Answer: It sounds as though your plants might be developing 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. Unlike most other fungal diseases, powdery mildew can be held in check by strong sprays of water that simulate a downpour.
Another effective treatment is regular sprays with compost tea solution. To make the spray, mix one part mature compost that contains some manure with five parts water. Let the mixture sit in the shade for two weeks, then filter through cheesecloth and spray on your plants. You can dilute it by half if you like. Repeat every two weeks.
If you can catch it in the early stages you can pick off the affected leaves to keep the disease from spreading to other plants.
I hope you have some success with these methods. Good luck.
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