Answer: Powdery mildew is a fungal disease that attacks plants when environmental conditions are right (warm days, cool nights, some humidity) and air circulation is poor. You can help avoid powdery mildew by giving plants the required exposure to sunshine, and plenty of elbow room so there's good air circulation all around them. Once plants are affected, there's really no control, other than picking off the infected leaves. Powdery mildew spores do not overwinter in the soil, so next year's crops shouldn't be affected.
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