Answer: Based on your description it sounds like there is a fungal infection at work. These can occur when weather conditions are favorable to them, or if the plants are overly crowded or in a spot where air circulation is not good. Evening watering can also encourage their development. Oddly enough, this sounds like powdery mildew which tends to happen in dry hot weather. Avoid handling the plants when they are wet but do remove infected leaves and discard them in the trash. Sometimes an early infection of this can be rinsed off with a spray of water from the garden hose. You may also be able to control it with a sulfur based spray, be sure to read and follow the label instructions.
Q&A Library Searching Tips