I look at the tomatoes every morning and every late afternoon. Something is chewing on the bottom 10-12" of the Rosella's main stem and about 2" out on the first branch and it happened since this morning. I have never seen this before. Hmmm, some kind of insect? Squirrel? Haven't seed any grasshoppers out and about yet. Should I wrap it with something like strips of bedsheet or row cover? Maybe sprinkle some Sevin on the leaf mulch in case something's hiding there? Hopefully it munched and left!

Any ideas are welcome!

I've had a vole that chomped a bunch of my seedlings & plants (eggplants, peppers, and tomato) in my garden until I put in a trap that got him. On a couple of the plant (the thicker ones), he did some gnawing that looks like the upper damage on your plant. For the thinner plants, it was a diagonal cut that lopped off the head of the plant. Didn't eat the leaves or anything. just toppled the plant.

Has it been dry in your area? Maybe something just in search of some moisture?

We've been having those afternoon pop-up showers now and then so I can't say we've really been dry. For once the pasture near the garden isn't brown yet. Nice and green.

Voles. I sure hope not. I've not had any problems with them before. Moles and shrews are out and about but they're carnivores. Voles are herbivores so that's a possibility. This morning I didn't see any additional chewing on that Rosella and still no damage on any of the other tomatoes. Maybe whatever did it hit the road. I'm just hoping that the gnawed spots heal over quickly so that no pathogen gets into those areas.

That sure is unique damage. Thought I have seen everything on the internet over the years. You win a prize! Could you go down late at night
with a flashlight? Can’t imagine that it is a big critter as wouldn’t they knock over the plant when they are chewing it up?

I think that it is some kind of rodent. Try wrapping the stem loosely with aluminum foil to deter them. The damage does not look to be too deep, hopefully they will heal over just fine.

Although I don't see that a terrible lot, it does look like something that's within the realm of what I've seen in my garden.

It could be mice, but I doubt it's anything larger. I thought it was just caterpillars, pill bugs, slugs, or some mysterious insect-sized critter. I never considered mice, as far as I remember. We do have mice, but I can't say with certainty that they've ever caused us a garden issue.

Wouldn't a mouse go from plant to plant more readily than focusing on so many differing points on one plant? Smaller critters aren't as fast as mice, so it makes sense that they'd stay on the same plant. But, I guess a mouse might like the taste and get fixated. Rosella is supposed to be a pretty great tasting tomato; maybe the plant tastes great, too.

I think I've seen this most on shaded plants, but I don't remember.

I think a tomato stem would make a mouse sick.... if it was a rodent, that should be an unhappy rodent.
OTOH slugs can cause deep damages like that too on eg brassica stems. And nothing makes them sick. Except those pellets
I had slugs in the greenhouse one year but they didn't eat stem iirc they exclusively fed upon large pink tomatoes.

Around here greenhouses have that trouble with mice.
One plant I bought had the bite marks and some of my own seedlings outside showed damage.
One greenhouse owner had to get cats.