Today I’m starting a test of two products for preventing and (possibly) curing powdery mildew. See the just-prior posts for background.
On the one hand, we have Bonide (Captain Jack’s) copper fungicide, 16 ounce bottle of concentrate. That’s mixed up at the rate of one ounce per gallon, and sprayed (as best I can) at a rate of about one gallon per 1000 square feet. (But see notes on accuracy, below).
Source: Is that his kitchen? Remind me never to eat at this guy’s house.
On the other hand, we have dilute citric acid solution. That’s mixed up at the rate of 0.52 gram citric acid per gallon of water, and sprayed (as best I can) at rate of about 2.5 gallons per 1000 square feet. Honestly, I’m probably just going to spray the heck out of that, given how innocuous the solution should be.
On yet a third hand, I’m leaving part of my squash patch unsprayed. That will serve as the control. Here’s the setup, in my squash patch.
As present, powdery mildew is just beginning to emerge on my winter squash, presenting itself as widely scattered little white patches. This is an ideal time to test whether these merely prevent the spread of it, or actually kill it once it’s already gotten started. Here’s the view in the three sections of the patch. As you can see, they are pretty much in identical states right now, with respect to powdery mildew.
As you can see, powdery mildew is just emerging. That’s the white spots on the picture below.
Methods
The only real challenge here is accuracy. Accuracy in mixing up the solutions, accuracy in spraying them per square foot of squash plants.
Spraying: Near as I can tell, the entire winter squash patch is about 100 square feet in area. Call each area to be sprayed 50 square feet. That means, with each spraying, I need to use:
- Copper spray: 1/20th of a gallon, or about 6.4 fluid ounces per treatment, or less than a cup. It is, however, acceptable to spray at around twice that rate, or up to around 12 ounces per treatment.
- Citric acid: 1/20th of 2.5 gallons, or 16 ounces (one pint) per treatment.
No matter how you slice it, that’s a tiny amount of liquid to meter out of a typical garden sprayer. I’m not quite sure how all that’s going to work out.
Measuring and mixing. I’m pretty sure that both sprays should keep just fine, once mixed. The copper spray is just a metallic soap, the citric acid spray actually comes as a dilute water solution when sold commercially. So I can mix it up by the gallon, and keep it on hand.
The copper spray is no problem. Two tablespoons (one fluid ounce) per gallon.
The citric acid spray, by contrast, is a challenge. I need a half-gram per gallon. In theory, my kitchen scale will measure out a gram, but in practice I’d bet that even if fully accurate, anything from half-a-gram to a gram-and-a-half will register as one gram.
I decided to do it by volume instead. One convenient home-canning calculator shows that one gram of citric acid for home canning is a quarter-teaspoon. (And 40 grams at exactly 10 teaspoons). I figured, all things considered, mixing this a a rate of a one-eighth level teaspoon citric acid per gallon of water was about as accurately as I could do it.
For as cheap as the citric acid spray is, I’m just going to mix up a gallon at a time and maybe spray a few more plants (e.g., pumpkins) on the off chance that it works.
I’ll do my best to spray the entire leaf surface, including the underside of the leaves. I’m not going to respray after a rain, because neither product mentions that. I’ll do two sprays, five days apart. Then one spray every seven days.
If I were exceptionally fussy about experimental design, I’d spray the “no treatment” area with plain water. There’s some chance that the mere act of spraying something will reduce the mildew load. But a) if that worked, I’m sure that would be a standard recommendation, and b) that increases the total amount of work that needs to be done with each spraying. I’m punting on that.
Conclusion.
Right now, those squash are in direct sunlight, so I’ll delay the first spray to this evening. I probably ought to test each spray on a few leaves first, before spraying (and possibly killing) my entire squash patch. So I’ll do a little test spray this evening, and assuming all goes well, I’ll start in earnest tomorrow evening.
If either one of these works, that will be a definite improvement in my gardening lifestyle.
But I’m rooting for the citric acid. It’s less harmful than lemonade. If that works, I can definitely see myself tossing a pinch of that in a hose-end sprayer and having at the garden every week or so.
I just can’t believe that getting rid of powdery mildew could possibly be that easy.
