It looks harmless enough – a light dusting like baby powder sprinkled on the leaves. But powdery mildew can attack new buds and shoots, stunt growth and distort plant development. If not controlled, the fast spreading fungus can cause billions of dollars of crop damage in California. For example, powdery mildew is the most significant disease affecting grapes in California, with all productive acreage treated to help minimize loss. Borne by the wind, its spores race through fields and can easily damage a season's crop, resulting in losses of 30 percent or more.

Growers combat powdery mildew with sulfur, fungicides, and other deterrents, but treatment is costly, and timing is difficult. But a much more precise strategy...