Edit: Copper soap works, citric acid does not (see Post G23-059).
It’s déjà mildew all over again. If you are looking for a way to kill powdery mildew cheaply, read Post G19 and Post G20. But if you have a powdery mildew problem, you might be well served to read this entire post.
Arguably the biggest expense in treating powdery mildew is the DeLorean. This, because once you spot powdery mildew on your plants, the only truly effective treatment involves a time machine.
By that, I mean that I should have been spraying something to prevent this, weeks ago. But I’m just not the sort of person to spray stuff on perfectly health-looking plants.
And so, I’m setting up a test of two sprays for preventing further spread of powdery mildew. In this post, I’m just showing the “before” pictures and setting up the background. It’ll be a couple of weeks before I have any results.
Powdery mildew …
- Is not a single organism, but is an umbrella term for a wide range of fungi that produce the same symptoms — the same look — when they attack plants.
- Attacks a wide range of plants, but cucurbits (squash, pumpkin, cucumber, … ) seem to suffer the worst. See this reference for the full list.
- Starts as little white dots, then eventually spreads to cover the entire leaf surface (as above).
- Arrives at different times of the year. In 2020 this was all over my garden by mid-July. In 2022, by contrast, I didn’t have enough powdery mildew to allow me to test methods for killing it.
- Is far harder to kill, with an eradicant spray, than it is to prevent, with a protectant spray (Post G15).
- Will come back, even if you kill it, so that if you find something effective, you have to keep spraying it.
- Is the subject of many absolutely useless home remedies.
- Damages, but typically does not kill, the host plant. That damage reduces yields.
The picture above is from years ago, not from the current test.
Can be kept in check, but …
If you have a powdery mildew problem that you are going to try to solve, the first thing you must get clear is the difference between protectants and eradicants (Post G15). Protectants will (ideally) stop it from taking hold, or stop it from spreading. Typically, these work by somehow preventing the mold spores from sprouting. Eradicants will (ideally) kill it, once you’ve already got an infection going. These will kill the mold itself, once it has started growing.
Most of the stuff you see either touted as home remedies, or sold commercially, is a protectant, and will do squat to kill an already existing infection. Hence my additional expense for a time machine, so that I could go back and spray a protectant when I should have.
In 2020, I tested a wide range of supposed powdery mildew cures. Almost all of them did absolutely nothing. This may be because they never work, or it may be because they don’t work on the particular fungus that is prevalent in my garden. FYI, the solutions that I tested, that did nothing to kill an existing infection, were: Sodium bicarbonate (baking soda),Potassium bicarbonate, hydrogen peroxide, milk, and Neem oil. Some of those might work as protectant sprays, but none of them worked as eradicants. By contrast, I got a small but noticeable reduction in existing mildew by spraying my plants with mouthwash.
I eventually mixed up something that would, in fact, kill my powdery mildew (Post G19, Post G20). Or at least knock it back pretty hard. But in the end, a) I had to keep spraying that, and b) all I was doing was salvaging badly damaged leaves. Once mildew has damaged a leaf, it’s not going to repair itself even if you can kill the mildew. As above. Those holes and de-pigmented areas in the picture above are mildew damage, and that’s permanent. The plant will not repair that.
And you have to keep spraying. You can knock the mildew back pretty well with a single spray, but it will return. And you have to spray all the leaves, not just those that are obviously affected by mildew. That gets to be a real chore, particularly for pumpkins, due to the total area those vines can cover.
At the end of the day, my conclusion is that if you can prevent it cheaply and easily, that’s a lot more effective than trying to cure an infection after-the-fact. Heck, I’ll settle for slowing it down a little.
This year’s head-to-head comparison.
Last year, I set up a fairly good head-to-head test of two ways to prevent the spread of powdery mildew in the garden (Post G22-040). Then Mother Nature intervened, by giving me a rare summer with no powdery mildew, to speak of, in the garden bed I was using for the test. Leading to my observation that the Garden of Eden was a lousy place for testing pesticides (Post G22-044).
So that was a wasted effort. But the research that pointed me to those two particular cures is still valid. This year, I’m picking up right where I left off last year.
This post is just the “before” pictures. In each case, I’ll be spraying a Bonide Captain Jack’s copper-based fungicide (mixed two tablespoons per gallon), and, separately, spraying a dilute solution of citric acid (mixed as one-eighth tablespoon per gallon). In each case, copper will be sprayed on the right side of the garden bed, citric acid on the left. And, in the end, I’ll be looking for a difference in the overall level of mildew that is large enough to be obvious to the eye.
I’m not expecting either of these to be an eradicant. In fact, I’m sure that the citric acid is not (Post G22-060). I’m just looking for some evidence that one spray slows down the spread noticeably more than the other.
Here’s how it looks at the start of the experiment.
Et tu, sunflowers?
While I am testing this on cucurbits only (winter squash and pumpkins), I note that powdery mildew can attack a truly vast array of plants. Even if, by some miracle, this should work, I still have no guarantee that the same spray would work on other plants. I have a suspicion that different plants attract different types of mildew-causing fungi.
For a full list, consult The Plant Lady.
It’s not your fault!
Finally, do not succumb to mildew-shaming. If you look for advice on the internet, you’ll read a lot of mumbo-jumbo about mildew being a consequence of poor gardening practices. Somehow, you weren’t watering carefully enough, or you didn’t properly prune your plants to create air flow, and whatnot.
In my experience, all of that is absolutely 100% un-tested bullshit. I’ve got mildew on my pumpkins. They are out in the open air, and the only thing that waters them is the rain. I have mildew on my butternut squash, on an un-crowded trellis, watered by drip irrigation.
Powdery mildew is a fact of life. Choose varieties that are resistant, or spend a lot of time doing preventative spraying. But don’t blame your gardening habits when it occurs.
Again, as The Plant Lady will tell you, it’s everywhere.
