When it comes to yellowing leaves, cucumber mosaic virus is a common culprit.
31+ Yellow Squash Leaves Turning Silver. John answers a question from a subscriber. My thai chili starter, my cucumbers, my squash, okra leaves are yellow looking and some edges are brown and crusty.
Earthbox Plant Problems from earthbox.com
Cold wet soil will do it. The white spots on the squash leaves are almost certainly powdery mildew. Tarnish first appears as a yellowish tint, then shifts to gray and black.
Zucchini plants are susceptible to a number of diseases.
I agree with ray about it being about normal for some lower leaves on mature plants to turn yellow and usually is not a problem. I'm including pictures of zucchini, acorn squash, cucumber, and sweet potato. Because zucchini are heavy feeders, they need lots of nutrients in their soil to draw upon. We're wondering if her hand cleaner might have the underside of my ring is silver and has not changed back.
