Post #G19: Three-part powdery mildew eradicant spray.

Posted on August 14, 2020

Another gardening post.  This one is about eradicating powdery mildew on cucurbits.  See my earlier posts to be clear about the difference between protectants and eradicants for powdery mildew.  Once your plants are already infected, you’re looking for something that will kill an existing infection, i.e., an eradicant.

I seem to have mixed together a spray that will kill powdery mildew on my cucurbits without killing the plants.  And, to my surprise, a mouthwash-based spray seems pretty effective as well.  Many others failed to kill the mildew.

Caveat:  If you try any of these sprays, test them on a small area first.  Wait a few days and see whether or not they kill your plants before you proceed.

Three-component spray:  Potassium bicarb, potassium soap, horticultural oil

The leaves shown directly below document the regression of powdery mildew after saturating a pumpkin leaf,  top and bottom, in the evening, with a spray consisting of:

  • one fluid ounce potassium bicarbonate
  • one fluid ounce light horticultural oil (Monterey brand)
  • one fluid ounce potassium-based soap (Dr. Bronner’s brand)
  • one gallon of water.

Pie pumpkin leaf with powdery mildew, 8/10/2020, pre-treatment.

One day post-treatment, 8/11/2020

Four days post-treatment, 8/14/2020

This appears to have killed the powdery mildew without killing the leaf.  I do not see the chlorophyll returning to the mildew-damaged portions of the leaf.  Maybe it’s too soon for that.  Or maybe this spray is too strong and has damaged those portions of the leaf.

All three substances in the successful spray were mentioned in academic references has having some ability to eradicate an existing powdery mildew infection.  Dosages were either based on commercial products (Kaligreen (r) for potassium bicarbonate, M-Pede (r) for the potassium-based soap) or label directions (light horticultural oil).

The theory is that potassium bicarbonate and potassium soap work by disrupting the cell walls of the fungus.  I’m not sure why the horticultural oil works, but it is mentioned as an eradicant, both in academic literature and per label directions.  It is ranked as superior to neem oil based on this page from the U California system.  (I consider that to be the best available reference on powdery mildew).

This is a stronger and slightly modified version of a spray that I tried and rejected a couple of weeks ago.  (The potassium bicarbonate version of the spray mentioned in Post #G11.)  Compared to that earlier spray, this has:

  • Twice as much potassium carbonate
  • Twice as much horticultural oil
  • Six times as much soap
  • Commercial horticultural oil (petroleum-based) instead of neem oil.

I can’t say why this one worked, and the weaker version failed.  But it’s a fair bet that at the higher concentration, this spray will be toxic to some plants.  If you use this, make a small batch and test a few leaves.  Wait a few days. See if this damages your plants before using more widely.  Spray in the cool of the evening, if only because use of horticultural oil in the heat of summer is inherently risky.

Mouthwash appears to work:  25% solution of Listerine (r)

The mouthwash-treated leaves didn’t stand out as clear winners as I was looking at them, live, in the garden.  They didn’t “hit the eye” as strikingly as the three-part spray leaves did.  But when I put the pictures together, it seems pretty clear that the mouthwash did, in fact, kill quite a bit of the powdery mildew.  This is based on a 25% solution of old-fashioned Listerine (one cup Listerine with three cups water).

For this one, I found zero data or gardener testimonials to back the claim that it killed powdery mildew.  And, it may be an expensive solution for a large area.  And it’s maybe not quite as obviously effective as the three-part spray above.

Unfortunately, I didn’t take this one very seriously, so the camera angle changes quite a bit from start to finish.  But if you’ll focus on certain individual patches of mildew, you can see that some of them fully disappear by the final post-treatment picture.  I’ve circle one patch that disappeared, to help you see this clearly despite the change in camera angle:

8/10/2020, pre-treatment

8/12/2020, post-treatment


8/14/2020, post-treatment

Milk:  A failed eradicant (40% solution of 2% milkfat milk)

If I didn’t tell you the order, do you think you could pick out, by eye, which was pre-treatment and which was post-treatment, below?  I can’t, so I have to class this as a failure to eradicate.  Or, at least, failure within this time window.  Maybe it reduced the surface bloom a bit (or maybe recent rain did that), but by eye, I can’t find a patch where I can clearly assert that the mildew disappears between the first and third pictures.So I judge that milk might or might not work as a protectant, but it does not appear to work well as an eradicant.

Note that this also serves as my control group.  That is, the mildew wasn’t naturally in retreat in my garden, on these pumpkin leaves.

8/10/2020, pre-treatment.

8/12/2020 post-treatment

8/14/2020 post-treatment.

Other failures

Finally, I should probably enumerate my other failed sprays here.  Because, science.  These might or might not prevent spread of powdery mildew, but failed to kill it (or, at least, failed to kill it quickly enough that I could tell they were working.)

Other sprays that failed as powdery mildew eradicants include:

  • baking soda (Post #G11);
  • potassium bicarbonate (same formulation as baking soda);
  • Bonide Rose Rx 70% neem oil;
  • hydrogen peroxide (6 oz. 3% solution per gallon, three successive day),