Squash – Downy Mildew
Q: My whole zucchini plant turned spotted overnight. I harvested 1 zucchini thus far this season. The plant is planted in a raised garden, with organic top soil. Should I pull the plant?
A: This looks like the beginning of downy mildew 🙁
The leaf disease runs rampant in squash when weather is warm and humid. Try to stop its spread by thoroughly spraying with chlorothalonil (click for sources).