When you see the first symptom of powdery mildew, that is, a white powder covering all of or part of a leaf, as if someone had dusted it with flour, it is already too late to save it. This “powder” isn’t something you can simply rub off. It results from the appearance of sporangia (spore-producing organs) from the fungal disease that has invaded the leaf already. It’s actually the penultimate stage of the disease (in the last step, the leaf blackens and dies!). The disease has already been at work inside the plant’s leaves for days if not weeks by the time the sporangia appear. So any home remedy used on powdery mildew can only prevent it, not cure it.
Of course, the very best “home remedy” against powdery mildew is to plant mildew-resistant varieties and to grow them under optimal conditions. That would include good ventilation and keeping the soil moist during periods of drought (powdery mildew is one of the rare diseases that is more likely to occur when soil is dry rather than moist). On the other hand, if you’re not yet at the point in your evolution as a gardener that you’re ready to yank out plants chronically subject to mildew to replace them by ones that are, here are two home remedies you can try while you’re waiting.
Baking Soda to the Rescue
Mix 5 mL (1 tsp.) of baking soda (found in the pantry) into 1 quart/1 liter of water and spray this solution on plants susceptible to powdery mildew. For the powder to better stick to the leaf, add a few drops of insecticidal soap or dish soap to the solution. I repeat that this treatment only helps prevent the disease and won’t cure it. If you start to the treat the leaves after they already show the typical dusty white appearance indicative of powdery mildew, you may be able to stop the disease from spreading further, but the damaged leaves will not recover.
Milk With That?
You’ve run out of baking soda? Try milk! Yes, milk has also proven to be an effective preventative fungicide against powdery mildew. The recipe? Just add one part milk to nine parts water and spray to saturation. And in case you wondered, skim milk has been found to be just as effective as whole milk.