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Mildew in Your Garden?
by Paulette Mouchet

Originally published in The Rose Garden newsletter

Mention mildew to a chemical gardener and a few minutes later he'll be decked out in a biohazard suit with a Hudson sprayer full of fungicide in hand. 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—a guy named Theophrastis wrote about it in 300 B.C.! It's considered one of the Big Three rose diseases. So what is it, why is it bad, and how do you get rid of it without resorting to chemical warfare?

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.

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

rose disease: downey mildew
Downey 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 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.

Powdery mildew affects roses grown in dry climates and is common in the Southwest. 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 temps, 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.

As with downy mildew, prevention is the best cure:

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