These are the long Sicilian zucchini and this year the leaves are turning yellow on a few plants. This is the first year I observed this. Does anyone have any idea if its a disease? Or something else. It has been very hot (South Ontario Canada) but it doesn't look like damage caused by heat. In my experience I never really seen these affected too much by strong sun, except a bit of wilting in the day which goes away in the evening. I've also never seen any insects attack these plants. The only other thing I can think of that has been different from previous years is the fertilizer, my father has been adding chicken manure pellets to the soil (mixed with water) Could this be causing an imbalance in the soil nutrients and damaging the plant?