Okay, so this year has been very hot in WNC, so we've been seeing some issues with the tomato plants. But our other bed has been going really strong: it's a thriving watermelon bed, mixed with a strong crookneck squash plant and a few tomato plants (my sister threw some seeds in from a little experiment she did and they took off). Also, there are a few marigolds. Again, they've all been doing great until recently, when suddenly the older watermelon vines began turning yellow with brown spots. An area that had so many leaves it looked like a rainforest would suddenly clear up, and look kind of bare. I thought maybe it was a natural process since the newer watermelon vines were doing great, but then I saw my squash plant doing the same thing: speckled yellow and brown leaves. Upon closer inspection today, I noticed one of the baby squash has just completely and bizarrely melted, and some of the squash "vines" have done the same thing: they're rotted mush. I've only had one of my watermelons wrinkle and kind of die off like that. I think we've had a few similar problems with the tomatoes. We also had a small problem with powdery mildew on the squash plant (I cut some leaves off before I learned that powdery mildew wasn't a major problem -- maybe that's why some of the vines are rotting?), but that seemed to go away.
On a little note, I've also had some trouble with snails and squash bugs but have managed to beat the latter away to a pretty good extent.
So, is there something wrong with the plants? We live in clay soil and added planting soil, so there should be enough nutrients. Is it blossom end rot? A nitrogen or calcium deficiency? A fungus? Any help and knowledge would be great, since I'm trying to learn to be a better gardener. Thanks guys!