It isn't a disease. The name is "Tomato Stem Primordia" and they result for overly wet conditions like if you have had a lot of rain and high humidity, perhaps heavy rains while you were gone, or if the plants are consistently over-watered.

Stems, just like fruit, can split due to excessive moisture and when that happens the primordia develop in the open split. Google photos of the term for many photos of the condition you can compare to your plants.

Dave

Did she water even though there was rain? Do you automatically water every 3 days whether the plant needs it or not? Fixed-schedule watering is a good way to drown plants and they can wilt from over-watering just as easily as from under-watering.

As to whether they will recover - normally TSP poses no long term threat to the plant as it is an effort by the plant to recover from the root rot in the ground roots. So it depends on if any splits seal themselves off quickly when given dry weather and if it can adjust/compensate enough before any disease process develops. Needless to say it needs the soil to dry out some and have time for some new soil roots to hopefully develop. I'd sure give them a chance to try and recover.

Dave

Given that they are grown in geopots next to my garage, most rainfall is usually never enough to sufficiently water my tomato plants. My mistake when arriving home yesterday was to water them even more assuming that she missed watering those two plants while we were away. I'll just have to wait and see what happens. Interesting that none of my other 10 plants were affected.

Those roots also form when there's vascular, stem, or root disease or injury. Basically, anything that causes a "blockage" in the circulatory system, which includes waterlogged roots. The mostly likely explanation is Dave's explanation. Did you check the moisture levels a couple inches down in the pot before watering? Fabric pots should have good drainage, but sometimes it's an issue with an inappropriate potting mix that holds too much water.

The only thing I'd add is that damage from certain herbicides can also stimulate it, as the hormone involved, auxin, is what a common class of herbicides mimic. Herbicide damage usually comes with other symptoms, though, but it could explain why those plants might be affected and others not. Maybe keep an eye out for distorted new growth, which would happen in the case of herbicide?