I was wandering around my garden this morning noticing how lush and green everything looks.
But something’s missing …
Aha. It must be time for powdery mildew.
What’s the schedule?
In 2020, powdery mildew arrived mid-July. As of the last week of July, I was already hard at work, spraying for widespread powdery mildew problems (Post G13). And by the end of July 2020 I had extensive powdery mildew on all my cucurbits (squash, pumpkin, cucumber), and traces of it on other plants as well.
I then spent much of early August trying various sprays to get rid of it. Everything I think I learned about powdery mildew is summed up in Post G20, from the end of August 2020.
In 2021, it had just started to pop up on the squash leaves as of July 29. (See “If only mildew were a crop”, Post G21-044.) That year, the only thing I did was trim out older leaves on the summer squash, once they became infected with powdery mildew. I didn’t bother to spray.
So, it wasn’t my imagination. Here in Virginia Zone 7, we’re just about to enter powdery mildew season.
What’s the plan? Notice of intent to do science.
This year, I actually have the opportunity to start spraying before powdery mildew is visible on the plants, using so-called “protectant” approaches rather than “eradicant” approaches (Post G15). Among the lessons learned in 2020 is that once it’s entrenched, it’s really hard to get rid of it. And even if you do, the damage to the leaf is already done.
Everything you’ll read about some mild, harmless, organic-sounding spray — that’s a “protectant” spray that’s supposed to be used before you actually have powdery mildew. It’s to prevent it from getting a foothold. At least, that’s what they say. Or maybe to prevent it from spreading. That’s also what they say. But they only say that after it fails to prevent it from getting a foothold.
For sure, those lovely, mild sprays do zip to get rid of it, once you have it. That’s my experience. I spent weeks in 2020 screwing around with (e.g.) baking powder, or peroxide, or skim milk, and having zero impact on my infestation. Killing entrenched powdery mildew takes some sterner stuff. I did find one formulation that knocked it back pretty well (Post G20). But even then, it required frequent re-spraying to keep it from coming back.
But do those sprays actually work to prevent powdery mildew, if you start spraying before the mildew develops? Or are they wishful thinking, just another bit of gardening folklore that gets passed around the internet, with nobody bothering actually to test it.
So I’m torn. I’ve vowed to do less work in the garden this year. And I hate spraying stuff. But maybe this is one instance where a little effort focused on prevention might pay off. (Plus, with a functioning irrigation system, I don’t really have that much maintenance to do in the garden now.)
But the critical factor is that, for the first time in my life, I’ve actually considered doing something about powdery mildew before it showed up. That degree of foresight is rare enough that I should probably take advantage of that.
If I’m going to bother to spray some relatively-harmless (i.e., seemingly useless) “protectant” on my plants, I’m at least going to take the opportunity to test whether or not that works. Too much garden lore turns out to be just so much untested and unuseful folklore. No sense in adding to the nonsense.
The notion that (blank1) works to cure (blank2) arises naturally from the normal random variation in (blank2). Somebody has a bad year with (blank2). Next year, they try some (blank1), and there is less (blank2). Or at least, they perceive it to be less. On the basis of that, they declare that (blank1) worked. Separately, nobody brags about their failures.
And so, there’s a constant stream of sincere messages that (blank1) is a sure cure for (blank2). Which might, in some cases, actually be true. But which, in many cases, is straight up post hoc propter hoc logic error.
In my case, after my dreadful 2020 experience, if I’d sprayed protectants in 2021, I might easily have fooled myself into thinking that the reduced mildew load in 2021 was a result of my spraying. Rather than just normal year-to-year variation.
Back on task: Right now, in the open air, the only plants greatly susceptible to powdery mildew are winter squash and (one) Dickenson pumpkin. So they’ve just volunteered to be test subjects.
The sprays that I recall include baking soda (sodium bicarbonate) solution, hydrogen peroxide solution, and potassium bicarbonate solution. And, no kidding, dilute skim milk. Oh, and mouthwash. And potassium-based soap (Dr. Bronner’s). All of which had tons of personal testimonials. Some of which also had some evidence basis beyond that. In any case, all of those qualify as cheap and mostly harmless. Absent anything better, that’s what I’ll be using.
So … I think I’ll divide my butternut squash into sections, spray it with various chemicals (and with nothing at all), and see what develops.
I guess I should get all intense about how I’m going to quantify the impact. E.g., how can I estimate the fraction of the leave surface covered in mildew.
But in fact, if the impact of spraying versus not isn’t immediately obvious to the eye, I’m going to judge that it’s not really worth spraying. I’m hoping to see some big, obvious differences across the sprayed and un-sprayed areas. That’s because it’s a lot of work to spray and re-spray all the leaf surface of (say) a patch of squash. If the impact is a maybe-sorta-kinda reduction in mildew, I’m not going to bother.
So that’s the plan. Mix up N spray bottles of different protectant solutions. Subdivide with butternut squash plot into N+1 sub-sections, in some fashion. Take some photos, spray, and see what develops.
Arguably the biggest complication is that, as with blight on tomato leaves, powdery mildew attacks the oldest leaves first. (And so, separately, you have to wonder how much of it is disease, and how much is just old age.) So I have to be mindful of that, as I divide up the plot, or I’ll mistake the effects of leaf age with the effects of the protectant spray.
So the theory is clear. I just need to work out the details and commence spraying.
As you can probably guess from the writeup, I’m expecting all of these protectants to be ineffective folklore. One of the reasons for that, not addressed yet, is that there is a commercial product sold for this purpose (Serenade ™). The commercial product — presumably proven effective — is quite expensive. If 25 cents worth of baking powder worked, I don’t think there would be much of a market for Serenade ™.
That said, it’s worth giving these D-I-Y preventatives one objective test, before I resign myself to a garden full of powdery mildew.
