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).
Downy Mildew Control Recommendations
Vegetable Disease and Insect Control