What is wrong with my plants

My zucchini and cucumber leaves have spots. Last year I had the same problem. What is causing it? How do I treat?

This is the beginning of downy mildew, a very common problem with all cucurbits. There is no cure but you can slow the disease with Daconil, which is not organic, or try milk sprays. There are directions for milk spray mixtures online. I have not had success with milk. Daconil delays the inevitable but is somewhat helpful in extending the productivity of the plants. I now do nothing but clean up and discard plant debris from spent cucurbits and rotate with any other vegetables for two to three years out of previously planted cucumber plots.