by Stefani Leto
If you garden in the Bay Area, you are either familiar with powdery mildew, or you soon will be. The fungus is ubiquitous and there’s a spore variety for seemingly every plant.
Powdery mildew is a catchall name for a group of airborne fungi that reproduce on plant tissues. Most often you’ll notice a few white powdery-looking spots on leaf surfaces. If unchecked, the powdery coating spreads over leaf surfaces and sometimes stems and fruit. Affected leaves may twist, yellow, and ultimately die off. While it rarely outright kills your plants, leaf loss can have negative impacts, from the plant being weakened by loss of food, to sunscald on vulnerable fruits and vegetables. The fungus thrives in dry, cool conditions. No wonder it’s a regular visitor in Alameda!
I think of powdery mildew as the scourge of squash, because that’s been my experience. Older leaves on zucchini, pumpkins, even melons, exhibit the spots to death cycle. This year, however? The stars of powdery mildew in my garden have been the tomatillos. Covered – leaves, stems, everything. I even saw a rollicking infestation on a sycamore tree.
Powdery mildew on sycamore leaves
There are ways to prevent powdery mildew, or to discourage it. Planting resistant varieties is of course the first step, although I haven’t noticed mildew resistance listed in seed descriptions for the things that I grow. (Zinnias, I’m looking at you.) You can rinse spores off of plants before they begin to send mycelium into the leaves, but top watering can cause its own problems.
Pruning, both to increase air flow and to reduce the amount of spores, is key. I don’t trust my home compost to kill spores, so zucchini leaves, downed zinnias, and trimmed-off stems all go in my municipal green bin. Moving affected plant material away also deters spore spread. Pruning allows easier rinsing of your plants early on a sunny day. The fungus thrives in dry weather, according to the UC Extension: http://ipm.ucanr.edu/QT/powderymildewcard.html.
Another prevention technique is sulfur powder. You can dust it on your plants or use a wettable type that can be sprayed. You’d have to repeat it as new growth appears, but it’s reportedly effective. It also seems to be effective against rusts and spider mites. I’ve never tried it, just assumed powdery mildew is the price I pay to garden, but if this year continues to be a fungal fest, I may consider it in the future.
Close up of powdery mildew on tomatillo leaves
What do you do once you notice spots on your plants?
First, remove material as noted above. But if you’re getting to the point that it’s spreading, there are treatment approaches (http://ipm.ucanr.edu/m/pn7406-3.html).The internet is full of homemade recipes, from milk to soap and baking soda. Feel free to try anything that appeals, but the gold standard of management seems to be Neem oil, derived from seeds from the Neem tree.
Neem is not a preventative, only a treatment, and you cannot spray it too soon after using sulfur. It has to be used early or late, to prevent accidentally wetting and harming pollinators. Also, don’t use it on drought-affected plants or if the weather is over 90 degrees F. Adding a teaspoon of dish soap to a tablespoon of oil in a gallon of water in a sprayer may increase the effectiveness, but I’m finding just plain oil and water is helping. Sprayers will run about $10 for a small one; a bottle of Neem oil big enough to make ten gallons of spray is also about $10.
The tomatillos aren’t fungus-free, but I’m spraying every few days and no more white powder and the leaves haven’t all died. I waited too long to react, but I may still have a batch or two of salsa coming.
Tomatillos after some treatment