While not as common with such large seedlings, damping off (a fungal problem) can also occur, and will girdle the stem at the soil surface as you've shown/described. Here is a link that might be useful: damping off

Back of leaf picture here This post was edited by ash1970 on Fri, Jun 6, 14 at 15:40

Hi Yennie. I had a similar problem this last year with the three tomato plants that I tried to grow in the ground instead of in containers. Something, and I never saw what, ate completely through their stems. Whatever it was never touched the leaves. I would just come out in the morning and an otherwise healthy looking plant would be on the ground with its stem chewed through. I've never seen a slug here, but have had problems with cutworms. But if cutworms were to blame, you'd think some leaves would be damaged as well. Anyhow, I think there are a few posts about it on my garden blog linked below. Here is a link that might be useful: Container Gardening in the Desert

To follow up on my question, If I were to allow a second or third main stem to grow (while still pruning all suckers on the secondary stems), could I tie 2 or 3 main stems to 1 stake without crowding the 2/3 stems too much?