Answer: From the description, I suspect your plants developed a fungal disease called Powdery Mildew. This disease 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. Although it's important to rotate your crops to avoid the same pest and disease problems year after year, powdery mildew spores do not overwinter in the soil. Good luck!
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