When I first saw these, I thought that my tomatoes had some kind of rabies. The good news is: they don’t. The lumpy, malformed black scars on the underside of the tomato are called Catfacing. There is a great article about catfacing at Veggie Gardener.
NOT A BIG DEAL: Catfacing is harmless. Affected tomatoes are gnarly, but totally edible unless it’s severe enough that it’s gone all the way through the tomato. And, at least you can safely compost what’s not salvageable.
WHY IT HAPPENS: Catfacing is the result of environmental stress during blooming, which causes developing fruit to produce too many cells. Other fruits and vegetables, including strawberries, can be affected, too. Stresses that cause catfacing are common in lower Hardiness Zones (4-6 in North America; I’m in Zone 6): drought, high winds, and especially temperatures below 13 degrees C (55F) or above 29C (85F). This makes sense, seeing as we’ve had a Stage 2 Drought here. Heavy use of fertilizers can cause catfacing, too.
MANAGEMENT: According to Rodale’s Garden Problem Solver (1988), the best way to avoid catfacing is to plant resistant varieties. You can also take climate/ exposure control measures for your tomato plants (floating row covers, cold frames, planting indoors, etc) but my thought is: I don’t mind so much if my tomatoes look like they lost a bar fight, as long as they are still delicious!
LESSON LEARNED: I shouldn’t have prematurely picked and wasted those poor, unsightly tomatoes. I’m going to let my straggle tomatoes keep growing, and only pick them if they’re looking severely catfaced. Those ones will just get frozen for future tomato sauces. (Still) Delicious!