zuchini leaf spot?
It appears only the veins are remaining green. The inner parts are losing all their chlorophyll. This makes me wonder about the roots and their environment. Can you tell me some about what has been done with the soil including sprays, watering practices and fertilizers? The plants appear to be mulched. What kind of mulch and from where?
Sorry I do not yet have a positive answer for you, but shall be glad to work with you on this one. As an alternative, you can access the excellent diagnostic hands-on group of MGs at your local extension office at 10th and Jefferson in Eugene.
I don't do over head watering and I water daily because my soil profile is super sandy. I have incredible drainage. I also have a heavy mulch layer, straw and leaves no disease, on top of my watering to prevent splashing unto the leaves and to maintain moisture. I fertilize with liquid fish and kelp every 10 days. The plot had a cover crop over the winter with a flush of minerals, ag lime dolomite, greensand, rock phosphate and some alfalfa. I covered the plot with compost in the fall with the minerals when I seeded the cover crop. The compost is made from disease and weed free yard waste and kitchen scraps. I also use compost from lane forest products.
I suspect a K excess which can cause problems since many soils already has excess K. I would suggest a soi test as it is possible to add even compost in excess and cause problems. This is new research. It surprised me also as I learned that compost solved all problems