Gymgirl said:Mildew. 
Ok.
Yeah, for me anyway, it's like fall blight on the tomatoes.. it just happens and you can't really stop it. You can just hold it off longer by keeping them dry and aired out as well as possible. Only water them on the ground, not on the leaves, keep them sunny, don't crowd them out.. that kind of thing. I like the climbing squash because it gets up off the ground. The biggest tell is the white blotches on the leaves before they get yellow.
