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I noticed this white fungus a few weeks ago on some flowers in my garden which I ignored. But now it's appearing on my zucchini leaves. I was under the impression the fungi are plant specific, but could this be spreading from the flower leaves to my zucchinis?

I'm not a milk drinker but I'm going to have to buy some to try and control it. I've removed the most infected zucchini leaves, and wondering if I should do the same to the flowers.

Flowers: flowers

Flower leaf close up: leaf close up

Zucchini leaf: zucchini leaf

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2 Answers 2

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Powdery mildew occurs only when it is relatively cool, still, and quite humid (maybe I should say foggy). Usually the mildew will vanish when these conditions pass.

You can spray with a solution of 2 tablespoons 3% hydrogen peroxide per quart of water. It should take care of it. Repeat as needed. Note that peroxide has no prophylactic effects - it only acts on an infection that exists at the time of application.

This spray has no cumulative effects. Peroxide releases reactive oxygen that kills fungi as well as bacteria and becomes ordinary water in the end. Most 'fungicides' (such as daconil), on the other hand, require careful use because of accumulative environmental and/or potential toxicity issues.

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Looks like some kind of Verbena in the top pic, possibly Verbena bonariensis - these are prone to getting powdery mildew, particularly towards the end of the growing season, and the usual cause is dryness at the roots, and yes, powdery mildew can spread to other susceptible plants. If the soil is dry, its worth watering, but otherwise, yes, the milk remedy will work - 1 part cow's milk to 9 parts water, doesn't matter whether the milk is skimmed or full fat, so long as its cow's. Repeated treatments may be necessary, as with hydrogen peroxide, works as a contact treatment only.

UPDATED ANSWER:

Following your comment, I did a bit more checking - it appears there's more experimentation with milk as a treatment for powdery mildew, so the recipe of 1 part milk to 9 parts water can be varied, using a higher ratio of milk to water, and yes, it seems the treatment is best applied on a bright day, link below

https://www.growveg.co.uk/guides/using-milk-to-prevent-powdery-mildew/

I'd add my personal preference is for using skimmed milk - tends to be fewer problems with being smelly than with higher fat/cream milks.

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  • Yes, it's Verbena bonariensis
    – Graham Chiu
    Commented Mar 11, 2016 at 16:02
  • I've thought that powdery mildew is caused in dry conditions and mould in damp conditions, both of which can spread to other plants. It is rust on plants that tends to be more plant specific. But I agree with bamboo regarding treatments with milk solution.
    – user13638
    Commented Mar 12, 2016 at 9:28
  • Arggh. I applied the 10% milk solution today but then read this which says it should be done on a sunny day sciencenews.org/blog/food-thought/dairy-solution-mildew-woes to activate the superoxides and peroxidases.
    – Graham Chiu
    Commented Mar 14, 2016 at 5:57
  • Interesting - there's obviously more research now, also found this link which suggests it works best in bright light, and even at different concentrations growveg.co.uk/guides/using-milk-to-prevent-powdery-mildew But its only a bit of milk, you can easily repeat the treatment on brighter day if what you've done already is ineffective. I might add this link to the answer...
    – Bamboo
    Commented Mar 14, 2016 at 10:50

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