Attack of the powdery mildew
What food gardener has not been horrified to watch a summer squash plant succumb to powdery mildew, a white coating that covers the leaves? Other plants can suffer from such mildew diseases as well.
The powdery substance is made of the spores of the disease, waiting to blow away and infect new plants. The body of the fungus is living throughout the leaf tissue.
In the case of summer squash, a wise move is to keep removing lower leaves of the plant as soon as you see the first little round spots of spore structures emerging on them. This will slow the spread of the disease to the upper leaves, letting the plant produce fruit longer than it would if the fungus overtook the entire plant.
The strategy of removing infected leaves works less well for peas, which tend to quickly become coated with powdery mildew spores in warm weather. It is best to remove infected peas and replant in cooler weather.
Put any infected leaves or plants you’ve removed into a municipal compost container if you have one, but not into your own compost.
When you buy seeds of squash, cucumber or peas, look for varieties resistant to powdery mildew. This may be spelled out or indicated with the code PM. ‘Cascadia’ is one resistant snap pea variety and ‘Oregon Sugar Pod’ is a resistant snow pea. Resistant summer squashes include ‘Dunja’ and ‘Spineless Perfection.’ Note that the species C. moschata has some natural resistance. This includes butternut and related winter squash, as well as ‘Tromboncino,’ a climbing summer squash.
The fungicide Serenade, based on the soil bacterium Bacillus subtilis, protects plants susceptible to powdery mildew if you spray before they are infected. Once the disease has solidly taken hold, growing deep inside the plant, eradication is unlikely. Some products, however, combine protection before infection with some ability to treat an infected plant. Gardeners once sprayed baking soda in water, but sodium is toxic to plants, so better would be commercial potassium bicarbonate products such as Kaligreen or Bicarb Old Fashioned Fungicide. Oil sprays, including neem oil, are also protective with some ability to treat infections. (The above ingredients are approved for organic gardening. Look for OMRI on product labels.)