To cut to the chase: Copper versus citric acid. That’s what I’m going to test this year, for control of powdery mildew on cucurbits.
This post shows how I got to this end point, from my original plan of a formal test of several “home remedy” approaches to controlling powdery mildew.
Using citric acid to control powdery mildew was news to me. There are, however, at least two commercially-available mildew-control products that are little more than a dilute solution of citric acid. Roughly one-twentieth the strength of typical commercial lemonade. So that’s surely worth testing.
Background
Powdery mildew is a common fungal disease that attacks a wide range of garden and other plants. It’s characterized by the production of a white powdery surface on the leaf. At the minimum, it can reduce growth and productivity of your plants. Worst case, it’ll kill them.
Here, in Northern Virginia Zone 7, you can pretty much count on it taking over parts of your garden, just about this time of year. It is particularly a nuisance for cucurbits (squash, pumpkin, melon, cucumber). But it’ll go after tomatoes and many other plants, given the right circumstances.
It isn’t caused by a single fungus, but by any one of a large family of fungi. Which means that your powdery mildew may differ from mine. And mine may well differ from year to year.
When I looked at controlling powdery mildew a couple of years ago (Post G20 and earlier), I immediately jumped to trying various “home remedies”. These were various substances (e.g., baking soda, milk) that were discussed in various places on the internet. I tried a bunch of them, and none of them did spit for the infestation of powdery mildew I had that year. I documented all of that in 2020. Maybe they work in some times and some places, but not for me, then. I finally got a little relief with a highly-concentrated mix of several of them.
I went the home-remedy route after a cursory glance at what was available for commercial (farm) use. It seemed like everything that farmers would use to control powdery mildew was just incredibly toxic to children and other living things (bees, fish, frogs, … ). Also, a lot of them were suspected carcinogens. My thinking was, if those were the commercial options, I’d go for home-made.
I think Daconil is what made me say “just don’t go there”. You can buy a bottle of that at your local Home Depot. But maybe you want to glance at its toxicity profile before you do (Wikipedia). The known extreme toxicity to bees, fish, amphibians, and aquatic invertebrates is absolutely no barrier to selling off-the-shelf to Joe Homeowner. This being America, the only real barrier to selling something is threat of an expensive lawsuit. Once upon a time, Daconil was sold in large quantities for use in lawn/turf care. That was stopped only after it became apparent that it probably caused cancer in children.
So, a pox on that entire class of antifulgal agents. If my only option is to use that crap, I’ll grow something else. The way I figure it, it’s not that just that one chemical is unsafe. It’s probably that they all are, and that’s the one they happened to catch.
But there’s a middle ground that I did not explore in 2020: Commercially-made anti-fungal agents for use in the home garden. Sure, you can go to Home Depot and buy yourself a bottle of liquid death, as above. But that’s not the full range of what’s out there.
The first order of business this year is to revisit that. I need to take a systematic look at everything that’s available off-the-shelf for the home gardener. If there’s a harmless, effective, cheap, and low-effort commercial spray that will prevent and kill powdery mildew, I see no reason to be messing around with home remedies.
And, sure enough, there’s not one, but several, that fit that description. Including one that I’d never heard of before — a weak citric acid solution. If that works, that would be a fraction of a cent per application, and safe enough to drink.
But the bottom line is that there appear to be several reasonably cheap, environmentally benign, and seemingly effective commercial products for controlling powdery mildew in the home garden.
Originally, I intended to test a few of the “home remedies” that are frequently mentioned on the internet. But with this new information, well, to heck with that. I’m already pretty sure those flunk, in my garden, based on my 2020 experience. Let’s test a couple of commercial products instead.
Finding likely powdery mildew treatments via systematic search on Amazon.
For this section, I’m going to do what I do with many consumer purchases, and systematically search Amazon. With a little care, this is the quickest way to get to a reasonably cost-effective, high-quality solution, if one exists.
My search strategy is to type various search terms (e.g., mildew plant spray) into Amazon, order the results by average customer rating, and see what shows up near the top. Then put the products into broad categories.
I exclude:
- chemicals only available for farm use
- products that don’t mention edibles plants (e.g., products for turf, roses, or shrubbery without mention of food plants).
- broad-spectrum products that do more than just kill fungi
At the end I will also rule out those with objectionable toxicity.
Walking down the resulting Amazon results, and focusing on the active ingredients, here’s the list, from highest-rated to lower-rated.
Amazon search results: Copper, sulfur, and …. citric acid?
1) Top rated: Bonide Copper Fungicide. The powder (via Amazon) is 7 percent copper sulfate, which has been incorporated into a copper soap (copper octanoate). It’s listed for vegetables. At four pounds for $16, the stuff is cheap enough to spread on your lawn, per the illustration.
When the EPA reviewed this in 1997 (Google search link), they seemed to find no significant toxicity issues, other than (e.g.) not ingesting it or getting it on your skin or in your eyes. It’s suitable for organic gardening, can be used up to the day of the harvest. There is a limit on use, however, to avoid poisoning your soil (earthworms) with excess copper — see below. Looking at those limits, it’s hard to believe I’d hit them.
It might be a bit difficult to use in powder form. But there’s a liquid concentrate form, which also appears to be on the shelf at my local Ace Hardware, roughly 4x as expensive per use, but likely easier to use in a sprayer.
Aside from the issue of how well this works, it ticks the boxes for cheap, nontoxic, and reasonably easy to use.
2) Next, another copper fungicide, Southern Ag Liquid Copper Fungicide. This uses a different copper compound (copper bound up in an amonium compound), has about twice as much metallic copper per ounce, and appears to be cheaper to use than the Bonide liquid.
It is advertised as easy to use in a hose-end sprayer. The label does not specifically mention powdery mildew, but there were several enthusiastic reviews that mentioned powdery mildew.
Ticks all the boxes, and cheaper to boot. The only question is how well it works.
3) Third in the list is a series of neem oil products. But these function not just as fungicides, but as broad-spectrum insecticides as well. I think I’ll put those aside for the time being.
4) Fourth comes a series of products designed for citrus groves, but they also get the boot for containing broad-spectrum insecticides as well as fungicides. Many of these appear to be copper fungicides mixed with broad-spectrum insecticides.
5) Fifth, we have a couple of standard bottle-o’-death industrial toxins: BioAdvanced 701270A Effective Fungicide with Disease Prevention Fungus Control for Lawns. The active ingredient is propaconizole. Possible carcinogen. Highly toxic to fish. Punt. Ortho MAX Garden Disease Control Concentrate,active ingredient chlorothalonil. Discussed above.
Nuh-uh.
6) Sixth, another copper product, Monterey Liqui-Cop. This one — and only this one — says that copper products are considered preventives, and not curatives, of fungal diseases. That’s good to know, and matches some additional research below. Same active ingredient as Southern Ag above.
7) Seventh, Bonide Sulfur. This is literally just a pound of very-finely-ground sulphur, with what might be some anti-caking ingredients and such. That’s about as basic as it gets, and apparently you can mix with water and soap and spray it. About $8 a pound. Looks like it would be pretty cheap, per application.
8) In eighth place, things start to get weird: Citric acid. Grower’s Ally Fungicide very specifically mentions powdery mildew, and touts its effectiveness against that. Interestingly, this is just an extremely weak citric acid solution. If I have done the math right, the final spray is just 0.013% citric acid. Again, doing the math, that’s roughly half a gram of citric acid per gallon of water. That’s … again assuming I did the math right — an eighth of a teaspoon of citric acid per gallon (?!).
By contrast, a gallon of commercial lemonade might easily contain 10 grams of citric acid. So this has one-twentieth the citric acid of lemonade? Homeopathic fungicide? Maybe I’ll just wave the bottle of citric acid over a gallon of water and see what the resulting solution will do.
I’m still not sure I believe that such a dilute solution of citric acid could possibly have an impact on powdery mildew. But, along comes, Earth’s Ally Disease Control Concentrate for Plants, for which the active ingredient is … citric acid. That mixes up at the rate of six tablespoons of 0.64% citric acid solution per gallon of spray, or … somewhere between one-eighth and one-quarter of a teaspoon per gallon. Thus validating the dosage.
9) In ninth place: Dr. Earth’s fungal control. This, and another one just like it, control fungi with a mix of various essential oils (rosemary, clove, mint). I’m not scoffing, as a lot of those oils have strong antibacterial and antifungal properties. Seems to get good reviews, but looks like it would be somewhat expensive to use for large areas of the garden.
Summary of Amazon results
The most promising approaches seem to be:
- Copper, delivered via various compounds, and various chemistries.
- Sulfur, delivered straight-up as ultra-finely-ground sulfur
- Citric acid, delivered as an almost unbelievably weak citric acid solution.
Each of these requires some commentary.
Copper?
Bordeaux mixture was the original way to control fungus on crops. It was a mixture of copper sulfate and quicklime. (Copper sulfate is most commonly sold in hardware stores as sewer line root killer, and is extremely toxic, as I recall.)
Bordeaux mixture fell out of favor because, among other things, heavy use of it poisons the soil with copper. That apparently is a negative for earthworms, and then for anything that eats earthworms.
I note that all of the modern copper products listed upper limits on application rate per 1000 square feet per year. Apparently the lesson of Bordeaux mix has been learned, and these are presumably set so that you don’t end up poisoning the soil with continual use.
The other interesting thing to note is that Bordeaux mix is strictly a preventative — it prevents spores from germinating. It won’t kill an existing infestation. Thus, I’d expect these modern copper products also to be preventative only. Only one of the products on Amazon made it clear that it was a preventative.
Sulfur?
Sulfur is an anti-fungal of long standing. It was used by the ancient Greeks. It is, in fact, what most plants use to defend themselves from fungi. And beyond gardening, sulfur was used as an anti-fungal in dermatology.
But sulfur appears to have some significant limitations. The most important of which is that, if you mess up, you can kill your plants. You cannot, for example, use this at the same time you are using any sort of horticultural oil. And, important for this purpose, it looks like it shouldn’t be used when temperatures are going to exceed 80F. Which, I think, all by itself, explains why there was only one sulfur product near the top of the Amazon listing. Because that’s exactly when you need it, in most places.
In addition, like copper, it’s a preventative — it keeps fungal spores from germinating. It won’t kill an existing infestation, but it might keep it from spreading.
Citric acid/reefer madness?
This is the one that floored me. I did a pretty thorough search for home remedies two years ago, and I’m pretty sure nobody mentioned weak citric acid solution. Even today, a quick Google search shows almost no discussion of it.
Citric acid is so cheap, and so commonly available (it is frequently used in canning), that had this been widely known at the time, I think I’d have come across it. I fact, I used citric acid just yesterday (Post G22-037). But not in the garden.
So this appears to be new.
I note that both of the citric acid products on Amazon were marketed to dope growers weed farms pot plantations the marijuana cultivation industry. So maybe this is something that pot heads ganjanistas dope fiends cannabis cultivators figured out, unique to their (typically) indoor grow-room cultivation methods.
Or, it may be bullshit. Ya never know til ya try it.
At any rate, the dope-centric focus for this anti-fungal would explain why this approach to controlling powdery mildew remained outside of the mainstream gardening blogs and university extension service advisories.
I do worry, though, that maybe the reason this hasn’t made it into mainstream gardening is that it’s only effective in an indoor growing environment. I’m pretty sure that most marijuana cultivation today is characterized by indoor grow rooms. Aside from the horrific energy use implications, there’s no rain in that environment, and there’s minimal ultraviolet radiation. Once applied, the citric acid will presumably remain in place, and it won’t be subject to photodegradation (being destroyed by the UV in sunlight).
How well it works in an outdoor environment may be less well tested. There is also a remote chance that this works well on strains of powdery mildew that attack cannabis in those closed environments, but not the strains in my garden.
The idea does appear to be relatively new. At least for home garden use. You can find some internet chatter. It is mentioned in at least one reference work on cannibis cultivation. But in terms of basic research on how well this works, I’m drawing a blank.
At any rate, if that works, it’s astoundingly cheap. Citric acid is routinely available at retail for $5 a pound. (As I recall, it’s a byproduct of the production of fructose syrup from corn, so it’s basically an industrial chemical. For sure, this wasn’t squeezed out of a citrus fruit somewhere). A pound would be enough to make up almost a thousand gallons of this dilute solution. That’s cheap enough that I could just run it through a lawn sprinkler instead of spraying by hand. Typical lawn sprinklers use 1000 gallons an hour, running full blast. Or hose down the plants with a hose-end sprayer.
I will once again underline how dilute this is. Commercial lemonade seems to come in around 10 grams citric acid per gallon, or about 20 times the concentration in this spray. A citric acid brine, for pickling, contains about 40 times as much citric acid as this anti-fungal spray would.
Finally, the sellers pretty clearly assert that citric acid will kill an existing infection. So, in addition to being cheap and non-toxic, uniquely among the options on the table, this one will actually kill powdery mildew, and not merely prevent its spread.
Conclusion and details of use.
Before I go wasting my time putting various “home remedies” to a formal test, I should try the two commercially-available products that seem to check all the boxes regarding cost, ease of use, and toxicity. That’s copper in some form, and citric acid.
I bought a bottle of Bonide Liquid Copper from my local Ace hardware. That was a little under $20 for 16 ounces. Within the range of application rates, I could choose an ounce per gallon, and a gallon per 1000 square feet. Directions for curubits say to apply twice at five-day intervals after emergence of mildew, and then at seven-day intervals thereafter.
Not to exceed 97 ounces per 1000 square feet per year. Which, given the recommended application rate, I don’t think I could achieve unless I applied this stuff with a lawn sprinkler.
Near as I can tell, the shelf life of this should be indefinite. It’s just a copper soap solution.
So, for my roughly 100 square feet of winter squash, if I spray at the recommended interval and recommended concentration, from now to first frost, that will be roughly one dozen applications, a cumulative use of 1.2 ounces of material in 1.2 gallons of spray, an annual dose of 12 ounces per 1000 square feet (or just over 10% of the annual allowable limit), and an annual cost of $1.50.
For citric acid, I’m using some that I bought for canning. Ball citric acid seems to work out to be about $10 a pound, full retail.
Based on this source, for Growers Ally, for curative use, spray every three to five days; for preventative use, spray every seven days. The recommended application rate is 2.5 gallons per 1000 square feet.
The actual citric acid content is 1.07% (which I believe is by weight), diluted 80:1 before use. So the weight of citric acid in one gallon of final solution is (0.017/80)*8.5 lbs/gallon =) 0.52 grams citric acid, per gallon (matching my rough calculation above).
Let me put this on the same spray schedule as the copper solution above. Then, for my 100 square feet of squash plants, for the entire season, I’ll use 3 gallons of spray, consuming approximately 1.5 grams of citric acid, for an annual cost of seven cents.
Looks like the only downside to either one of these is the small scale of operations in my garden. I’m either going to have to mix this up in really tiny batches, or I’m going to have to dedicate a couple of sprayers to these two mixes, and try to guess how much I’m spraying each time.
At any rate, let me state the obvious: At those annual costs, why on earth would anybody mess around with un-tested “home made” solutions?
So, based on this little bit of research, I’m going with an off-the-shelf copper product, and a home-mixed version of what appears to be an extremely simple commercial citric acid product. Next, I’ll set up the relevant squash bed and commence testing the effectiveness of these two products.
Got milk? An interesting footnote.
Here’s an odd fact that came out of the one reference work on cannibis cultivation that I found. (Integrated Pest Management
for Commercial Cannabis in BC, from the British Columbia Ministry of Environment and Climate Change Strategy. Just the idea of having a ministry of climate change strategy shows these folks are light-years ahead of the U.S.).
In their brief discussion of antifungals, they put lactic acid on the same footing as citric acid, emphasis mine:
Chemical: Registered pesticides for control of powdery mildew include the active ingredients: potassium bicarbonate, sulphur, garlic powder,citric acid/lactic acid, Reynoutria sachalinensis and hydrogen peroxide. They should be applied at the early stages of disease development
Apparently, in BC, somebody has registered an anti-fungal using lactic acid.
This brings me back to the apparently effective practice of using diluted milk as a preventative for powdery mildew. I tried that in 2020 without notable success. But now I wonder whether the mechanism of action is, in fact, the souring of the milk and resulting production of lactic acid, in situ, on the leaves.
Anyway, that’s just one more weird footnote, on what has been an admittedly weird search. If I can, in fact, kill and prevent powdery mildew in my garden, for 7 cents per year, by hosing it down occasionally with extremely dilute citric acid, that approach would surely dominate any other method.
Sounds too good to be true.
All that’s left to do is test it.
