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I think I'm dealing with powdery mildew, but I want to confirm that diagnosis.

Context: I'm in Portland, OR. I've got several raised beds with a couple kinds of kale, broccoli, brussels sprouts, some beans, cucumbers, and few other things. The soil is a combination of high quality store-bought and my own home-cooked compost. I'm not using any chemical pesticides. Only OMRI products and Cornell spray, etc. I'm struggling with aphids on many of the brassicas these days. I'm keeping them under control, but it's a daily struggle. Meanwhile...

My Russian kale started looking unhappy. The main symptom is a white, dusty, hazy complexion. enter image description here enter image description here enter image description here

If I rub with my finger, the white haze wipes off to reveal a shinier, greener leaf surface. enter image description here

In addition to this symptom, I also see brown splotches along the stems. Usually worse on the outermost leaves. enter image description here enter image description here

Interestingly, it only seems to be effecting the Russian kale. I am also growing Portuguese kale in the same beds, and they look pretty darn happy... enter image description here I do see a slightly different kind of brownish flecks along these stems of the Portuguese kale, but I don't know this variety well enough to know whether that's normal. The leaves seem totally fine.

I think I'm dealing with powdery mildew. So I sprayed the leaves with Cornell formula (mostly water + a little baking soda). After a short amount of time, I do see a change. The dusty white is gone and the leaves look greener and healthier. enter image description here The before/after effect is a bit exaggerated in these photos because the light was different when I took the photos.

In some cases, the treated leaves look sort of brown. It's usually more pronounced underneath and the texture feels a bit "crispy". enter image description here enter image description here

What are your thoughts? Is this powdery mildew? Am I handling this correctly?

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2 Answers 2

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Yes this looks like mildew, probably because the weather has chanced and the moisture levels in the air are higher now.

I have treated mildew once, not on kale but on an oak bonsai tree. I used a mix of milk and water (40/60%), and sprayed the affected leaves. It worked very well, and is organic without chemicals. I don't know why it works, but it does work. I don't have experience with your baking soda method, seems harmless as well.

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  • Interesting and fairly available method; if could mention if was it 0% or 2% or 8% milkfat milk, how long it was left on after wetting, was it then rinsed off, was the treatment repeated after x number of days & how many times, and was the soil also wetted with the mix, would be appreciated. Thank you.
    – M H
    Commented Sep 1, 2020 at 15:34
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    @MH don't try to overthink this, it's not rocket science. Just use the milk available in your fridge, mix it with water and spray it on the leaves. The color of the spray bottle is not relevant.
    – benn
    Commented Sep 1, 2020 at 16:09
  • @benn I think baking soda works because it's alkaline, and makes the surface of the leaf inhospitable to the mildew, which needs acidity. At least that was my understanding. It's possible milk works similarly, but I'm not sure. I did a quick Google about the acidity of milk and it seems to depend on a variety of factors.
    – emersonthis
    Commented Sep 1, 2020 at 18:25
  • @benn Is the brown discoloration on the stems consistent with powdery mildew? I've seen the dusty whiteness before, but not the speckles on the stems or the brown "char" after treatment.
    – emersonthis
    Commented Sep 1, 2020 at 18:27
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    Big chance that the brown discoloration on the stem is caused by the mildew too. When it looks like it's rotting and beyond saving, I think you might better remove it to prevent further spread.
    – benn
    Commented Sep 1, 2020 at 19:24
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+50

Yes, it is Erysiphe cruciferarum - powdery mildew. I have not treated this myself. Bicarbonate sprays have been used successfully on moderate infections. If the plants are in pots then keep them out of the rain, otherwise you have to repeat the applications. If you can find it easily, Potassium Bicarbonate is better, and is sold commercially for this purpose (e.g Milstop, Armicarb-100, Kaligreen)

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  • Thanks! Is the brown discoloration also the result of this same parasite? I'm trying to figure out if that's the result of the mildew, my treatments, or maybe something totally different. Also can you say a bit more about potassium bicarbonate products. Baking soda is sodium bicarbonate. I'm interest in how that differs from the potassium bicarbonate products you recommended.
    – emersonthis
    Commented Sep 6, 2020 at 19:58

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