If I may guess -- and this is a guess -- I'd say it's a small animal of some sort. Small, because it's eating no higher than 1" off the ground. Only in early spring, because after that its preferred food is available.

Try the sort of protection people use against cutworm damage: 2-3" pieces of cardboard tube, with a slit cut in the side, wrapped around the stem and inserted just far enough into the soil that a small animal can't dislodge it. Some people buy the plastic covers for shower curtain rods (which already have a lengthways slit) and cut them into short pieces; being plastic, they can be reused multiple years.

I'm assuming these are inground plants and it sounds to me like it's post-emergent damping off/ There are three different fungi that can cause it and if it's pre-emergent damping off the seeds in the sowing mix can rot, and with post emergent damping off one normally sees it shortly after the seedlings emerge when the wee seedlings develop a brown constricted area at the soil ine and then fall over and die.

But, post emergent damping off can also affect plants up to 4-6 inches tall perhapos a bit taller. The main problem I see here is that you say the plants are 40 cm tall and since 1 cm = about .4 inches that would make your plants about 16 inches tall now.

At what height do you first start seeing the brown constricted area at the soil line?

I'm assuming you raised all of these plants from seed. The pathogens can be in the starting mix, which is why one has to use an artificial mix, but they can also be found as natural flora in the soil and contract it there.

And there's no reason that OP heirlooms would be any different than hybrids as far as delayed post-emergent damping off is concerned since no hybrids have any bred in tolerance to those three fungal pathogens .

Below I've linked to Google images of damping off and you can also do a straight Google search and read more about it.

So that's my best guess as to what's going on and of course any stresses to the plants, such as cool cloudy weather and wet soil is going to make it worse.]

Without a picture it's usually very hard to make a good diagnosis. Do you ever see the plants fall over just as the new seedlings come up? Or is this condition more likely to develop only after the plants are set out which would then point to soil as the source.

Carolyn

Here is a link that might be useful: Damping Off; Google Images

It could be cutworms. Here's a helpful link.

Kim

Here is a link that might be useful: Cutworms

I would say an unusual case of damping off...it happens after cold spells (damping off is worse when it's colder), it happens at the soil line, and it happens when the seedlings are young. I'd say it's unusual because I don't usually hear about it happening on plants larger than a few cm tall, but it can still happen.

I'd go with what Carolyn says...she is very experienced with tomatoes.

Thanks for your thoughtful ideas.

To answer various questions:

1. All my plants are from seed. I don't put them out till they are 20 cm or taller and well hardened off. I transplant them in pots at least three times beforehand so that the stems are stout and the roots strong. Post-planting damping off seems unlikely - the stems are always thicker than a pencil.

2. The problem occurs at a particular time, not at a particular height. When I see the problem, I pull out the plant. Antifungal agents haven't ever helped.

3. The dry, corroded area looks a lot like alternaria stem canker (except it's only at the soil line) or the stem damage of wilt (except that the inner stem is very healthy looking and the leaves full and green). I wish I'd taken a picture before I dissected it...

4. The recent cold spell is my primary suspect(a few of the nights were below 10 degrees C, which is the cut-off for tomato tolerance, although the daytime temps were 15 or higher). But if it is fungal in the soil, then why are some cages disease-free? It is the same soil, same weather. Conversely, it can't really be a critter, since the greenhouse coverings go right to the ground. Also, what critters like to chew on tomato stems, when there is very tasty cabbage and lettuce right next to it?!

5. I have never had damping off in pots. I plant seeds in vermiculite, which is pretty close to sterile. The soil is about .5 cm below the vermiculite, and the roots eventually find it. This prevents any fungal spores from sitting at the soil line.

6. For the first time, yesterday I lost a young cabbage seedling in the same way. The stem at the soil line dried out and dried through. This seeding was about 10 cm tall and very healthy. It could have been damping off, although cabbage is usually pretty hardy in the cold. Strange.

My answers are assuming it's late post-emergent damping off.

(1. All my plants are from seed. I don't put them out till they are 20 cm or taller and well hardened off. I transplant them in pots at least three times beforehand so that the stems are stout and the roots strong. Post-planting damping off seems unlikely - the stems are always thicker than a pencil.)

Stem width has nothing to do with damping off.

(2. The problem occurs at a particular time, not at a particular height. When I see the problem, I pull out the plant. Antifungal agents haven't ever helped.)

Antifungal agents wouldn't help if it was damping off b'c the spores are in the soil. If you leave the plants in do they usually flop over and die?

(3. The dry, corroded area looks a lot like alternaria stem canker (except it's only at the soil line) or the stem damage of wilt (except that the inner stem is very healthy looking and the leaves full and green). I wish I'd taken a picture before I dissected it...)

While you spoke of a gnawed look above I'm actually glad you now speak of a corroded area b'c that's exactly what the symptoms look like and that corroded area is often constricted and has a brown tinge to it. Since wilt is a general term and doesn't relate to any one disease I'm not too sure I understand that part of your answer.

(4. The recent cold spell is my primary suspect(a few of the nights were below 10 degrees C, which is the cut-off for tomato tolerance, although the daytime temps were 15 or higher). But if it is fungal in the soil, then why are some cages disease-free? It is the same soil, same weather. Conversely, it can't really be a critter, since the greenhouse coverings go right to the ground. Also, what critters like to chew on tomato stems, when there is very tasty cabbage and lettuce right next to it?!)

Fungi are not evenly distributed in the soil so I can understand why some plants get it and some don't. And I too would pin it on low temps/bad weather.

(5. I have never had damping off in pots. I plant seeds in vermiculite, which is pretty close to sterile. The soil is about .5 cm below the vermiculite, and the roots eventually find it. This prevents any fungal spores from sitting at the soil line.)

The fungal spores don't sit at the soil line, they infect the plant through the roots of the plant. But I'm not sure what soil you're talking about under the vermiculite in pots, especially since you say you pot up about three times.

(6. For the first time, yesterday I lost a young cabbage seedling in the same way. The stem at the soil line dried out and dried through. This seeding was about 10 cm tall and very healthy. It could have been damping off, although cabbage is usually pretty hardy in the cold. Strange.)

I haven't taken the time to research what specific pathogens might be involved with damping off of cabbage, assuming it does occur.
Cold lousy weather does encourage germination and growth of soil fungal pathogens so that makes sense to me as well.

Again, have you ever seen damping off as your seedlings just emerge and if so, what artificial mix or whatever do you use for seed sowing?

Alternaria Stem canker occurs almost exclusively in CA and I don't know where you live and garden, but the general color of the lesions would be about the same except that the lesions of Alt Canker are found all over the stems, one sided, not around the stem as you describe, and there are prominent leaf lesions as well.

Just trying to help here. If what I've suggested doesn't resonate with you then I'm afraid I have no other ideas to put forth. Sorry.

Carolyn