Post #G15: Powdery mildew: Protectants versus eradicants.

Posted on August 3, 2020

Some people view their garden as place of peace and tranquility.

But I say, if the lion lies down with the lamb, we’ll soon be overrun with sheep.

Source:  By Edward Hicks – National Gallery of Art, Washington, D. C., online collection, Public Domain,

For the persnickety, per the actual Old Testament reference, that’s wolf/lamb, leopard/kid, lion/calf.    But in terms of predator/prey relationship, no functional difference.

This is yet another post on gardening.  If you have no interest in gardening, move on.  The actual subject of this post is powdery mildew, below.

Trying to remain rational


Yesterday afternoon, I was sitting in my kitchen, exhausted, dripping sweat.  I calmly explained to my daughter that I was engaged in a struggle, adopting the best of modern science, testing the limits of my ingenuity, spending significant amounts of cash and copious amounts of labor. Hence my sorry state at that time.

All of that, to defeat a beetle a bit larger than a grain of rice.  The cucumber beetle.  Likely, it has fewer neurons in its brain than I have in my little toe. And yet, here we are, slugging it out.  Toe-to-toe, so to speak.  Damned if I’m going to let that pest beat me.

I have tens of dollars of produce at stake!

Next up, I’m getting ready to buy a $60 bottle of Serenade ™, to treat powdery mildew in my garden.  In order to save maybe $20 worth of pumpkins, in a reasonably environmentally-friendly, bee-friendly manner.

So, fair warning, my gardening may have passed the point of rationality.


A short treatise on powdery mildew.

Let me cut to the chase:  The two most important words you need to know, regarding powdery mildew, are protectant and eradicant.

Protectants keep the mildew from taking hold on your plants.  They are preventive medicine only.  They don’t do you one bit of good on the plants / leaves that already have powdery mildew.  What you need to kill powdery mildew is an eradicant.  That’s something that kills the mildew on the plants and leaves that are already infected.

So, in a nutshell, boy did I screw this up.  Because the things I have used so far have been, I believe, mostly protectants.  (Although, see notes below).  What I need is an eradicant.

This somewhat dry page from the University of California system is the best explanation of this I’ve stumbled across.  The upshot is that when you read about “a treatment for powdery mildew”, you don’t know whether you’re looking at a protectant or an eradicant.  If you read research that says, e.g., use of such-and-such reduced powdery mildew damage by 70% , you still don’t know if it worked by acting as a protectant, or as an eradicant.

I’ve tried three things so far.  At least two of those are generally classed as protectants, not eradicants.

  • Sodium bicarbonate solution is classed by most as purely a protectant.
  • Potassium bicarbonate solution is said to have “some eradicant activity”.  Which I guess means if functions poorly as an eradicant.
  • Hydrogen peroxide solution is discussed the same way, as having “limited eradicant activity”.

The commercial spray I was getting ready to buy — Serenade ™ — is a protectant as well.

The only thing in my possession that appears to be classified as an eradicant — both by discussion and by the label — is 70% hydrophohic extract of neem oil.  In particular, I have a bottle of Bonide Rose Rx concentrate.  It is in fact advertised as a cure for powdery mildew.

Interestingly, I added some of that to my sodium and potassium sprays.  I am now reading the label directions, and it says not to add soap, but merely to keep the solution agitated while spraying.  And I only had a third of the minimum recommended amount of oil, in those sprays, to serve as an eradicant.

So, only as a last resort, I’m going to use a product that is actually an eradicant for powdery mildew, and I’m going to use it according to label directions. 


Postscript:  What I’ve tried, what other treatments are available, and what I’ve noticed.

I’m not going to bother to explain what powdery mildew is.  If you don’t know what it is, that’s great, because you apparently don’t have it on your plants.  This post is for those of us who have suffered the heartbreak of powdery mildew.

What I’ve tried (that hasn’t worked).

I’ve tried three “home remedy” leaf sprays so far with limited success. Limited success being a euphemism for failure. As in, I still have powdery mildew all over my pumpkin plants.  Unfortunately, I  was so focused on trying something that I didn’t document the pre-treatment state of the leaves with photos.  So it’s possible there was some improvement.  It’s possible that it slowed down the spread.  But in terms of killing the @#$@# mildew, no, none of these appeared to work.

The three apparent failures (with one-gallon recipes) were:

  • Sodium bicarbonate (one heaping tablespoon) + oil (neem oil 70%, one tablespoon teaspoon)  + soap (Dr Bronners, couple of one teaspoons) + water (one gallon).
  • Potassium bicarbonate + oil + soap + water, same recipe as above.
  • Hydrogen peroxide (6 ounces 3% H202 solultion) + water (one gallon), sprayed on three successive days.

By eye, the H202 may have knocked it back.  Some.  A bit.  But not enough to make a real dent in it, for my pumpkins, which are my hardest-hit plants.  For the two bicarbonate methods, if there was an effect, it was not large enough for me to perceive it and/or it takes a long time for the effect to show.

In all fairness, these are often billed as largely preventative measures.  They might keep powdery mildew from taking hold, or spreading, if sprayed early enough.  But they may or may not be curative, particularly for a plant that is overrun with the stuff.

A big upside here is that, while I didn’t kill the mildew, I didn’t kill the plants, either.  So these, at the mixes shown, sprayed in the evening (NOT in direct sunlight), appeared safe for all my cucurbits.  I might continue spraying with these, to see if there is an effect.

As an interesting side note, a fairly hot anti-mold product (Concrobium) is based largely on sodium carbonate and bicarbonate, per this analysis of products for marine use.

What other treatments are available.

Home remedies.  There are  two more commonly-mentioned home remedies that I might try.

  • Skim milk, diluted 1:10 with water
  • Mouthwash, diluted 1:3 with water

The skim milk has some research basis behind it.  So that is known to have had some effectiveness, on some plants, in some situations.  I have not seen (nor will I likely see) any research behind the mouthwash approach, as that would likely be impractically expensive for a farmer.  And agricultural research, by and large, serves the farming industry.

Finally, there are at least two modern commercial products that can be used (in addition to old-time remedies like bordeau mix or wettable sulfur.  I am pretty sure that I don’t have the knowledge and skill to be able to use the old-time products effectively and safely.)

Presumably, either of these would be curative, for badly infected plants.

  • Daconyl.  This is cheap enough, but toxic, as in banned-in-Europe toxic, as in implicated in large fish kills toxic, as in toxic-to-bees toxic, as in possible carcinogen toxic.  This being America, of course Joe Homeowner can buy this off-the-shelf at Home Depot.
  • Serenade ™.  This has a reputation for being effective, and it is allowed in organic farming, so I have to presume the toxicity is low.  But it’s expensive.  The smallest bottle of concentrate available is about $60.

Neem oil.  And now I have come to realize that neem oil spray targets powdery mildew.  For this, I want a commercial product that is 70% neem oil extract, because I’m not using it as an insecticide.  I want something labeled for use as a fungicide.   So I’m using Bonide Rose Rx concentrate, diluted per label directions.

Some observations.

I have a number of observations from my garden, that, in the end, may all boil down to one thing:  For cucurbits, this disease attacks the oldest leaves first, and most completely.  I don’t know if that’s due to senescence of the leaf, or to the spores taking some time to grow.  But the bottom line is that newer leaves seem to be largely free of this.

Interesting, the squash pruning strategy espoused by the Rusted Garden amounts to systematic euthanasia of the oldest leaves.  He take all the leaves off the vine once that portion of the vine has flowered.  In theory, this is to promote air flow and light penetration.  But it also has the effect of continuously removing all the oldest leaves.  I think that this, alone, might help control powdery mildew on summer squash.

That doesn’t work for pumpkins and vining squash no new leaves grow back, I think, once you strip the leaves off a section of vine.  So for those key plants, I’m trying to save the infected leaves.