powdery mildew - causes/control

coengJuly 23, 2012

So I've never experience powdery mildew on my cukes, zukes, and squash until this year. I don't even understand HOW I got it considering we've had a VERY LONG dry spell here in NJ this summer. Explanations?

Yesterday I decided to cut off the really badly affected leaves. I mean a lot of them. Did I effectively kill those crops or will they recover? I figured I had nothing to lose at this point and I wasn't about to spray anything (milk solutions, store products, etc) on the leaves this late into the season.

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I think PM actually thrives in hot dry weather. The "mildew" part of the name is misleading.

Why wouldn't you want to spray your infected plants?

    Bookmark   July 23, 2012 at 5:57PM
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I use Serenade and it works very, very well if you double the recommended dosage.

    Bookmark   July 23, 2012 at 7:18PM
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I agree that the most effective spray in the world won't cure an advanced case of PM. It is one of the few diseases that does not need a damp leaf surface to grow, because it sinks its "roots" into the outer cells on the leaf and takes moisture there.

We have moderate PM pressure on cucurbits, so I use resistant varieties and run my crops kind of late. Young plants are more resistant than old ones, so I succession plant summer squash, four plants at a time. I plant winter squash after garlic, peas and other spring crops. It's just staring to bloom now. These scheduling tweaks help a lot with PM in my garden.

Photo from Max Planck Institute. Not the cucurbit strain, but close enough.

    Bookmark   July 24, 2012 at 8:04AM
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It seems like cutting the affected leaves off had minimal negative impact. A week later I am starting to see new leaves and a couple of nice straight english cukes growing. The bush and other varieties are still roundish, yellowish, and curling, but the english cukes are much improved. I have also been watering more deeply and it helps that we've had rain here in NJ lately. I did notice a few more leaves had mildew again yesterday so I cut those off.

    Bookmark   July 31, 2012 at 11:12AM
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This was my first year gardening, and we got hit with a nasty bout of powdery mildew. I kept cutting back the affected leaves, and eventually I had to begin generously cutting back as it still spread to about half a dozen leaves per squash plant a day despite cutting back. I think cutting back would work for a mild infection, but once I realized I had barely any leaves left, I knew we needed some fungicide. It has definitely helped. The plants have bounced back and are producing well, but I am still having to reapply almost weekly. I will definitely space these squash out better next year. They're about 18 inches apart from one another, and the fungus spreads so easily when they're this crowded. I wouldn't worry about cutting the leaves back either. They should grow back fine.

For those who have been there, what are some things that you do differently now to prevent powdery mildew?

    Bookmark   July 31, 2012 at 12:25PM
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I'm seeing a lot of difference in the resistance of curcubits to the dreadful PM hitting them this year - worst I've ever seen. Alas, charentais melons don't seem to have any resistance at all. They're the worst hit and doubtless infecting the rest. Right next to them, the row of Early Gala Galia melon is barely touched by it.

    Bookmark   July 31, 2012 at 2:33PM
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Had to give up on the cucurbits when the dense fog became an every night phenomena. I handpicked leaves when infected and used diluted milk spray, but I was losing entire 8 foot vines in the space of a few days in July. The most favorable conditions for powdery mildew are supposedly nighttime temperatures of 60F and relative humidity of 90 to 99%, with 80F temps and 40 to 80% relative humidity during the days, which we were getting 4-6 days/week. Cherry tomatoes are barely hanging in there, but I lost 10 Brandywine tomatoes, 10 acorn squash, 5 butternuts, 10 cukes and 20+ charentais and petit gris melons. So I feel your pain, Coeng.

Yank plants, clear litter, add compost, turn soil, let sit, add compost tea, replant and don't look back is my new motto, I've got 100 melon seedlings at the 2 leaf stage right now that I'll try in 4-5 locations, but who knows what the weather will serve up in August.

RIP, little buddy. You were great while you lasted.

    Bookmark   July 31, 2012 at 3:23PM
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