Looks like powdery mildew (a mold) to me. remove all the affected leaves and bag them to the curb. Be sure to inspect all the other leaves for any beginning signs of infection too.
List of Bryant RedHawk's Epic Soil Series Threads We love visitors, that's why we live in a secluded cabin deep in the woods. "Buzzard's Roost (Asnikiye Heca) Farm." Promoting permaculture to save our planet.
if you think brussel sprouts are yummy, you should try any other food. And this tiny ad: