This plant looks healthy. (Photo: Flickr/ezioman)
Q: The leaves on my zucchini plants have developed a white powdery substance on them. The stem of the leaf is getting the same powder and then rots. The zucchinis themselves seem to be OK. Do I need to add nutrients, or is it OK to cut off the affected leaves?
A: It sounds like you are describing powdery mildew. This is fairly common on zucchini plants. If it is causing the leaves to die, then you can spray a fungicide on the leaves to mitigate the problem. Copper- or sulfur-based fungicides are good organic options. There are stronger chemical fungicides, but they are more toxic.
As long as the plant continues to produce, cutting off the affected leaves is one way to deal with the problem. By cutting them off you reduce the chance that the new leaves will be affected. You will need to leave at least a few leaves on the plant, or it won't be able to support new squash.
-- Answer from Rick S., home improvement specialist on JustAnswer.
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