Yellow leaves, what's the cause?

[Blue Canary]

We have some tomatoes planted in a half whiskey barrel that have, for no apparent reason, begun to wilt and shrivel. Neighboring plants in the ground are unaffected, so I don't think it's related to pest damage. If it's fungal I can't begin to guess which fungus it might be: I scraped the skin to check for fusarium wilt, but even the dead limbs were a healthy green in their skins.

My only other guess is accidental over watering due to the amount of rain we've had lately-- the weather's been blisteringly hot for a month and a half, but it's drizzled every other day or so for the last two weeks. I don't remember if the wilting started before then or not; I wrote off the withering of the older leaves as a natural part of the plant growing.

[img]https://i27.tinypic.com/2ah7ac5.jpg[/img]

[Blue Canary]

I snipped off one of the dead stems; it was still green and healthy on the inside, so I'm tentatively labeling this as a case of over watering. The soil in the barrel isn't sopping wet but it is damp. I'm guessing that combined with overcast skies and cooler temperatures (80's for the last week or so) haven't given the soil enough time to dry.

If that's true, is there anything I can do to help the plants? It's supposed to rain for the next two days, and I'm hoping they can pull through to the next dry spell, but they are looking quite ill. Will they be able to recover?

[Gary350]

It looks like blight. Water plants with copper sulfate once a week. Spray plants with copper sulfate once a week.

Mix 1 tablespoons of copper sulfate with 1 gallon of water.

[cottonpicker]

Most likely is Late Blight.

[Blue Canary]

Thanks guys. Once I checked up on the symptoms I realized that it had started spreading to adjacent plants as well. I've started picking off infected leaves on the healthy plants and spraying them to try and stave off further infection. I think the plants in the barrel might be lost causes at this point-- they're almost entirely infected; even the new shoots are turning fast-- but there are still some tomatoes on them, and I'm wondering if it's worth pulling up the plants now, or if I should just wait until the current crop ripens, by which point the plants themselves will most likely have died off.

I'm just glad that none of my favorite varieties were in the barrel. Only two of them even managed to produce tomatoes before the infection kicked in.