I have two zucchini plants that were doing quite well. But today when I checked on them, their leaves were all turning yellow with white stuff on them. And the zucchinis that were growing, are suddenly rotting it seems. (they were nice and green yesterday, now they're half-mush) The yellow-and-white-stuff seems to be moving to my spaghetti squash nearby. What's going on? Deficiency? Disease? Pest? Thanks for any help! pics

I always start here . I don't always get my answer but I'm too lazy to be of more help this morning.