Roses And Downy Mildew: Identifying And Treating Downy Mildew On Rose Bushes

Roses And Downy Mildew: Identifying And Treating Downy Mildew On Rose Bushes

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

Downy mildew on roses, which is also known as Peronospora sparsa, is a problem for many rose gardeners. Roses affected by rose downy mildew will lose beauty and stamina.

Symptoms of Downy Mildew on Roses

Initial leaf symptoms of roses with downy mildew are light green to yellow spots, called “oil spots” because they may appear greasy. Downy mildew on roses seems to attack the new foliage growth first and works its way down the rose bush. You may also see a reddening of the stem or sepals on the rose bush.

Treating Roses with Downy Mildew

Rose downy mildew can be a very tough customer to get rid of once it gets started. When using fungicidal sprays to gain control, it is best to alternate fungicides used at each spraying using a fungicide with a different mode of action. Fungicides containing Metalaxyl seem to provide some control.

The Oospores of rose downy mildew can be either wind or water borne, thus infection of other rose bushes in the same rose bed is highly likely. Spraying all of your rose bushes from first onset of the infection and continuing for at least four to five sprayings 7 to 10 days apart should stop any spreading of the downy mildew. Here too, a preventative fungicidal spraying program carries much value.

Downy mildew on roses will over winter in rose bushes. For this tough customer, the best cure is truly prevention. A good spring clean up of all old foliage is very important in the prevention of this disease.

With downey mildew on rose bushes and any rose disease, maintaining good air flow through and around your rose bushes will help prevent this disease. Do not let them become so overgrown or tight with foliage. New cane growth throughout the center of the rose bush can become its worst enemy in the fight against diseases. Indeed, a rose bush with full foliage and loaded up with blooms is a pure delight to behold; however, you can get this same beautiful look with just a bit of thinning out that will allow the air movement needed.

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Downy mildew, techniques and organic treatments to avoid it

Famous for being vineyard owners’ #1 enemy, downy mildew is a constant threat to grape harvests. This blight also threatens certain other vegetables from the vegetable patch such as tomatoes and potatoes, too.

Regularly applied preventive treatment should help you produce a beautiful harvest without any trace of downy mildew.

Symptoms and Causes of Downy Mildew

Downy mildew can be festering on your plants for some time before being noticed. That is because it usually first appears on the undersides of leaves. The leaf bottoms can appear to have white or bluish-white fuzzy or fluffy looking growths.

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After some time, the tops of the leaves may develop small green or yellow translucent spots. If these spots remain untreated, they leave a gray colored lesion in their wake. Eventually whole leaves, then branches, even flowers and fruit will die. These deceased brown or bronze colored portions will continue up the plant, if not controlled and the climatic conditions do not change or are otherwise unaddressed.

Downy mildew is caused by several related, but host-specific fungal species of pathogenic water molds (oomycetes). This means the type that infects one kind of plant may not be the same species of fungus affecting another plant, though the symptoms may appear the same. Downy mildew spores are produced only on living plants and spread primarily through wind displacement and air movement, though they can be spread by insect pests moving from plant to plant. As spores land on a host plant, they can germinate and infect within as few as eight to 12 hours if the plant or foliage is wet. Some of the more commonly occurring pathogens for downy mildew are Peronospora spp., Plasmopara spp., and Pseudoperonospora spp. It should be noted, however, that finding downy mildew on one type of plant typically means that the conditions are favorable for the development of other downy mildews on other types of plants as well.

If unsure what kind of disease you have, take a large sample to your local cooperative extension service or larger garden center for identification where it can be viewed by a trained professional with a keen eye or viewed under a microscope. Downy mildew appears under the lens in a branched pattern whereas powdery mildew appears in a chain-like formation when view under extreme magnification.

How It Works

Hydrogen peroxide is really water with an extra oxygen molecule, according to Agron. As it naturally decomposes, it acts as an antimicrobial. The Food and Drug Administration regards the solution as environmentally friendly, and it's considered a natural alternative for pest and fungus control.

Hydrogen peroxide, with its extra oxygen molecule, also helps plant roots take in more nutrients from the surrounding soil. These roots become more efficient at absorbing nutrition and therefore help the plants grow faster and more vigorously and stay healthier, because they are better able to fight off fungus and bacteria.

Make sure you store hydrogen peroxide in a cool, dark place. Do not get it directly on your skin, because it can bleach or burn skin.


SERIES 16 | Episode 38

Discovering a disease in the garden is often a trigger for a gardener to use a chemical, such as a fungicide to get rid of the powdery mildew. But chemicals often cause other problems. They can affect beneficial micro organisms in the soil, and can kill pollinators, like bees, and without those there probably wouldn't be many tomatoes.

As an organic gardener, I believe pests and diseases should be kept in their place, so all the remedies used in my garden are safe. They're safe for pets, for wildlife and for kids. The ingredients for some favourite organic remedies are hiding in full view, probably in every kitchen. They include milk, coffee, bicarbonate of soda, vegetable oils, detergent and white vinegar. Let's see how to mix these up to make some effective controls for a range of plant problems.

First lets make a milk fungicide. Use organic milk because it contains all the antibiotic qualities necessary to make it work. The magic ingredient is one part of the organic milk to 10 parts water. Give it a good stir and it's ready for use. The objective of spraying is to cover every part of the plant, both sides of the leaves and coat the stems. Fungicides work best as a preventative, not as a cure. Now this has been proven to work on plants within the cucumber family, so the choko is fine. It's also good on begonias. Research in South Australia is proving that it's also effective in controlling mildew on grapevines. Research has also shown that too much milk in the solution will encourage sooty mould, so stick to the recipe one part milk, 10 parts water.

This organic fungicide is particularly useful on soft leaf vegetables. Use 2 litres of water, and a drop of vegetable oil, which helps to fix the spray to the leaf when it's dried. A drop of detergent helps to spread the mix over the leaf. Then add the active ingredient - bicarbonate of soda. Put in two teaspoons per litre. The bicarbonate of soda makes the leaf surface alkaline and this inhibits the germination of fungal spores. Use it on tomatoes, and Chinese celery. The Royal Botanic Gardens in Sydney has found this to be effective on powdery mildew, rust and black spot on roses. So oil, detergent and bi-carb is all you need for a great preventative fungicide.

Algae ruins paintwork and if it grows on footpaths they become slippery and dangerous. So to make a straightforward organic algae killer, use one part of vinegar - use white vinegar because it's so cheap - add three parts water and simply spray on after it's been mixed. The acid in the vinegar kills the algae. Normally people would use chlorine bleach to get rid of algae, but of course when chlorine touches the soil, it damages plant roots and seedlings, and when it combines with organic matter it forms chloramines, which are highly toxic. So one part vinegar and three parts water and it's perfectly safe

Coffee. It's a great slug and snail killer. Add to 10 parts water, one part espresso coffee. It has to be espresso because instant coffee is just too weak. Spray this solution over the surface of leaves and over the surface of the soil, where snails and slugs might crawl. The snails absorb the coffee through their skin and the caffeine in it kills them. So that's 10 parts water and one part espresso coffee. That's got to be better than using chemical snail baits in the garden. But remember to reapply all these remedies after heavy rain because they are not residual.

So for a successful and safe garden without any poisons why not try these simple and effective organic remedies at home.

Ever-Blooming Roses

Do you know the difference between black spot and downy mildew? I am just now realizing I’ve been battling downy mildew. So not cool!

I get emails from many rose growers in spring and fall who think they have blackspot. Many of them have been treating for this disease and don’t understand why the fungicides they are using are not working. In most cases I find they have misdiagnosed their problem and what they actually have is downy mildew.

I will talk about how to control both diseases in future blogs, but for today let’s focus on what they look like.

Advanced stages of blackspot

Blackspot (Diplocarpon rosae) starts as a small black spot/spots on otherwise green healthy leaves. As the disease progresses, the leaves get more spots and the green tissue begins to be replaced with yellow. Usually within two weeks of infection the leaves will begin to fall and the entire plant may be defoliated. Left untreated the plant will try to put forth new…

  • Remove and dispose affected leaves and buds as soon as possible. Clean equipment afterwards.
  • Spray roses during the growing season with a fungicide that states the treatment of downy mildew. All roses in your garden should be treated. Follow up treatment will be necessary.
  • When winter pruning has been done and all waste removed, spray with Lime sulphur on the plant and ground around. This will clean the area of fungal problems giving you a fresh start in the new growing season.

If you are living where roses are known to have this problem, make sure there is good air circulation around your rose and prune to take away dense foliage from the heart of the bush.

Start your chosen spray routine before you see the problem on the leaves.

Make sure your roses are well watered and fed regularly. A happy rose is a healthy rose.

Choose varieties that are strong growers and either newer varieties or those recommended by the rose society in your area.

Important: signs and symptoms will vary significantly between varieties, even within similar categories. The information provided here is a basic summery of the most common affects and will not always be applicable to all rose varieties.

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Watch the video: See how Grape Downy Mildew develops