Looks like a form of blight. There are a few different types, you should read up on them. In the past I've pruned the bottom leaves off, don't let them drop and stay on the soil, blight will transmit from year to year.
Afterwards I mulched with straw and sprayed with an organic spray for fungicide problems. It didn't stop it but it slowed it down. Some blights can't be stopped and you'll have to pull the entire plant up and dispose of, don't compost it. Good luck!
If you do determine it's blight, try rotating your crops for three years to rid it from the soil but be careful not to plant something from the same family as tomatoes there next year or you'll continue the cycle.