This particular disease is totally vexing! I admit, I love cramming plants together, especially in a garden. I hate bare soil. I do prune for aeration. I do use free water to wash off leaves early in the day so they can dry by evening. I have used milk; to water plants for more strength (seems to work well) against fungus and virus. I have sprayed with milk. No difference that I could see. I do use mega fans all day and night for aeration. This is a virgin garden and typically all the squash, summer and winter, the cucumbers and tomatoes are always infected. As soon as I see any sign (usually the first cool nights of the summer) I spray with neem oil. It does not stop powdery mildew. Just slows it a bit. I wash vegetables to get rid of any Neem, I hope!

Is there an expert out there that has figured out how to control this fungus?

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    Move to a drier climate ;-) That's what commercial hops growers in the USA did, anyway...I grow mine in the wet east and it's often a problem. – Ecnerwal Sep 1 '14 at 1:14
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    when you tried the milk solution as a spray, what ratio of milk to water were you using? – Bamboo Sep 1 '14 at 9:53
  • I have some copper fungicide, some sulfur fungicide, neem oil, and a chemical fungicide, and some mildewed peonies. I will run an experiment for ya, with before and after pics, to see what's best. Please allow 2-4 weeks. – J. Musser Sep 1 '14 at 22:44
  • Ha ha!! This is the driest climate I could imagine...other than the real desert!! It would be fine if this garden were outside but this by necessity is in a hoop house. With FANS. Heat at night, now as it has started to go below 40 degrees. I can't believe how insidious this stuff is! Bamboo...I used 1/4 cup to 1 gallon...too much? I miss being able to 'graze' as I work in the garden!! Grin...help!!! – stormy Sep 1 '14 at 23:28
  • Great J.! This is the ONE thing I'd like to be able to control A BIT MORE... – stormy Sep 1 '14 at 23:30

Stormy, you've said you used a quarter of a cup of milk to one gallon of water - I've no idea how that works out, but the ratio of milk to water is 1 part milk to 9 parts water, at a push, 2 parts milk to 8 parts water, though there is no benefit in using a higher milk ratio. I suspect even a cup of milk doesn't work out at one part - if you're making a gallon mix, you'd need more milk than a quarter cup (assuming a gallon in America is 8 pints, as it is here, though I don't know what your 'cup' holds. Milk spray is very effective for powdery mildew particularly (if you've got the ratios right), though you need to spray under and on top of the leaves and all stems, till run off, as well.

  • OK! I'll try using this ratio and I also use NON-fat milk. Is that right? Don't need the cream? Thanks Bamboo!! – stormy Sep 3 '14 at 17:37
  • @stormy - my personal feeling is that semi skimmed is more effective than completely skimmed, but the research suggests the level of cream makes no difference to effectiveness, so maybe I just sprayed more effectively when it happened to be semi skim. It's the lactose in the milk that works, so it shouldn't matter if there's no cream. – Bamboo Sep 4 '14 at 13:05
  • Bamboo...this one friggin' treatment has stopped the powdery mildew...almost completely! It is the LACTOSE that is working whoa! It works BETTER than NEEM! I am so dang impressed...so 1/4 cup in one gallon. How often? I haven't sprayed since this question. Thanks!! – stormy Sep 9 '14 at 21:16
  • One to 9 so how much milk to a gallon? 9 parts h20 would be 115.2 ounces plus 12.8 ounces milk...a little more than 1 1/2 milk per gallon. How do you figure it in liters? Arghh. Why don't we nations get this together? Grin... 128ounces = 10 parts, yes? 1 part = 12.8 ounces to 9 parts (X12.8)= 115.2 oz. I used 1.5 cups milk added h20 to make 128 oz or one gallon. Gotta get this published... – stormy Sep 9 '14 at 21:43
  • So reading this back I DID NOT USE 1/4 CUP per gallon I used 1 1/2 cups to make one gallon. 1/4 cup per gallon was not enough! I think I am getting everyone confused...sorry. – stormy Sep 9 '14 at 21:47

That time of year and inevitable.

For summer squash, you can cut the worst affected leaves off, leaving the stem. We make sure all the cucumbers grow up on frames so they're well off the ground and ventilated.

Foliar spraying a baking soda solution of 1 Tsp per Quart of water can help raise the pH and hold it off. Potassium bicarbonate will actually be more effective if you can get it.

Also, milk whey has an active ingredient that will attack powdery mildew. It's a byproduct of making cheese. Applied as a foliar spray as well.

In the end, you will only delay it as it's an end-of-season issue where plants slow down with less heat, are less able to defend themselves and succumb to changes into fall.

Addendum: Final harvest, November 7. Back in September, the mildew started in earnest so we clipped the worst affected leaves off one by one. Longest squash is 12 inches, enough for there for a final couple of squash dinners. Between the four plants, we couldn't give them away fast enough during the height of the season.

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    Methinks you mean "raise" the pH - baking soda was a base the last time I took chemistry. – Ecnerwal Sep 1 '14 at 15:08
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    Yep, thanks for catching that. Basic highschool chemistry no less. – Fiasco Labs Sep 1 '14 at 16:29
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    Wait, baking soda will raise pH! Isn't this an alkaline? But Fiasco! I'd forgotten Baking soda...and I am cutting off a lot of leaves from down below that don't really help with photosynthesis... – stormy Sep 1 '14 at 23:23
  • Fiasco...love your name...I will try baking soda next time. Dang milk worked so well I am just agog! Forget neem! My frogs are gone. Next year it will be milk and baking soda! Have you tried milk? I am so amazed... – stormy Sep 9 '14 at 21:20

Powdery mildew is worse where sunlight is limited, as is downy mildew. You said cramming plants can increase the risk, and you've had fans on to increase aeration. Humidity would affect it. Dry leaves may make it worse, strangely. The opposite seems to be true of downy mildew, about wet leaves.

Here are some potential ideas that might help:

  • Plant resistant varieties. They may or may not be hard to find, but they exist. Resistant tomato varieties that I would recommend looking into include Striped Stuffer, Granadero and Tropic. Striped Stuffer is the only heirloom I've found for sure.
  • If you have trees giving shade, trimming them or getting rid of them might help, as it would increase the light.
  • Consider using trellises for your plants to keep them off of each other and provide more air flow. This may give shade to plants near the trellis, though, and cause problems there. However, higher plants may get more light, too.
  • There may be optimal soil nutrients to help protect against it. I don't know if this extends to powdery mildew, but sufficient potassium can help protect against disease, insects and other ills. Calcium and silica availability may also be factors, since they play a role in plant strength and structure. Interestingly, calcium silicate is supposed to suppress powdery mildew. Also, potassium bicarbonate is supposed to be an effective fungicide that can work with organic gardening. I'm not sure how well potassium silicate and potassium bicarbonate work for treating potassium deficiency, though.
  • Artificial UV and/or infrared light may help to inhibit it.
  • You might consider planting in southwestern Texas. The UV index is pretty high there, and land is inexpensive. Or there's always Arizona or southeastern Utah for an arid climate. That's maybe a severe action for combating powdery mildew, though. :)
  • Weeds and such may or may not be hosts. Pulling them might help.
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    Great answer Shule! Got a chuckle about trying new locations! Powdery mildew is a weird one...plants beneath a roof/eaves that keeps their leaves dry are most susceptible. I have to grow anything/everything in a greenhouse where space is limited and I really overplant, lots of fans, aeration...etc. I tried the milk at 1:9 ratio and THAT MADE A HUGE DIFFERENCE. Downy mildew is no problem, it is just a natural decomposition gig. Planting powdery mildew resistant varieties is probably the best preventative! Thanks! – stormy Mar 27 '15 at 7:05
  • @stormy I updated my answer with some resistant tomato varieties. – Brōtsyorfuzthrāx May 22 '15 at 10:15
  • I've found some resistant varieties although oddly none were on your list. No biggie! The greenhouse environment is so condusive to powdery mildew. Doesn't matter that you've got fans blowing constantly or that everything is on trellises and you've pruned as much as one dares! I am trying to grow a small area outside of the green house, so we shall see. I've started the milk therapy earlier this year so we'll see...thanks for your info! – stormy Jun 24 '15 at 21:24

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