Right at the soil line or how high up the stem? Mice and rats will do that if it isn't at the soil line. And don't rule out cutworms completely as they will often ignore one plant and take another. Slugs/snails is another possibility tho the damage is usually right at of just above the soil line.

Have you tried any of the stem protection methods? What works may often reveal who the culprit is.

Dave

julia42, I don't think you have a stem borer, but in case anyone would like to know about them, here are a couple of links:

Photos and info on page 3: http://www.btny.purdue.edu/Pubs/vegcrop/VCH2009/VCH508.pdf

More info on the bug here, but not tomato-centric: http://ipm.ncsu.edu/ag295/html/stalk_borer.htm

I had a stem borer once, about a foot above the soil. There was a small entrance hole, and the stem above that point stayed narrow and never got much longer. After a few weeks I just cut it off.

Actually, I think I may have found the culprit. Let me know if you think I'm way off.

I found a few flea beetles in the vicinity of the hacked plants. I have a pretty heavy straw and leaf mulch around them, and my guess is that larvae in the mulch hatched and chewed at the stems for a while. I've found pretty minimal leaf damage, but I've read they can damage stems too... I may go pick up some DE.

OK so I think it is cutworms in Arizona. How do you get rid of them?

I rule out slugs. They eat the tender leaves not the stems.To verify, go out after dark with a flash light and looks very closely.

Flea beetle eat the leaves also, leaving small holes. I don't think they harm the stems.

I suspect earwigs. The are nasty. They hide under mulch, under rocks, like slugs do. Remove the mulch and you should see them.

For cutworms put a few nails, bamboo stick .. around the stem. That should prevent them. Mouse and rats also chew the stems.

Spray with Neem Oil spray . it might deter some insects.

Sey

I like to use cutworm collars made out of toilet paper tubes. I put the collar around the tomato when I first plant it. You would need to sink it about 1/2 way into the ground. I only use about a 4" section of tube. I've always been curious how the nails/ bamboo stakes treatment works.