Post #G20: Powdery mildew, what I have learned.

Posted on August 21, 2020

Another gardening post.  This one is about eradicating powdery mildew on cucurbits.  See my earlier posts to be clear about the difference between protectants and eradicants for powdery mildew.  Once your plants are already infected, you’re looking for something that will kill an existing infection, i.e., an eradicant.

This is a followup to Post #G19, where I found a spray that appeared work as an eradicant on my pumpkins.  You can find the recipe there.  I’m now spraying that around my garden.  This post is my summary of what I think I’ve learned by this process.

But first, pictures.

Three-component spray:  Potassium bicarb, potassium soap, horticultural oil

Pie pumpkin leaf with powdery mildew, 8/10/2020, pre-treatment.

One day post-treatment, 8/11/2020

Four days post-treatment, 8/14/2020

11 days post-treatment, 8-21-2020

What do I think I have learned?

1:  Yeah, this definitely knocks the mildew back fairly well.  But …

As you can see, 11 days after spraying, the mildew load still appears much lower than it was before I sprayed the leaf.  And the leaf is still alive.  So that’s a success.


But 1:  But I have to keep re-spraying those leaves to keep the mildew off. If you look closely, you can see that I now have a bit of mildew where I had none before.  So this isn’t a one-and-done, as with some plant diseases.  I still have mildew spores all over my garden.  This spray doesn’t provide immunity.  Instead, this is now a weekly chore I need to do, to allow my pumpkins to put on and finishing ripening their fruit.

But 2:  That leaf is still pretty badly beaten up.  Sure, I got the mildew level down.  And I guess that leaf is making a net contribution to the overall health of the plant.  But, with pumpkins, at least, it’s not like that’s going to return to a nice green picture of health.  It will be, at best, a leaf that was crippled by mildew, but was rescued.   And bears the scars of that infection.

But 3:  I am now spraying everything, not just the obviously infected leaves, and that takes a lot of spray and a lot of time.  I note that the powdery mildew attacks leaves in order from oldest to newest.  I don’t think that has to do with the leaf senescence.  I’m now guessing that it just take that long between the time the powdery mildew spore infects the leaf, to the point where I notice the infection.

In other words, to a close approximation, given how much mildew there is my garden, I should probably assume that every leaf on the susceptible plants is infected.  It’s just that I can’t see it yet, on the newer ones.

By spraying just the obviously infected leaves, I was always playing catch-up.  The only sensible thing to do is to spray everything.  And that is a chore, given the extent of my pumpkins and other cucurbits.

I just did a round of spraying, but for the next round, I’m going to work out how to do this with a hose-end sprayer.  Making up gallons of spray, for a one-gallon sprayer, and then tediously coating the leaves — that’s a dead loser when I have this many vines to treat.

2:  There is only one rational way to go about this.

A second big realization is that methods that seemingly-reputable gardeners swear by did not work at all, for me.  Just cruise through YouTube looking for powdery mildew cures, and you’ll see everything.  And every possible cure has somebody standing behind it,saying that it works.

Part of the problem is preventative versus eradicative treatments, discussed in earlier posts.  Most of what you see will help keep powdery mildew from spreading.  But what I needed was something to eradicated it on already badly-infected plants.

But I think it goes deeper than that.  Likely, there are many strains of powdery mildew.  Certainly, different plants are affected differently.  And both your  weather and your individual micro-climate may differ from mine.

To the point where I think it’s fair to say that what works for you may not work for me.  And vice-versa.  Probably there is no one universal cure for this.

Given that, there is only one rational way to go about dealing with this.  Make up small batches of several suggested sprays, and try them on your particular problem, all at the same time.  Do NOT do what I did, which was to take what seemed to be the best available advice, spray the entire garden, find out that it didn’t work, and then repeat with a different formula.

Part of the reason my pumpkins are such a mess with mildew is due to the way I approached it.  Every time I tried a spray, that was another couple of hours spent spraying and another few days waiting to see whether or not it worked.  And you wake up two weeks later and realize you have a far worse problem than when you started.  Even though you’ve been working on it all the time.

The only reasonable thing to do is to get (say) five quart sprayers, make up small batches of five supposed mildew cures, and try them all, right now..  Then take out your camera, find five similar-looking patches of mildew, and test and document those five sprays, all at the same time.  Then, assuming one works, spray your garden with it.

3:  Prevention is vastly better than cure.

Finally, it goes without saying that I should have been spraying preventatively.  Not only are there more things that work as preventatives than as cures, it’s not clear that the cure is worth the effort.  See above.  If you let a plant get badly infected, those leaves are going to be permanently damaged.  You can, with effort, kill the mildew.  You cannot repair the damage.

My rule now is that if more than half the leaf has mildew, it’s probably not worth saving.  I’ve gotten much more aggressive at cutting out the infected leaves, then spraying what’s left.


  1.  Prevention looks like a vastly better strategy than cure, in this case.  But that’s a lot of work.  To cover a large area, I’m now looking at using a hose-end sprayer.
  2. What works for me (or for some YouTuber) may not work for you, and vice-versa. 
  3. The only sensible way to address widespread infection in your garden is to mix up small amounts of a variety of sprays, test them all at once, and choose the best from what you tested.  Don’t assume that just because someone assures you that a mix works, that it will work in your garden.  And don’t test them one-after-another, because mildew may have consumed your garden by the time you find one that works for you.
  4. Aggressively prune infected leaves if you are dealing with fast-growing plants like cucurbits.  For one thing, each infected leaf is a source of spores.  But mostly, once a leaf is badly infected, it’s badly damaged whether you manage to kill the mildew or not.  Sure, you might be able to save it, but so what.   And a final reason to remove infected leaves is:
  5. There is no home-brew cure.  You can knock the mildew level down considerably, but you can’t make your plants immune.  So you have to keep re-spraying, even once you’ve reduced the mildew load.  You aren’t going to be in the mildew-eradication business.  You’re really in the mildew-maintenance business.  At least until the cooler weather hits.

The upshot is that if you have a plant or two, you can adopt some fairly labor-intensive methods for saving those plants from powdery mildew.  But at some point, with hundreds of square feet and up, the amount of spraying required becomes fairly punitive.  Right now, I’m just trying to make it through October so that the large pumpkins will ripen.  I’m at the point where a one-gallon sprayer just isn’t cutting it, and I’m moving to a hose-end sprayer.

I suppose I could switch to something more toxic.  But I’m more comfortable with materials that I don’t have to worry about a) spilling on my skin, and b)ending up eating via the pumkins.  So I’m going to stick with what I’ve been doing.