I've never heard of these ailments for zuccini. My first though is over watering. I'd like to know too in case it comes up. My enemies are squash vine borers and powdery mildew but I still get a lot of squash before the plants fail.
Here are some pictures of my zucchini that I took tonight. Watch out for fungus or powdery mildew. The way to prevent this is to not water after 12 noon. Do not spray the water on the leaves only at the base of the planet.
I had problems with my zucchini's this year also. The plants never really got that big and in most years they are so large and full that I have to move leaves aside to see if there are zucchini's growing. and about 3/4 of my zucch's got what I think is blossom end rot - where they start out growing looking healthy but then the end of it turns yellow and ends up being mushy. I had quite a few leaves also that turned yellow so I just cut those off. I bought mine as starts this year and maybe they just weren't hardy starts, or maybe I didn't properly amend my soil with the right nutrients. not sure.
I am losing my squash every year to the squash vine bugs. How do you get rid of them? When we lived up North you had so many zucchini's it was crazy. Down here in Eastern OK the vine bugs get the plants just as they're producing. I've cut the vines and pull the insects out but I am losing the plants every year. Is there something I can put on the ground to begin with?
We have to battle squash borers and powdery mildew as well. Last year I did surgery on my plants, too little too late... and it was nauseating pulling those horrible grubs out of the vines. This year I am spraying the plants with Bt to see if we can kill anything on the way in. If I see signs of infection, my plan B is to pull the plants and replant. I read that some gardeners have success doing that, because the insects only infest at one time of the season, and when you destroy the infected plants, the second planting will not be infected. So squash sacrifice.
For the powdery mildew, I have been spraying with a homemade recipe of 1/4 cup skim milk, 2 1/4 cup water, 1/8 tsp baking soda, few drops soap. So far, so good... the mildew seems to have subsided for the moment. I am continuing to spray once a week on my zukes and cukes.
It worked well last year, but as with the squash borers, it was too little too late. The cukes never fully rebounded.