Post #G16: Another fail on powdery mildew

Posted on August 7, 2020

Edit:  See Post #G19 for something that actually worked.  That said, once your pumpkin or squash leaves are as heavily infested as these are, a) even if you kill the mildew, those leaves are going to remain badly damaged, and b) you have to keep re-spraying, otherwise the mildew re-infects them.  So, yeah, I found a fairly strong spray that appears to kill the mildew without killing the leaf.  See G19 for the recipe.  But I’m still pondering just how much that gained me, in the context of pumpkins.  Above all, I’m now spraying everything — healthy leaves too — so as to prevent powdery mildew, instead of just spraying the obviously infected leaves.

In short, I’ve learned a lot, none of it was enjoyable, and the moral of the story is that an ounce of prevention is worth a pound of cure.  Right now I’m figuring out something that will work with a hose-end sprayer, because once you get to the point of having hundreds of square feet of pumpkin vines, going around with a one-gallon pump sprayer on a weekly basis is just tedious.

I’ve also come to the conclusion that the ONLY way to go about dealing with powdery mildew is to get a bunch of one-quart spray bottles, make up small batches off the recipes suggested by various sources, and try them in small amounts, all at the same time, to see if any of them solves your problem. 

Don’t do what I did, which was to pick one plausible recipe and spray the entire garden.  Then see that fail.  Then pick another one and spray the entire garden.  And repeat.  To my credit, I only did that three times before I woke up to how stupid that was.

Instead, make up a half-dozen small batches of a half-dozen plausible anti-mildew sprays, spray a half-dozen leaves in your garden, see what works, and take it from there.  That’s a lot less work and gets you an answer a lot faster.

There are probably different strains of mildew, there are different plants that get it, we all have different weather and different micro-climates on our plots, and when you get right down to it, what works for somebody else may or may not work for you.  And that’s why you’ll see people swearing by cures that did not work at all for me.  And that may be why what worked for me — see G19 — may or may not work for you.

The original post follows:


August 4, pre-treatment






August 5, one day post treatment






August 6, 2 days post treatment.




Based on the early returns, I’m calling this one in favor of powdery mildew.  I can now add Neem oil (70% hydrophobic extract, Bonide Rose Rx) to my list of things that haven’t stopped powdery mildew on my pumpkins.

I mixed and sprayed this meticulously according to the manufacturer’s directions.  If you look closely, you can see that not only did this last treatment leave the existing patches of mildew untouched, it didn’t even keep it from spreading.

Maybe this works for other plants, or under other conditions.  But it pretty clearly failed for my sugar pie pumpkins this year.

That’s now a total of four failed attempts to find a powdery mildew eradicant, including:

  • Sodium bicarbonate (baking soda) mix
  • Potassium bicarbonate mix
  • Hydrogen peroxide solution (three days running)
  • Neem oil solution

No point repeating the recipes (given in prior posts) because none of them seems to have worked.  Not where I need it, which is on my pumpkins.

On the plus side, aggressive pruning seems to be the complete solution for powdery mildew on summer squash.  The idea of pruning summer squash was new to me this year, but I’ve now run across several seemingly competent YouTube gardeners who do that.  I can now attest that summer squash will grow new leaves as fast as you can reasonably prune them.  In particular, under current conditions, they’ll grow new leaves much faster than powdery mildew can destroy the old ones. 

The upshot is that my summer squash look fine.  Not because they are completely resistant to powdery mildew, but because all the infected leaves are in the trash,  not on the plant.  I don’t think I’m even going to bother to spray the summer squash any more, because vigorous pruning seems to be an adequate solution.

I don’t think pruning is a solution for pumpkin vines, as they won’t regrow leaves.  Or, rather, all the new leaves are at the trailing ends of the vines.

I stumbled across this fairly comprehensive list of potential treatments.  Most appear to be protectants, not eradicants (see Post #G15).  I will continue to experiment.

Addendum:  Up to now, I’ve been testing these substances serially, one after another, spraying all the relevant plants in my garden every time.  Sometimes, waiting to see if there’s an effect. 

And as a result, a) my powdery mildew problem just keeps getting worse, b) time is slipping away, and c) I’ve wasted hours of time by, in effect, spraying my entire garden with a placebo.

If all I’m going to do is stumble through a list of possible cures, none of which works on my pumpkins, it should be adequate to spray just a few leaves each time.  Failure to cure this on a few leaves will tell me just as much as failure to cure it in my garden generally.

Which brings me to the obvious face-palm moment.  The smart way to do this is to test all the remaining candidates at the same time, making up micro-batches and spraying just a few leaves with each substance.  Test them in parallel, and test them as soon as possible.

I’m not seeing any commercial product I can get that advertises itself as an eradicant.  Other than neem oil, above, which did not work.  Daconyl is just too toxic to use.  Serenade ™ is a protectant.  Triazole fungicides and sulfur are recommended only for the earliest stage of infection.  I assume the same holds for copper.  Various systemic fungicides are unappealing because I’m going to eat the produce.

I think the list of potential eradicants that I can get is a fairly short one.  Might as well try them all.