Powdery mildew appears like white death on the leaves of my pumpkin plants and peonies at the end of every summer.
I’ve made at least one futile attempt to get rid of it with spray fungicide but it was way too late. This year, I cut back my peonies before it appeared.
Experts say there are several ways to prevent and/or slow the spread of these fungi, which can reduce yields and impact fruit.
The easiest thing to do is purchase plants that have resistance to powdery mildew, said Debbie Courson Smith, a University of Idaho advanced master gardener.
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That does sound easy.
I wondered if seed packets contain that information, and I just never noticed it. She checked out some of her packets, and they had only growing information. It turns out that some are resistant to powdery mildew, even though that information isn’t on the seed packets.
So where do we find this information?
The way that Debbie and other expert gardeners figure out whether plants that are resistant to powdery mildew — and a whole host of other diseases — is to consult lists compiled by researchers, including Cornell University’s Department of Plant Pathology.
“There are sites available online,” Debbie said. “Cornell has a very nice site with a grid that will show you which types of pumpkins are resistant to powdery mildew.”
There are many pumpkins listed as having powdery mildew resistance, including: Touch of Autumn, Prankster, One Too Many and Orange Rave. Slicing cucumbers with resistance include: Sweet Slice, Speedway and Marketmore.
Cornell’s site has tables of disease resistant varieties for everything from French beans to zucchini.
Powdery mildew thrives in shade, high humidity and moderate temperatures (60 to 80 degrees), according to Oregon State University Extension Service.
Researchers recommend that gardens be planted in full sun, with plants spread apart enough to allow for good air circulation (also, circulation is helped by keeping beds weeded). Slow-release fertilizer also helps.
Fungicides are most effective at the first sign of powdery mildew. So if you see a leaf with a spot, it might be worth spraying the rest of the plant to prevent the spread.
Removing infected leaves and/or whole plants (after you’re done harvesting) will also prevent the spread of powdery mildew. It’s not something you’re going to eradicate, just keep at bay.
“I went back through my garden journal and photos — I get it somewhere in my yard every year,” Debbie said.