Answer: If it is a white powdery growth on the foliage and stems, it is probably powdery mildew. This is caused by environmental conditions, typically cool nights with high humidity and poor air circulation. I would suggest rinsing the plants to clean them (use tepid water) and place them in a location with better air circulation. (If you can't move them, try using a very small fan at lowest speed, you want to move the air slightly and not create a big, windy draft.) Take care not to over or underwater them, and do not crowd them. I hope this helps.
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