My zucchini leaves turning white, edges are browning and leaves are dying! Help!

Ummm well they need a lot of water, and are they climbing, where are they planted, how long have they been in the ground, tell us more

The plant is about a month in the ground (Baltimore) in a raised bed. It is not climbing. It has sun 10 hours a day, is watered regularly. Has produced eating sized zucchini already, but fruits still on the plant are not growing as quickly as I thought they would. This is my first vegetable garden, and it is very crowded with tomato plants. I have inspected as carefully as I can and see no borers. New leaves are healthy and green until they get about 6 inches wide--then the white patches start. I thought is was powdery mildew, and am spraying with the bicarbonate and water, but other writers are not reporting the brown edging and leaf death that I am seeing. The plant appears healthy except for this which I think will kill it since there is so little chlorophyll left in the leaves.