White, powdery spots develop on both leaf surfaces and expand as the infection grows.
The fruiting part of vegetables usually isn’t affected
Powdery mildew is common in warm, dry conditions. Unlike many diseases, powdery mildew doesn’t require moist conditions to grow. Moderate temperatures (60º to 80ºF) and shade encourage the disease. Powdery mildew spores are carried by wind to new hosts.
Today's weather report: Mostly sunny, with a high near 70. South southwest wind 5 to 10 mph.
A perfect fit weather-wise to spread the fungus: a little wind, a little shade (overcast), moderate temperature, no rain
The good news
Why is this good news?
Because while I have no qualms about using gray water from rinsing the dishes or the laundry, I had reservations about using the wash water especially on vegetables. No more. I couldn't put the soapy water to better use than controlling powdery mildew.
Oh, happy day!
Powdery Mildew on Vegetables Management Guidelines University of California Integrated Pest Management (UC IPM)