I have three large zucchini plants in my garden, and all three have been producing nice fruits for a few weeks. The last couple days, one of them has started to turn yellow and sickly:
I've never grown zucchini before, so I'm not sure if this is just the natural progression, and the plant's life is over... or if this is a sign of some damage or disease. If it is the latter, can I nurse the plant back to health? Or should I destroy it so it doesn't infect the remaining healthy plants?
In response to @EdStaub's answer, here's a new photo of the base of the plant, at three magnifications: