This is one fast growing crop that I didn’t intend to grow. I think I “bought” this problem of powdery mildew when I did a foliar spray with some fish emulsion that was then followed by multiple damp dark gloomy depressing sunless days.
Within a week, it has really done a number on my vine crops.
It doesn’t really upset me because this represents a teachable moment. My family will not starve due to a reduced number of spaghetti squash and mini-pumpkins and I will, hopefully, learn something about how to cope with powdery mildew.
Lesson #1: Once it starts, its not going away so you have to “deal” with it if you do not want it to spread everywhere.
Lesson #2: It might start in the shade but it will be happy spreading to sunny locations.
While searching for an organic way of dealing with this problem I came across this paper by Crisp, Wicks, Troup, and Scott called “Mode of action of milk and whey of in the control of grapevine powdery mildew” (PDF format) in the journal called Australasian Plant Pathology, published in 2006 (Australasian Plant Pathology, 2006, 35, 487–493).
Powdery mildew takes out all sorts of crops and can be especially vicious to wine grapes. Control of powdery mildew in wine grapes has been done by spraying with sulfur containing fungicides. This is why wines are contaminated with sulfites (and why I can not drink wine, I am violently allergic to these sulfites).
Its been known for some years that spraying a 10% skim milk or whey solution on affected plants is very effective in the treatment of powdery mildew.
This paper reports that sunlight interacts with the milk to form oxygen free radicals that collapse the hyphae of the organism grapevine Erysiphe (Uncinula) necator which we call powdery mildew (zucchini is effected by Sphaerotheca fuliginea). It also damages the conidia (asexual, non-motile spores of a fungus) within 24 hours of treatment.
Some people use hydrogen peroxide, a rich source of oxygen free radicals, but this paper reports that H2O2 doesn’t do anything to the conidia and that it encourages germination of this fungus.
Must be that the free radicals produced by photo-oxidation of something in the milk acts by a different mechanism than the free radicals in hydrogen peroxide. Or does it?!
The answer lies in complexity. Read on.
When they looked at a specific component of milk called lactoferrin (a globular multifunctional protein with antimicrobial activity (bacteriocide, fungicide – WIKI, an 80 kDa iron-binding glycoprotein, binds to the membranes of various bacteria and fungi, causing damage to membranes and loss of cytoplasmic fluids – paper), a part of the innate defense, conidia were ruptured but hyphae were not effected until 48 hours out.
This indicates, as per these authors and seems intuitive, that milk has a bipartite action against powdery mildew:
- photo-induced free radical activity
- lactoferrin mediated rupture of conidia
The authors close by saying that the mechanism by which lactoferrin is effecting the conidia is not clear. They also think that there many be other parts of milk which could be having an anti-mildew effect.
I will be spraying my unfortunate mildew-infested plants with a 10% skim milk solution (shown below) and I will let you know how it goes!
Drop me a comment if you have had success with this method.
Measure a 1:10 milk:water solution into a sprayer. I am using skim now but I might try a whole fat to see if there might be some fat-soluble or fatty acid aspects. I might even use raw if all else fails!
If I were being scientific about this, I would leave a section unssprayed, spray one section each of – skim, whole commercial, whole raw. This is not scientific, its going to be anecdotal and descriptive and with no controls because I started out just desperate to attack the problem so I do not have any untreated plants right now.
As I am not a mycological physiologist and I am not working of a government grant, I think its going to be ok.
Pour in the milk.
Pour in the water, mix it all up in a bucket.
Spray on a SUNNY DAY, not cloudy.
Spray twice a week.
Lets hope I can see SOME result otherwise there will be no follow up to this post and you will be left hanging!