Tuesday, August 6, 2013

Preventing Powdery Mildew on Cucumbers and Squash

It’s been a while since my last post (this summer has gone by in the blink of an eye!) so I thought I would address my ongoing battle with powdery mildew. Powdery mildew and I have a history. Saying that I don’t like it is too passive—it’s more of a profound feeling of hatred! For whatever reason it seems particularly virulent here, and every year it completely wipes out my cucumbers and squash by August 1st or so. Any type of resistance I offered in the form of sprays and treatments was completely futile. When you smile while reading the description of a powdery mildew product because it states that the spores’ cell walls rupture upon contact (fungus can feel pain, right?), you know it is getting to you! This year I was determined to turn the tide, and I might have finally found a cheap, safe, and fairly effective solution.

Last summer's sad cucumbers... Powdery Mildew 1, Me 0
The first thing to realize is that 100% effective treatment/prevention methods do not appear to exist. The mildew will come if you are in an area where it is prevalent. It’s just a given around here. With that in mind, the goal becomes control and mitigation, so that whatever mildew you end up getting in the garden can be treated without killing the plants.

My approach this year was to begin prevention measures as soon as the plants got true leaves. Do not wait until you see the white spots on your plants’ leaves! When it comes to powdery mildew, not letting it become established is key. After reading hundreds of articles and websites about preventing powdery mildew, I kept coming across the now fairly well-known milk and water spray. Given that there aren’t very many commercial products that a home gardener can buy (or would want to apply to a crop that is going to be eaten) I thought I would give it a try. I admit I was skeptical, since many of these organic solutions turn out to be useless or even harmful. But given the utter futility of growing cucumbers here in past summers, it was worth a try.

This year's cucumbers... much better!
Here is my regimen. I sprayed once a week with a 50/50 mixture of skim milk and water. I used skim simply because that’s what we use, so we always have it on hand. Also, the milk fat adds nothing to the preventative nature of the spray, and would leave behind a nasty residue that would spoil in the sun. I sprayed with this mixture until the plant was completely soaked, making sure to spray the undersides of leaves as well as the tops. Interestingly, many of the sites recommend spraying on a sunny day, as there is something about the milk and sun that causes a chemical reaction that results in a strong fungicide. I just wouldn’t spray on a very hot day (you could wait until early evening to avoid leaf burn). I began using a handheld mister bottle, but that quickly became a pain as the plants became large, so I switched to a handheld pump mister bottle (about $6 at Home Depot).

I found that this mixture was pretty effective at keeping powdery mildew away. I sprayed the three types of cucumbers I am growing (all in different containers) as well as the green and yellow zucchini. Effectiveness varied as seen below. Please note that this is just my unscientific (but reasonably accurate) guess as to how effective the spray has been, based on experience in previous years.

Plant Type

Effectiveness (% reduction in mildew)
Straight 8 Cucumber
Lemon Cucumber
Bush Champion Cucumber
Yellow Zucchini
Green Zucchini

Notice the Bush Champion number. I am only guessing on this one, since this is the first season that I’ve grown it, but it still had a pretty bad case of powdery mildew. Perhaps it would have been completely killed by it had I not sprayed; I’m just not sure.

Mildew in the Bush Champion container... EVIL! Evil I tell ya!
Either way, the milk solution kept it at bay on the other plants pretty well for the most part. Only having about 15-20% of the mildew I would normally have has meant that the plants are still alive and producing in August! It also makes manual removal of affected leaves a viable option, since it doesn’t involve stripping the vines.
Still, I was looking for a knockout blow. I researched a great many alternatives to employ, and I came upon two: neem oil, and a product called Green Cure. Green Cure is tough to find, and the only nursery around here that had it was closed on Sunday, which is the one day I had free this week to spend any amount of time messing with spraying plants. It’s also the one that mentioned exploding spores, so I will definitely be picking some up for my next spray. Still, for the time being I went with neem oil. I sprayed it in place of my normal milk mixture this week, and it seems to have killed about 80% of the mildew on the Bush Champion cucumber (I  manually removed the affected leaves on the other plants, but I did spray them as well as a preventative measure).

All in all, I found the milk spray to be very effective in preventing powdery mildew, understanding that 100% prevention is unrealistic in this area. Also, the milk is mostly a preventative measure—not curative. It creates an environment on the leaf that is not hospitable for powdery mildew spores to take hold. However, once they set in you need to call in the reinforcements! Manually remove the most affected leaves (being careful not to strip your cucumber vines of too many leaves) and then spray with a fungicide as needed.

Lemon cucumbers! August and still going strong!
If I have had reasonable success using these methods, I would assume that anybody can. I had never seen powdery mildew before I began trying to grow cucumbers here. I don’t know if living in the city makes it worse, but there are houses down the street with bushes in front that are completely white by mid-August with the stuff. Still, my squash is nearly mildew free, whereas it would normally be completely white with not just spots but full-blown leaf coverage by now. The fact that it is August 6th and there are still green, healthy cucumber vines on the porch is a major victory in itself.
Until next time, happy gardening, and have no mercy on powdery mildew!