Could be very advanced powdery mildew but an expert will be along shortly.
Many plant/leaf problems mock others.
I keep a small arsenal of products on hand and try to identify the problem asap, with
help, before applying anything.
Has it been damp and hot?
I always, every year, get powdery mildew on my summer squash so i treat in advance as a preventative.
Thanks Ginger...(posting at the same time). Makes sense Inspector Ginger.
