Answer: The best method of control is prevention. Avoiding the most susceptible cultivars, providing plants full sun, and following good cultural practices will adequately control powdery mildew in many situations. However, some ornamentals require protection with fungicide sprays where conditions are most favorable for mildew.
Powdery mildew spores are carried by wind to new hosts. Although relative humidity requirements for germination vary, all powdery mildew species can germinate and infect in the absence of free water. In fact, spores of most powdery mildew fungi are killed and germination is inhibited by water on plant surfaces for extended periods. Moderate temperatures (60? to 80?F) and shady conditions generally are the most favorable for powdery mildew development. Powdery mildew spores and mycelium are sensitive to extreme heat and sunlight, and at leaf temperatures above 90?F, some may be killed.
I think in your case, a preventative spray is required. Bordeaux or lime-sulfur is the most effective (Safer's Garden Fungicide), applied in the spring just as your ninebarks are leafing out. (Follow label directions - repeated applications may be necessary.) Good garden sanitation such as raking up the fallen leaves in the fall and removing them from the garden will go a long way in keeping the fungal spores from splashing up and re-infecting your plants. Once the plant is fully infected, control is difficult and spraying lime-sulfur when temperatures are above 80F can completely defoliate them. For now, I'd simply rake up the fallen leaves and launch a fresh attack in the spring to protect your ninebarks from powdery mildew next season.
Best wishes with your plants!
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