Wonder if it isn't something like a cutworm or beetle feeding on the plants as opposed to fungi -- though the conditions you mentioned are ripe for fungi. If you haven't already done so, also post your query on the tomato forum -- pics would be beneficial.

Toms can reroot though I would have completely cut off the area with the lesion. How tall are the plants in question?Keep the soil lightly moist and warm.

Thanks, Paul.

The plants in question were quite tall -- about 40 cm. I grow all my own seedlings so that they're very robust when I put them out. I'd thought of cutworms, but the plants are far too big, and I don't know of any insects that feed on the stem of tomatoes. Besides, that wouldn't explain the dry streaks running up the stem.

As for the rerooting, I can't cut off the part with the lesion, otherwise I'd be cutting off the roots entirely. The plants will have to reroot above the lesion before it completely dries through. They're not looking very good today, I don't expect them to make it.

Where is the tomato forum?

Hmmm, the thought also occurs to me that perhaps you have some sort of stem-borer (generally a grub like larva)

Either way, forget the roots, cut the stem off above the dry streaks. Remove all but a couple leaves. Pot the plants up in new soil. Keep lightly moist, in a warm location and out of direct sun. If your plants are on their last legs you have nothing to lose by cutting them. If it is a bug or disease, NOT cutting off the healthy material will just let it spread. (I would also recommend pulling up the lower stems -- roots and all -- and burning them. Do not compost them as if it is a disease you will possibly be allowing it to 'hide out' in your compost.)

The link to the Tomato forum follows

Here is a link that might be useful: Tomato forum

I think I have photos of this problem attacking another tomato plant. but I can't see a button to upload photos. Is there one?

I figured it out. Here are the photos:

Tomato Photos

I have had that happen to my tomatoes from time to time. I have been able to save them by taking a one gallon plant container with the bottom cut out, sleeving it over the plant to surround the wound and filling it with compost. The tomato reroots and also keeps some of the original stem. It is not insect damage. Looks more like fungal or a kind of crown rot.

tj

I am not sure if I should pull my plants or not with something that sounds almost identical except the rot is not at the stem/soil line. It looks like dark brown rot on the stem (some have 2 rot spots) and many leaves are affected as well and the growth is stunted. The plants have been part of the same mud party the rest of everyone else's plants have been attending in the Northeast. (I think I'm growing gills myself.) I thought I would try (if the weather should behave) to save the (entire) plant by using liquid copper fungicide spray in case it is fungus. I've included a photo (if it worked) of one that is about halfway from the ones that still look decent to the really bad ones. I wonder if I should just pull all the affected (25 of the 30 plants) and get a late start on some larger store bought plants. Could it be that it's simply rotting from the wet soil? Or most likely a fungus? They look pretty stressed... I don't know if plants can come back from this... Any ideas would be greatly appreciated.

Here is a link that might be useful: Sad tomato plant

Ah, the unknown culprit. If it's a bug the cure is often as easy as putting a barrier around the stem at soil level (plastic, cardboard etc.) If it's a disease, do not plant in that same soil again. Water your transplants with chamomile tea, which reportedly staves off fungal diseases through it's tannins.