It started with that one. First, there were a few leaves yellowing on the bottom. I wrote it off as older leaves dying as the plant grew, and didn't think anything of it until I realized that leaves up the entire plant were turning yellow, then crisping up and shriveling. And now the same thing is slowly but steadily afflicting the other plants in the barrel. Fortunately, the heirlooms we have planted in the ground adjacent to it seem to be doing fine-- for now.

I've looked up the various potential diseases that can afflict a tomato plant. While plants with similar symptoms seem to be suffering from psyllids, I've seen no sign of insect damage other than stinkbugs, and I don't think I live in a region where psyllids are prominent. I also suspect over watering, though not on purpose-- we've gone through a good month of solid 90+ temperatures and dry conditions, but for the last two weeks it's been raining every other day or so. The plants in the ground aren't afflicted by the yellowing, so I'm thinking the ones in the barrel aren't draining well? I've noticed that the speckled roman is covered in bumps that seem to be the type a plant puts out when it's trying to grow new roots; one of the other tomatoes has some as well.

Someone on another forum told me it was Late Blight. After reading more about it I unfortunately reached the same conclusion, and now that I know what to look for I realize that it's started spreading to adjacent plants as well. I've since aggressively been picking off and disposing of infected leaves in the still-healthy plants, and will at least let the current crop on the infected plants ripen before tearing them up. To help stem the spread of the infection to the healthy plants I've started spraying them with Daconil. Organic gardeners will want something with Copper Sulfate, which I've read is less effective than the chemical in Daconil.