The huge zucchini leaf looked as if it had been dusted with flour. The man holding it was looking at me expectantly, waiting for my diagnosis. I was volunteering at our county’s Master Gardener helpdesk, providing free gardening advice to the general public. Sometimes we get stumped, but this time I immediately knew exactly what the problem was.

“Your zucchini plants have powdery mildew,” I told the man. “It’s pretty common around here, especially this late in the season.”

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