I'm no expert but looks similar to fungal or bacterial diseases which appear similar to what is shown in your photo in many plants. (Black Stem, among others) It's often associated with a too-wet root environment. You could try removing damaged parts, cut back on watering. Rule of thumb: allow soil to become dry between waterings, say 2" or so before watering. Tomato plants like the soil to become fairly dry in between watering - they can survive desert conditions. Even if they wilt midday, so long as they recover, they don't necessarily need water.

Important to disinfect cutting tools (dilute bleach or use alcohol) and toss plant trimmings - don't put in your compost pile.