Black spots on roses

Black Spot On Rose Bushes – How To Get Rid Of Black Spot Roses

By Stan V. Griep
American Rose Society Consulting Master Rosarian – Rocky Mountain District

A common rose disease is known as black spot (Diplocarpon rosae). The name is very appropriate, as this fungal disease forms black spots all over the foliage of rose bushes. If left unchecked, it can cause a rose bush to totally defoliate. Let’s look at what causes black spots on rose bush leaves and steps for treating black spot roses.

What Causes Black Spots on Rose Bush Leaves?

Many frustrated gardeners wonder, “What causes black spots on rose bush leaves?” Black spot and roses usually go hand in hand. In fact, many roses get a little black spot, which can even be tolerated to some degree without any harm to plants. However, heavy infections can seriously defoliate plants.

Rose black spot is caused by fungus. Dark-brown to black leaf spots develop on the upper leaves, which eventually become yellow and drop. Black spot can be distinguished from other leaf spot diseases by its fringed edges and dark black color. Raised, reddish-purple spots may also appear on rose canes. Warm, humid conditions favor its germination and growth.

How to Control Black Spot on Roses

Once your rose bush gets attacked by the black spot fungus, its markings are there to stay until the marked leaves fall off and a new leaf is generated. The fungus that causes the black spots can be killed and not do any further damage to the foliage but the marks will remain for some time. In my rose beds, a rose named Angel Face (floribunda) was a black spot magnet! If I did not spray her when her leaves first started to

form in early spring, she would most certainly get the black spot.

My fungicidal spraying program for the last several years to prevent black spot in roses has been as follows:

In the early spring when the leaf buds on the rose bushes first start to push out the little leaves, I spray all the rose bushes with a black spot treatment fungicide called Banner Maxx or a product called Honor Guard (a generic form of Banner Maxx). After three weeks and then at three week intervals, all rose bushes are sprayed with a product called Green Cure until the last spraying of the season. The last spraying of the season is done with Banner Maxx or Honor Guard again.

Should the dreaded roses black spot get ahead of you in the rose beds, a product called Mancozeb fungicide will stop black spot on rose bushes in its tracks. I found out about this great product a few years ago when rose black spot got ahead of me and the rose Angel Face was well under attack. The Mancozeb does leave a yellowish powder on all of the foliage, but that is part of how it works. This product is applied every 7 to 10 days for three sprayings. After the third spraying, the normal spraying program may continue. The black spot fungus should be dead, but remember the black spots on the rose leaves will not disappear.

The Mancozeb product may be mixed with another fungicide called Immunox and then applied to the rose bushes to lessen the amount of yellowish powder left on the foliage. Both are added to the spray tank as if they were the only product in the tank mix. I have personally used both of these application methods and both worked very well.

Preventing Black Spot on Rose Bushes

Treating black spot roses begins with prevention. Black spot rose disease control includes adequate planting sites, the use of resistant cultivars, and pruning. Roses should be planted in areas with plenty of sunlight and good circulation.

Good garden hygiene is important for treating black spot roses. During the growing season, overhead watering should be avoided. Removal of leaf litter and pruning of diseased canes (back to healthy wood) is also important. Keeping the rose bushes thinned well at pruning and deadheading times will help the airflow through the bush, thus also helping to prevent black spot on roses and other fungal disease outbreaks.

With any of the fungal diseases, an ounce of prevention truly is worth a pound or more of cure! Either having a routine spraying program or keeping a close eye on your rose bushes is a priority. The sooner roses black spot treatment starts, the easier it is to gain control of it. I like to use the Green Cure as my main fungicidal spraying product, as it is earth friendly and does the job it needs to do. Neem oil can also be used, which helps control many rose pests as well.

Some people also use baking soda, which helps change the pH level on leaf surfaces, making it more difficult for black spot to infect plants. To make this organic solution, mix a couple tablespoons of baking soda with a gallon of water. Adding a drop or two of bleach free dish soap will help keep the baking soda on the leaf. Spray both sides of the foliage. Reapply weekly and repeat after any rain.

Where we live in the deep south, the reason most people don’t grow these beautiful roses is that they can’t keep the blackspot off the rose bushes. Blackspot is a fungus called Diplocarpon rosea, and travels via water to infect plants. It our wet and humid environment, blackspot is a given. It will cause leaves to turn yellow with black spots. These leaves will eventually die and fall out, leaving your plant weakened.

I have found that the only fungicides that keeps blackspot off of the roses are propiconazole, which is found in Banner Max, or in generics such as Honor Guard and others. Most of this fungicide is only offered in large quantities. The other fungicide is Ethylenbisdithicocarbamate which also includes manganese and zinc. This product comes under a number of names, such as Manzate, Diathine M-45, and others.

The two products we use are Honor Guard (a systemic fungicide) and Diathine M-45 (a contact fungicide). These are available for sale. Honor Guard is in a 1 pint container for $49.95. Diathine M-45 is in a 2 lb container for $34.95. The recommended use for Honor Guard is ½ teaspoon to a gallon of water. The recommended use for Diathine M-45 is a tablespoon to a gallon of water. These will last you for many sprays.

We spray our roses every 7 to 10 days, and this controls the blackspot. We use both products in the same spray as one is a systemic and the other is a curative or contact. We also add a couple drops of dishwater detergent to a gallon of water as a sticking agent for the spray. We spray early in the morning or late in the afternoon so as not to spray during the hot hours of the day ( this prevents the leaves from burning).

We also recommend protective gear that in on the label of the product as well as following all the other directions on the label.

Every 4 times we spray our roses we omit the Honor Guard and spray only Diathine M-45. This keeps the roses from building up the growth inhibitor that is in the Honor Guard. Then the next time we spray we use the Honor Guard as usual.

In all of the years we have grown roses, this is the most effective way we have ever used. You should also use a sprayer that will spray a fine mist so as to really cover the leaves. There are many types available on the market such a backpack sprayers or the easy rechargeable 12 volt sprayers.

How to Control Black Spots On Roses

The Diplocarpon rosae fungus causes black spot, one of the diseases that most commonly affects roses . Black spot begins as small brown or black pinhead-sized spots on leaves. The spots grow and the leaves turn yellow and fall off. The disease weakens the plant, making it produce fewer blooms and more susceptible to winter kill .

Black spot spores germinate when it’s around 65 degrees Fahrenheit (18.3 degrees Celsius). The disease spreads rapidly when the temperature reaches around 75 degrees Fahrenheit (23.9 degrees Celsius). Temperatures above 85 degrees Fahrenheit (29.4 degrees Celsius) will keep the disease from spreading .

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The best way to control black spot is to prevent it altogether. Here’s how to prevent black spots on your roses.

  • Choose resistant varieties of roses. Some varieties are more resistant to black spots than others.
  • Grow your roses in a sunny spot, so the foliage will dry rapidly. Spores only grow if they’ve been wet for at least seven hours .
  • Leave room between your rose plants so the air circulates freely .

Here’s how to control black spots on your roses:

  • Remove infected leaves as soon as possible, so the disease won’t spread.
  • Prune and discard any canes that are obviously infected.
  • Avoid wetting the foliage.
  • Rake and discard all fallen leaves. This is essential because the fungus can survive the winter on fallen leaves. It cannot survive in the soil.
  • Spray your roses with fungicide regularly throughout the growing season. (Liquid sprays work better than dust formulations.) Don’t wait until you see black spots on your roses. Preempt them by using fungicide before the problem begins .

The following fungicides will help keep black spots at bay.

  • Captan
  • Chlorothalonil
  • Maneb
  • Mancozeb
  • Myclobutanil
  • Propiconazole
  • Thiophanate-methyl
  • Trifloxystrobin
  • Triforine
  • Ziram

If you don’t want to use chemical fungicides in your garden, here are some alternatives:

  • Copper products
  • Hydrogen dioxide
  • Lime Sulfur
  • Neem oil
  • Potassium bicarbonate
  • Sulfur

Because rose leaves are waxy in nature, adding a spreader to the spray will help give better coverage.

Black Spot Control

SERIES 18 | Episode 07

Roses are the world’s favourite flower, and black spot is the scourge of rose growers. Black spot is a fungal disease and it thrives in warm humid climates. It starts off as a black spot in the leaf and then it turns yellow and eventually the leaf falls off, and if it’s really bad, the plant can die.

The first thing to do is to open up the rose bush to create more air movement through the middle and that minimises the risk of humidity.

Watering the rose foliage creates the right environment for the proliferation of the disease, so never water the foliage. Always water around the roots of the rose and give it a good soaking.

Plants are no different to people – the better fed they are, the more likely they are to resist disease. And that’s also true with roses and black spot so that means give your roses a good feed every six to eight weeks through the growing season with an organically based rose fertiliser.

When the black spot spore lands on the leaf, it germinates and sends its little root system through the cell wall into the sap stream below and it proliferates. If you thicken that cell wall, the spore lands on the top, the root system germinates and it goes halfway through and then fizzles out, and so you get less black spot. But how do you thicken that cell wall? It’s easy – just use sulphate of potash and give them about 100 to 150 grams per bush about four times a year – this should guarantee a lot less black spot.

Even after attending to the nutritional needs of roses, there will still be a need to spray. There are plenty of safe and organic ways to treat black spot. Try using two teaspoons of bicarb soda in 5 litres of water, add a couple of drops of detergent or a couple of drops of seaweed extract. This makes an excellent and inexpensive fungicide. Or use Bordeaux mixture or one of the other copper based fungicides.

Nutrition and spraying will control most black spot but from time to time some bushes will be chronically affected. The only thing to do is to rip these out so they don’t infect any of the others.

Remember, garden hygiene is of vital importance. Go round on a regular basis and pick off any black spot affected leaves, put them in a plastic bag and tie the top tightly. Then leave it out in the sun to cook and that will kill the spores. Don’t put them in the compost heap, instead put them in the bin. When you control black spot, you can have some beautiful roses.

Many gardeners are experiencing problems with Black Spot on roses at the moment. This isn’t surprising as it’s spring and September was very cool and wet.

Black spot produces tell tale, irregular black spots on rose leaves. As the disease progresses leaves turn yellow and can fall off. On young roses and very susceptible varieties, black spot, can also infect canes.

I think as the days slowly get sunnier and warmer, you’ll probably see even more black spot out there. It’s sort of that “honeymoon” period for the fungus, there’s still a fair bit of moisture around, cool night temperatures and warmer days.

As we enter the more normal late spring-summer weather pattern, foliage tends to dry out quite quickly so conditions don’t suit fungal growth, so we don’t see it as much.

The best way of dealing with black spot is some preventative spraying to keep the fungus at bay or to treat existing outbreaks. I’d recommend fortnightly spraying with a suitable product to check black spot during this it’s most prevalent period.

Black spot is virtually a natural part of rose growing. It’s an environmental disease, as the spores are usually always out there, just waiting for the right weather conditions to get started.

Products suitable for black spot control are:

Yates Rose Shield – A good all in one rose doctor which treats all the major rose fungal diseases as well as insect pests.

Sharp Shooter – Another all in one product like Rose Shield.

Eco Fungicide – Organic registered. Used in conjunction with Eco Oil. Eco Fungicide has an interesting mode of action it actually changes the pH of the leaf surface controlling existing infections and inhibiting new fungal spread and infection.

Triforine – (Fungicide only) for most rose fungal infections.

Yates Liquid Copper – Anti fungal effect, works on black spot.

On the do it yourself front, there is quite a bit of evidence that milk sprays also work. A spray of 1 part milk to 10 parts water applied weekly.

Remember with rose spraying, good coverage over all surfaces (above and below foliage is essential).

Cultural things also help; avoid overhead watering where possible, space plants appropriately to get good ventilation, good feeding, plenty of sun, good hygiene (cleaning old fallen leaves particularly in winter).

Enjoy your roses,

The Dawson Gardener

Prevent and Treat Blackspot

How to Prevent and Treat Blackspot

Blackspot is a fungus that affects rose bushes. Black spots appear on leaves which enlarge over time and make the foliage around the spots yellow. The leaves will go from green to yellow and then drop to the ground. If left untreated, it can defoliate the entire plant. The disease spreads by rain or overhead watering and can affect other nearby plants. Heirloom Roses’ Head Grower, Don Merrick, provides some tips on how you can prevent and treat Blackspot.

  1. Plant disease-resistant roses: There are many varieties that have strong resistance to Blackspot and other fungal abnormalities. Some of our favorite varieties that exhibit good to great disease resistance are: Apricot Abundance, Electron, By Appointment, Welsh Gold, Morning Has Broken, Carefree Beauty, Highfield, William Baffin, Amber Abundance, Lawrence of Arabia, Sharifa Asma, Velvet Abundance, Soaring Flight, Black Pearl, Belle Epoque, and Berolina.
  2. Find the right spot: Plant roses in an area that gets 6-8 hours of sunlight daily. They also need good air circulation. Space them out to give good air movement and prune out some of the inner branches to allow more air movement into the center of the plant. Also, plant them in a spot that has good drainage. You can also add well-composted organic matter into the soil to make the soil friable and well drained. This will also encourage an abundance of beneficial organisms.
  3. Water correctly: Too much water and water at the wrong time of day will encourage the outbreak of Blackspot. The best rule of thumb is to water when the soil is dry to the touch at about 2-3″ below the surface. If it is during the cooler spring months, a deep thorough soaking once a week is sufficient. If it is raining, check the soil to determine if watering is necessary. During the hotter summer months, it will be necessary to water more frequently and deeply.
  4. Keep the foliage dry: If using an overhead sprinkler, it is best to water mid to late morning, giving the roses a chance to dry off during the day. The best method to keep foliage dry is to water the soil only. This can be accomplished by using any of the excellent drip systems or soaker hoses on the market. Also, avoid standing water around your roses and keep the area around your roses debris free. Blackspot spores will fall to the ground and stay in the leaf matter/mulch at the base of your roses. Avoid splashing water as the spores can reattach to the undersides of the leaves when they are carried by splashes of water.
  5. Prune properly: Remove any weak or damaged branches to keep your roses happier and healthier. Cleaning up the debris is paramount to keeping Blackspot at bay. All trimmings, debris and dead leaves need to be removed and destroyed immediately.
  6. Know you enemy: Recognizing the disease quickly allows you to nip it in the bud. Look for circular black spots that are serrated in appearance on the surface of the leaves. Always check the lower leaves, as they will become infected first. Upper leaves will be yellow and fall off easily. Roses with Blackspot start to grow less vigorously and blooming will be reduced or stop all together.
  7. Treat immediately: If your rose has been affected, remove all infected leaves from the rose and the ground. Do not compost these leaves. Keep the ground surrounding your roses free of leaf debris and weeds. Then, apply the right type of chemical controls at the right frequency and duration during the most critical times.
  8. Prevent early: An ounce of prevention is worth a pound of cure. If you can get ahead of the game and have your roses sprayed before there is noticeable damage, then your problems will be reduced or possibly eliminated. It is a good idea to spray a fungicide once every 7-14 days during the growing season. There are many different chemicals to use and there are several organic sprays that can be used with fairly good success. It is vitally important to change up the chemistry, or alternate chemicals throughout the growing season to avoid chemical resistance by the fungus.
  9. Prune in spring: Not all varieties respond the way we want them to with spring pruning. The once blooming types of roses will need to be pruned hard just after flowering in the spring and summer to encourage flower bud set for the next spring. This is also a good time to clean them up and take out any diseased wood. Make your cuts well below the Blackspot-damaged area of the plant to ensure that you are removing anything that may be on or in the canes. In the spring, be sure to cut back the canes that are infected with Blackspot. It should be fairly easy to see the black spots on the canes and easy to remove them. Again, whenever you are pruning or cleaning up around your rose, it is imperative that you clean your area up completely.
  10. Clean your pruners: Another good tip is to disinfect your pruning tools with Lysol disinfectant spray every 15-20 minutes. A 10% bleach solution is also a great way to keep the spores at bay.

To the rose enthusiast (especially those in the more humid parts of the country) the appearance of black spot can be an endless headache.

However, with a few proactive health strategies for your roses, black spot needn’t darken your mood.

Black spot taking hold on rose foliage

What is black spot?

Black spot is a fungal disease characterised by the appearance of circular spots over the foliage of affected roses. Black spots with yellow-fringed edges, up to 12mm across, will appear on the leaves of affected bushes. These affected leaves will eventually become yellow and fall off. If left untreated, black spot can cause a rose bush to completely defoliate, leaving frustrated gardeners scratching their heads over the demise of their favourite rose bush.

What causes black spot?

Black spot is caused by a fungus, which thrives in warm, humid conditions. It is more common in particular rose varieties, particularly those with yellow genetic parentage.

The first yellow rose was introduced into rose breeding in Paris in the mid-19th century. This Persian rose, R. foetida ‘persiana’ – which was most likely introduced by Andre DuPont, Empress Josephine’s rose-breeding confidante – went on to play a significant role in the development of new rose cultivars, creating never before seen colour variations. It is reported that with the introduction of R. foetida, the Empress’ collections at ‘Malmaison’ increased from 182 varieties, to over 6000 between 1814 to 1850.

Sadly, over time R. foetida proved to be very susceptible to black spot. Today the susceptible gene is the cause of many frustrated rose growers worldwide. So prevalent was the Persian rose in breeding, that many modern rose varieties, especially yellow varieties, are believed to have R. foetida ‘persiana’ as part of their ancient parentage.

Managing black spot on rose bushes

Controlling black spot in roses requires a multi-faceted approach. Rather than treating the symptoms in isolation, a holistic approach at managing the disease will yield better results.

1. Consider the weather conditions

Many fungal diseases proliferate in warm and wet conditions, particularly is the leaf remains wet for extended periods. Reduce humidity by avoiding overhead watering. Watering in the evening should also be avoided, as it allows moisture to remain on the foliage, creating ideal conditions for fungal spores to germinate and cause disease. Make sure there is good air circulation around your rose bushes.

2. Select resistant rose varieties

As most roses are genetically susceptible to black spot it is important to choose resistant varieties that are well suited to your climate.

3. Choose your planting location carefully

Growing conditions can play a big part in a plant’s susceptibility to pest and disease. Most roses require a minimum of five to six hours of direct sunlight each day to bloom properly, so it is best to avoid semi-shaded positions when planting. Roses also prefer to grow without root competition from other plants, such as large trees. For example, if planted under large gum trees roses will compete for water and nutrients making the plant even more prone to infestation.

4. Maintain good plant hygiene

Good sanitation is important to eliminate contamination by fungal disease. Remove and dispose of diseased leaves, including those on the ground, and put them in the rubbish, not the compost. Leaves left lying on the soil have the potential to pass fungal spored on to other roses. Apply a layer of mulch prior to spring so that there can be no splashing of remaining fungal spores from the soil to the lower foliage of the plant.

5. Keep your roses healthy

Just like humans, the healthier your roses, the stronger their resistance to black spot. By improving the general vigour of your rose plants with generous applications of a specific rose fertiliser that includes potash, as well as improving the growing conditions, you can reduce, if not overcome, the incidence of pests and diseases.

Suitable products for treating black spot

Members of the Rose Society of NSW, and other Australian rose societies, have conducted trials of the rose fertiliser, Sudden Impact for Roses, which consistently show an improvement in overall health of the roses trialled, with increased resistance to fungal disease, resulting in a significant reduction in preventative spraying; up to 66%.Rose sprays with tau-fluvalinate and myclobutanil as the active ingredients (like in Yates Rose Gun, and Yates Rose Shield), will be effective in the control of black spot and insects such as thrips and aphids.OCP Eco-Rose is effective in the control of black spot because it contains a specially formulated potassium bicarbonate that alters the pH of the leaf, dehydrating the fungal spores. It can also be combined with OCP Eco-Oil for further benefit. It is good practice to spray roses after pruning in winter with lime sulphur, to disinfect them and clean up fungal spores and insect eggs. If untreated fungal spores can multiply over winter then germinate in spring in the warm, humid conditions.

Remember… prevention is better than a cure

High humidity combined with warm weather encourages black spot, so it is important to start treatment early. Start a program of preventative spraying every two weeks, beginning early in the growth season (October) and continuing through spring, summer and Autumn (April). Alternate each fortnight between Yates Rose Shield and a mixture of OCP Eco-Oil & Eco-Rose. A regular watering with a foliar seaweed tonic like Organix Ecoguard will increase the health and vigour of roses by thickening cell walls, making them inherently stronger. Even with the best hygiene practices, if your roses become stressed for any reason, they will get black spot. Healthy and well-nourished plants grown in the appropriate environmental conditions are always naturally better equipped to fight off pests and diseases.

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