Your tomatoes need calcium. Crush egg shells and soack them in water for a day or two. Then use that to water your tomatoes. I take calcium pills and sometimes I crush a few of those and put them in water to water the tomatoes. It won't help the tomatoes that are already brown on the bottom, but hopefully will keep the new ones health.
From Birdy: Sour milk poured around the base of the plant will also do the trickl. -Birdy