Powdery Mildew

How do I have powdery mildew already? In California?! This photo is from my squash plant. Both the zucchini and winter squash have it on a couple of leaves.

I thought powdery mildew was supposed to happen in humid environments, and the Bay Area is anything but humid… hmmm… I have some things to learn about powdery mildew…

Powdery mildew is a disease caused by several different types of fungi (437 different species within the order Erysiphales). The spores or conidia of the fungi are carried by the wind to a host plant. According to the Penn Extension, ideal conditions for powdery mildew are high humidity at night and low humidity during the day with daytime temps of 70-80 degrees. Additionally, the University of California Master Gardener’s website says that placing plants in full sun can help prevent or kill the fungus that is causing powdery mildew, since powdery mildew fungi don’t like really intense heat.

From the Ohio State Extension

Here in Palo Alto, we have low humidity during the day and 70-80 describes pretty much every summer day here. I occasionally water in the evening, after I get home from work and see that the garden looks a little dry, which may make for relatively humid local conditions during the night. The squash plants are in the most sunny spot we could find, but there are several trees in the area, so it is mostly shaded in the afternoon. Overall, not ideal for my squash plants, but pretty ideal for powdery mildew fungi.

Fortunately, it’s a very mild case at this point, so I’m not going to bother trying to treat it, but it’s a good lesson for me to water in the morning rather than at night. Ideally, I would have a drip system set up on a timer, but those are dreams for a garden a few years from now.

One other thing I learned about powdery mildew is that the fungi are obligate parasites, which means they need a living host to survive. Therefore, (apparently) it’s ok to throw powdery mildew-infested plants in the compost pile, since when the plant dies, so does the fungus. Huh. Is this really true?

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