. They leave a sticky residue on foliage that attracts ants. Burpee Recommends: Introduce or attract natural predators into your garden such as lady beetles and wasps who feed on aphids. You can also wash them off with a strong spray, or use an insecticidal soap.

Leaves turn yellow or brown and may drop off: When the lower leaves turn yellow and the growth slows down this can indicate that the plant needs to be fertilized, or that it is rootbound. Burpee Recommends: If the roots are circling around in the pot (rootbound), clip them and repot the plant. For a nutrient deficiency, use Garden-tone as directed on the packaging.

Mealybugs: Flat wingless insects with a white waxy shell that form cottony looking masses on stems, branches and leaves. They suck the juices from leaves and stems and cause weak growth. They also attract ants with the honeydew they excrete, and the honeydew can grow a black sooty mold on it as well. Burpee Recommends: Wash infected plant parts under the faucet and try to rub the bugs off. They may also be controlled by predator insects such as lacewings, ladybugs and parasitic wasps. Check with your Cooperative Extension Service for pesticide recommendations.

Scale: Insects similar to mealybugs, they feature a waxy outer shell and suck the juices from plant stems and leaves. They weaken the plants and make them more susceptible to other pests and diseases and environmental stress. Burpee Recommends: If they are in the crawler stage (moving insects) they may be controlled with a cotton swab dipped in rubbing alcohol, or you may be able to scraping them off with your fingernail. Young plant parts that are infested may be removed. Check with your Cooperative Extension Service for pesticide recommendations such as horticultural oils.

Spider mites: These tiny spider-like pests are about the size of a grain of pepper. They may be red, black, brown or yellow. They suck on the plant juices removing chlorophyll and injecting toxins which cause white dots on the foliage. There is often webbing visible on the plant. They cause the foliage to turn yellow and become dry and stippled. They multiply quickly and thrive in dry conditions. Burpee Recomends: They may be able to be controlled with a forceful spray every other day. Check with your Cooperative Extension Service for miticide recommendations.

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Need help with plant identification, visit the plant identification page.

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Monday – August 13, 2012

From: Hartwell, GA
Region: Southeast
Topic: Plant Identification
Title: Identity of plant that resembles rosemary in Georgia
Answered by: Nan Hampton

QUESTION:

Hi, I am an herbalist, but have been unable to identify this plant and it’s driving me nuts since I cannot find any information on it. I live in NE Ga and have a plant that grows here that looks like rosemary it grows wild in very unkempt yards or fields, it also has a smell like rosemary, but with a slightly sweeter smell. It only grows in one stalk (as opposed to a bush like plant), but the leaves are slightly bigger and a lot denser than that of rosemary. I have a friend that lives south of me in Madison Ga who has a farm and has fields of it..we would like to know exactly what it is and if we can use it. Thank you!

ANSWER:

You may have stumped Mr. Smarty Plants because I can’t find anything that exactly matches your description. Here are some suggestions, however:

Eupatorium capillifolium (Dog fennel) has foliage that looks similar to rosemary, but the scent of crushed leaves is described as “rather unpleasant.” Here are photos and more information from Plants of Southern New Jersey and North Carolina State University.

Conyza canadensis (Canadian horseweed) looks somewhat like rosemary but is not described as smelling like rosemary. Here are photos and more information from Illinois Wildflowers.

Pycnanthemum tenuifolium (Narrowleaf mountain mint) has foliage that looks like rosemary; however, it doesn’t tend to grow as a single stalk. Its leaves do have a pleasant minty scent. Here are photos and more information from Illinois Wildflowers and Connecticut Botanical Society.

Pycnanthemum virginianum (Virginia mountain mint) with rosemary-like foliage that is mint scented. Here are photos and more information from Illinois Wildflowers and Plants of Wisconsin.

Hedeoma hispida (Rough false pennyroyal) has foliage that looks somewhat like rosemary and is fragrant. Here are more photos and information from Plants of Wisconsin and Southeastern Flora.

Hedeoma pulegioides (American false pennyroyal) looks a bit like rosemary, although the plants are are only about a foot tall. They have a strong, pleasant scent. Here are photos and more information from Illinois Wildflowers and Purdue Horticulture Service.

If none of these is the plant you are seeing, take photos and then visit our Plant Identification page to find links to plant identification forums that accept photos of plants for identification.

From the Image Gallery

Horseweed
Conyza canadensis
Rough false pennyroyal
Hedeoma hispida
Narrowleaf mountain mint
Pycnanthemum tenuifolium
Virginia mountain mint
Pycnanthemum virginianum

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Silvery plant looks like rosemary

You’ll probably not want to hear this, but better you know before you get to invested in the path you’ve chosen. Trying to recreate the bog or riparian settings in which some in situ plants thrive, in a pot, almost always ends in disappointment or failure. The short version of the reason for that is, plants in bog and riparian settings are able to grow roots specifically structured with aerenchyma at the root pith that allows the plant to get the oxygen that drives root metabolism from above the water line. Oxygen then diffuses through the roots to fuel the roots’ functions. Whereas, roots of terrestrial plants are filled with a parenchymous tissue at their core, and derive oxygen from air spaces in the medium. Since water absorption is an energy-driven process – no oxygen = no water uptake = a very sick plant. Your plant wants an evenly moist medium with NO soil saturation at any depth in the pot. That means the soil should be based on a very large fraction of chunky ingredients, and should never be ‘boggy’ no matter what the plant prefers in situ. Something like this: (misc articles on soil surface to provide size perspective) Unfortunately, the roots are unable to switch back and forth between the variations of conventional container culture and some form of aquaculture fast enough to ensure ongoing viability. You might not like the lecture, but I see you’re designing the care of your plant around a personal ideology that is limiting, so in order to help you, It would be necessary to change your mind about several things and go into growing methods that are overly complicated; this, because I’d be working around the limitations you’re saddled with and I’m not sure how you’d tolerate that. FWIW – All parts of the moorwort are toxic. This plant is not a houseplant, in fact, no plants are naturally found indoors – there are only some outdoor plants that are able to tolerate indoor conditions to varying degrees. Your plant will need a cold winter rest (period of dormancy) at temps below 42* in order to survive the next growth cycle. Deprived of that, it will perform poorly until it collapses. My suggestion would be to pick something easier and to start researching plant physiology and soil science if you intend to pursue the joy of growing things. Best luck – and Happy New Year. Edited to say: DUH! I just looked at when this was originally posted. Al

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