Looks like it might be a virus, though not something I have experienced so definitely can't say for sure.

Peter might have nailed it. One other possibility is that someone watered while the sun was high and hot. Here in Oklahoma, this can cause scalding of leaves. I once killed a young fruit tree just by laying a hose at its base and watering while the sun was high: cooked it.

George
Tahlequah, OK

Thank you Peter1142 and maxmex for your thoughts. I think you both could be right. We decided to pull the zucchini plant. Because we were not sure what it was and as our garden is at a community garden we didn't want to take a chance and have it spread to other plots. Our raised beds are built close together. Thank you again, I appreciate your replies.

I've had that happen last year. I thought it was a virus. But I also suspect it might have been some residual herbicide effect. I mulched with grass clippings,and wonder if some of those might have herbicide residuals.
In any case, this year I haven't had any of it. If it was a virus, I think its the mosaic virus. Google it.

I canôt see any signs of powdery mildew.
All the symptoms are alternaria blight.