4 sick plants (tomato, cucumber, watermelon, zucchini)

Hi!

I'm a pretty inexperienced gardener, and this year I find myself dealing with a big group of sick plants—and I have no idea what to do! I need a visit from a veggie doctor. =) Any advice or suggestions (on what to do this year or how to prevent this from happening next year) would be greatly appreciated.

My garden: Since I don't have ground space, I planted a container garden. I used an organic potting mix (lightly fertilized) and a variety of containers. I planted seedlings in early June and have fertilized with a very mild mixture of fish emulsion twice since. I'm currently having trouble with my tomatoes, zucchini, watermelon, and cucumber.

Tomato: an indeterminate heirloom plant that's grown very tall. I pinch back suckers and removed all leaves below the first fruiting branch. I have a few green fruit, some of which are quite big. In the past two weeks, most leaves turned brown and dry around their edges; it seems to be slowly spreading into the leaves. Just yesterday I noticed soft black spots—fungus?—forming on some of the leaves.

Zucchini: I know, who has problems growing zucchini?! I didn't until 2 weeks ago, when the older leaves of the plant began to yellow, droop, and drop very quickly. The younger parts of the plant still seem very healthy.

Cucumbers and watermelon: I've trellised both of these and they seemed quite healthy, growing faster than I could keep them trellised, until about 3 weeks ago, when the growth on both slowed significantly. It was at the same time they began producing fruit, so I assumed that was the reason for the slow growth. Then about a week ago both began to have some yellow leaves. Leaves that looked perfectly healthy in the morning would turn light yellow by the time I got home from work and would sometimes shrivel and turn black. This happened to leaves all over the plants, not just at the bottom or on one vine or something. I've removed these leaves as much as possible, but they keep coming and the plants seem less healthy.

As I mentioned, I'm looking for any advice I can get, at all. (Do veggie doctors make house visits?!) Thank you so much for taking the time to read and consider. =)