Hi, what’s the best way to deal with powdery mildew?
Let’s start with a little background first, for all the readers who aren’t familiar with powdery mildew. Powdery mildew is a fungal disease which affects plant leaves, turning them blotchy or (you might have guessed it) powdery-looking. Over time, the infection can get worse and the plant will lose its leaves, which can be pretty bad if you had been dreaming about those tomatoes all summer. If you have sharp eyes, you can usually find powdery mildew as grey streaks or gray water stains on many trees in the fall.
Ordinarily, I’d give you a Latin name for this fungus, but powdery mildew actually refers to a number of different fungi. Your cucumbers might be attacked by exotic-sounding names like Erysiphe cichoracearum or Podosphaera xanthii, while your wheat plants might get Blumeria graminis f. sp. tritici. The trees we were talking about in the last paragraph probably have Sawadaea tulasnei. Luckily, they can all be treated in the same way.
Powdery mildew (like all fungi, really) likes moist conditions and mild temperatures. Mildew spores will float around in the air hoping to land on a leaf and find enough water to sprout, which will let it dig inside the leaf and establish a little home. This is why you’ll usually find powdery mildew striking in the late summer/early fall when the rains start to come in. The height of summer is too dry for mildew, but once the air is humid enough, all of the spores which have been lucky enough to land on a leaf can sprout.
There are actually dozens of different species of fungi that are called powdery mildew, affecting a huge range of plants, but there are a few methods which work well on all of them. Once again, we don’t recommend any chemical treatments for your garden plants, but you might find some at a gardening store.
First off, let’s try changing up our planting practices to avoid getting any mildew in the first place. Try to situate your garden in a nice sunny spot that doesn’t get shaded over the course of the day. We want to avoid cold, dark areas which might cause condensation to form on your plants’ leaves. Moisture on leaves is a sure-fire recipe for fungus. Secondly, try spacing your plants out. A lush canopy of leaves might look attractive, but if the inner leaves on this canopy don’t get proper air circulation, it might get humid in there leading to mildew.
Okay, maybe it’s been a really wet summer, you’re finding mildew on leaves, and of course you can’t change your planting practices because the plants are already in the ground. Let’s talk home remedies for mildew. The easiest two methods for dealing with mildew are milk and potassium bicarbonate, which are actually effective enough to be used for organic farming.
Milk can be diluted to a 1:10 ratio (100 mL milk to 900 mL water for 1 L) and sprayed on leaves to treat powdery mildew. Why? Research by Peter Crisp and his team from the University of Adelaide suggests that a protein, ferroglobulin, in milk whey produces oxygen radicals when exposed to sunlight, damaging the fungi. Another protein, lactoferrin, acts as an antimicrobial. Microbiology nerds can check out the paper Mode of action of milk and whey in the control of grapevine powdery mildew here.
An important thing to keep in mind is that it’s very difficult to completely ‘cure’ your plants of powdery mildew (like all diseases). Spraying milk, potassium bicarbonate, and even some chemical treatments won’t save a heavily infected plant. Instead, you should start spraying milk (or other things) as soon as you see an infected leaf. That leaf might not make it, but you can still save the rest of your plant.
Powdery mildew is just one of those diseases that you’ll have to anticipate every year if you’re planting susceptible crops. With a bit of knowledge and experience, you can minimize the damage to the point where you don’t even have to worry about it. Good luck and happy planting, friends!