Mildew Malady
Organic Alternatives for Preventing Mildew on Roses

By Paulette Mouchet

Mention mildew and it's enough to make any rosarian cringe. It's considered one of the "Big Three" rose diseases and is why many rosarians follow a routine spraying program. Even organic gardeners who follow good cultural practices will sometimes find an outbreak that needs eradication. Mildew has been a known problem since long before Christ when a guy named Theophrastis wrote about it in 300 B.C. So what is it, why is it bad, and how do you get rid of it?

Mildew is a disease caused by a group of fungi that damages the leaves, flowers, and fruits of many plants including roses. Different fungi strains cause different mildews on different plants. A severely infected plant will eventually die.

There are two types of mildew common to roses: downy mildew and powdery mildew.

Downey mildew is caused by the Peronospora sparsa fungus, and occurs under cool, moist, cloudy conditions. Common in coastal gardens, it produces irregular, reddish-purple blotches on the rose leaves. Advanced infections will produce yellowing of leaves and brown, dead areas within the purple blotches. In severe cases, there will be irregular purple blotches on the canes. Plants eventually die. Note, there is NO white powdery coating on the leaves.

The fungus can over-winter on infected plants and when conditions are right, be in full swing in as little as 3 days. Since the fungus develops in free water on the plant's surfaces, it's important to avoid overhead watering when weather conditions favor the development of downy mildew.

The best cure is prevention:


Temperatures above 80 degrees and relative humidity below 85 percent suppress downy mildew development so it is not often found in the drier areas of California. 

Powdery mildew affects roses grown in dry climates and is common in much of California, including the Santa Clarita Valley. Caused by Sphaerotheca pannosa var. rosae, it forms a white, powdery film on the leaves, stems, and flowers of infected roses. Young peduncles, sepals, petals, and stems become twisted and distorted and new growth buds and growing tips may be killed.

Powdery mildew is common in the spring and fall when warm days are followed by cool, damp nights. Spore maturation and release usually occurs during the day when the relative humidity is low. At night when temperatures drop and the humidity increases, spores germinate and the fungus penetrates the plant's surfaces. Spores need low humidity, warm temperatures, and dry leaves to mature and be released, hence the advice to hose off the leaves of infected plants during the day. The only caveat being that the plant must be dry by nightfall.

Again, prevention is the best cure:

One of the easiest ways to control powdery mildew is to give your roses a thorough wash at least two mornings a week. A morning bath raises the humidity so spores can't mature. Free water left on the plants also discourages spore maturation. Plants must be dry before nightfall so residual moisture does not provide favorable conditions for spore germination.

Powdery mildew is attracted to lush, new growth. Rosarians growing for spring shows feed their plants heavily early in the year in order to have roses ready to cut. If you are not pushing your plants for a show, try cutting back or postponing the early spring feeding.

Chemical Fungicides
There are several chemical fungicides on the market that will control and eliminate downy and powdery mildew. They upside is that they do kill fungi. The downside is that they kill all the fungiincluding mycorrhiza, tiny fungi that live in symbiosis with the delicate feeder roots of plants. Mycorrhiza are responsible for the transportation of nutrients and water into plant roots and without these fungi, plants will wither and die.

According to Dr. Elaine Ingham, a professor at Oregon State University, with proper fungal and bacterial populations in the soil, "you will see plants with higher nutrient levels in them." Grapes growing in healthy [soil], for example, had three times more protein than those growing in a depleted system. Wheat had 10 times more protein. Concentrations of every micronutrient are increased when plants are growing with mycorrhizal relationships, says Ingham.

If you'd like to skip the chemicals, there are several options available. Jan Weverka, former editor of The Rose Garden, has long recommended using a vinegar rinse to eliminate powdery mildew.

The Vinegar Rinse
2 tablespoons white or cider vinegar (5 percent)
1 gallon water

Pour undiluted vinegar into the container of a Gilmour Metering Dial (dial-a-spray) hose-end sprayer. Set the dial to deliver 2 tablespoons per gallon and start spraying. Soak the entire plant, making sure to get the undersides of the leaves. Spray in the morning. Apply once a week as needed until the weather warms up.

Baking Soda and Insecticidal Soaps
In the late 1980s, Dr. R. Kenneth Horst of Cornell University began testing the use of baking soda and insecticidal soap to control powdery mildew and blackspot on roses in New York. The soap helped the baking soda solution spread over, and stick to, the plant leaves, but had no effect in suppressing disease. Roses were sprayed every 3 to 4 days and Dr. Horst found the mixture was most effective in preventing blackspot but was also effective on powdery mildew. The formula that Dr. Horst developed is now available commercially under the name of Remedy [available at Amazon].

Mildew doesn't have to ruin your day and neither does getting rid of it. The time involved in maintaining a regular chemical spraying schedule is one reason I prefer the organic way. It takes a lot of time to put on protective gear, mix up the chemical, apply it, then clean up. The organic way takes a lot less time and I don't have to worry about getting cancer from the product I'm using. As an organic gardener, I'll be sitting in my lawn chair admiring my beautiful roses long before a chemical gardener stows his bio suit.

Paulette Mouchet is editor of The Rose Garden, a monthly newsletter devoted to fine organic gardening for roses in temperate climates. She can be reached at Crown Valley Press, P.O. Box 336, Acton CA 93510, 661-269-1525, geomouchet@Qnet.com

© Copyright 2012 Paulette Mouchet. All rights reserved.

This article was originally published in "Rose Ecstasy," bulletin of Santa Clarita Valley Rose Society, Kitty Belendez, Editor.

Photos © Copyright by Kitty Belendez

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Updated December 11, 2012