Post #G21-044: Stealth mildew and a farewell to squash

Posted on July 29, 2021

Squash vine borer, and an observation.

Source:  U Wisconsin Vegetable Entomology.

As was the case last year, the squash vine borer season has been slowly tapering off.  My last sighting of a squash vine borer was a lone female about six days ago, or circa 7/23/2021.

I’m calling that the end of the season, unless I see another one.

This year the season ran just over five weeks, from 6/16/2021 to 7/23/2021, here in Zone 7 Northern Virginia.  That’s a bit shorter than last year’s seven-week season (7/5/2020 – 8/25/2020, Post #G-27).  The season was earlier this year owing to warmer spring temperatures (Post #G21-043).

Last year I managed a successful defense of all my squash plants.  This year, by contrast, I lost four summer squash plants to the SVB.  I sprayed the stems with spinosad on a regular basis, same as I did last year.  The mix of varieties was about the same as last year.

Four plants succumbed, all in the exact same way:  The borer entered the stem, near the base, where the stem was lying on the ground.  In each case, I couldn’t see or feel the borer entry hole, but in each case the stem broke off at more-or-less the exact same point when I pulled up the dying plant.

(In passing, I’ll say that I tried a neem oil soil drench on one infected plant, and it failed.  The theory is to poison the borer by having the plant take up azidarachtin (the pesticide in neem oil) through its roots.  I took a tablespoon of neem oil (pure neem oil, the kind that contains the pesticide azidarachtin, not the “hydrophobic extract” neem oil), combined with a tablespoon of Dr. Bronner’s soap, mixed it up in water, and poured it in a 12″ diameter circle around the base of one of the infected plants.  That plant lasted longer than its neighbor, FWIW.  And that might or might not have eventually killed the borer.  But the plant continued to wither, and I pulled it out.)

Here’s my observation, and a bit of a calculation.  I had eight summer squash in a nice, orderly, weeded and mulched bed.  I pruned them and sprayed them, staked them up and weeded them.  Prowled the patch regularly removing squash bug eggs.  Talked to them, occasionally, as the mood struck, although honestly the tomatoes are a much more colorful lot.

Separately, I had five others — extras — that I just plopped down in other areas and never gave a second thought.  They were the unwanted step-children of my excesses with the seed-starting supplies, passed over because they were puny, ugly, or otherwise deemed unworthy.  Those five were left to fend for themselves, in among weeds and other garden plants.

Each set of squash plants — eight in a bed, five not — was a mix of zucchini, yellow squash, and patty pan.

All four plants killed by the SVB were together in the well-tended bed.  By contrast, all the ugly ducklings survived.

First, what are the odds of that occurring by chance?  That’s (8/13 x 7/12 x 6/11 x 5/10) = ~10%.  So that’s improbable.  Only about one year in ten would I see that purely by chance.

I wonder if there’s a lesson there.  Was the SVB was systematically better able to find and deposit eggs on plants clustered in that nice, neat bed?

I’ve been told that insects find their host plants by smell.  Dispersing the plants, and interplanting them with other species should make them harder to find.  I’ve seen at least one gardener swear that interplanting squash and cucumbers keeps the SVB off.  (Where cucumber is the least desirable cucurbit target for the SVB).  I’ve seen gardeners swear that spraying your plants with aromatics such as peppermint oil keeps the SVB and other insects off.  And I’ve seen gardeners swear that interplanting within beds (rather than devoting a bed to a type of plant) keeps the insect population down.

I’ve always planted plants of a given type in one spot, for good reasons.  First, how can I rotate crops if every bed contains some of every crop?  Second, it simplifies maintenance and harvest.  Third, I imagine it aids pollination. Plus, real farmers do it that way.

But maybe that’s not such a smart thing to do.  At least not for some crops, in my tiny backyard garden.  Maybe it might make more sense to spread the squash plants out, interplant with other species, and so on. I’ll consider that next time.

If only mildew were a crop,

I’d never have a crop failure.

It’s the time of year for all my cucurbits to get powdery mildew.  Looking back at last year, I first mentioned it in a post around July 19, at which point it was already well-established in my garden.

In theory, I should have been spraying against powdery mildew well before it arrived.  And I did, to the exact same extent that I rotate my tires every 5,000 miles, change my home air filter every 30 days, check the pantry weekly for expired food, and in general, lather, rinse, and repeat.

Even absent preventive spraying, that problem hasn’t arisen this year.  I’ve seen mildew on a leaf or two of squash, which I immediately removed and took away from the garden.  But nothing more than that.

Naturally, I assumed my good mildew fortune was the result of diligence and, surely, my exceptional flair for squash-pruning.

But it was all just a fool’s paradise.  It was an arid, and apparently mildew-free fool’s paradise. Rain has been non-existent here for the better part of a month  (Post #G21-043).

Then we had a little under a half-inch of rain two days ago, and another half-inch last night.  Today, powdery mildew has bloomed all over my squash plants.  Presumably the spores were already there, just waiting for the right conditions.

Hence, stealth mildew.  I swear this leaf was an unmarked green just yesterday:

Crunch time

That certainly sounds better than squash time.  But what I mean is, now that reality has displaced my fool’s paradise, I have to decide what to do about my rapidly-mildewing summer squash.

Last year, I chased after that powdery mildew with a vengeance.  I learned a lot about powdery mildew and powdery mildew treatments, all of which I summarized in in Post #G20 I even managed to whip up something that did, indeed, kill the powdery mildew but not my plants.  And all of that was the opposite of fun.

This year, I had planned to euthanize all my summer squash and cucumbers on or about Bastille Day.  Based on last year’s timings, that’s the time when everything hit:  squash vine borer, cucumber beetle, and powdery mildew.  I figured I’d enjoy a nice, pest-free spring and just hang it up when the pests came calling.  Pull the problem plants and re-plant with some mid-summer crops.

My wife said I’d never do it.  I’m just that stubborn, or perhaps greedy, to tear up plants while they were still bearing.  She figured I’d fight to the last cuke, regardless.

We never did get to find out.  That plan blew up with the early and unexpected arrival of cucumber beetles.  I went into the details of why that happened in Post #G21-027. In theory, we have distinct cucumber beetle seasons around here.  In practice, they come early and stay late.

As it stands, I’m just now seeing flowers on my second planting of cucumbers, after bacterial wilt (spread by cucumber beetles) killed off the first planting.  I chose the second planting (Little Leaf and Cross Country) because they were advertised as resistant to bacterial wilt.  I’ll be leaving those in to see if they can give me some yield yet.

But the squash have already done yeoman’s duty.  I’ve rung the changes on squash-related dishes this year.  Pickled, fried, casserole, lasagna, salads, several crock-pot recipes, and tomorrow, I’m going to try pie.  (Yep, like pumpkin pie, but with summer squash puree.  Who knew?)

And I don’t even know how much I still have sitting in the fridge.

This is a high-maintenance crop.  Even though SVB guard duties appear over for the year, I would have to:

  • Rotate the protections on male blossoms (Post #G21-042).
  • Hand-pollinate the female blossoms.
  • Continue to remove squash-bug eggs and such.
  • Crush any cucumber beetles in our about the blossoms.
  • And now, spray on a regular basis for powdery mildew.

I think I’ve had enough.  It’s time to have a graceful ending.  I’ll use up my last refrigerated male blossoms to fertilize any females in the next couple of days.  I’ll remove the guards and let the birds have the rest of the male blossoms.  I’m not going to do a thing about powdery mildew, squash bugs, or cucumber beetles.  I’ll let the plants produce whatever squash they can.  In general, get their affairs in order.  And in a week or so, as the mildew gets bad enough, I’ll pull them and replant with something that likes that heat and requires less maintenance.

I’ll break the news to them tomorrow morning.

I might even re-plant some summer squash.  In theory, we still have more than 90 days in our expected growing season here, before first frost.  I should probably have used up my squash backlog by then.