Late Summer - The Season For Powdery Mildew In Utah

Sorry to have to tell you this but powdery mildew is a problem during the more humid and rainy times of late summer.  I have seen it on several flowering annuals and perennials but it seems to do the most damage to squash plants.  Sometimes there is just a little bit and it's no big deal.  Sometimes it will kill entire plants.  There doesn't seem to be a cure for it but there are ways to control it's spread, like using baking soda with water.
Some squash plants have green leaves with light green or gray markings/coloring on them.  This can sometimes be mistaken for powdery mildew.  Inspect the leaves closely and you will see that normal leaf coloring will have a pattern that follows the shape of the leaves - beautiful.  Powdery mildew will be blotchy and spotty and dusty - ugly.
Here is an article about powdery mildew from Organic Gardening.

squash leaf with normal grey markings - no powdery mildew

squash leaf with powdery mildew

squash leaf with powdery mildew