It may be fusarium wilt, as I have a hard time telling it from veticillium.
http://www.tomatodirt.com/verticillium-wilt.html
When compared with fusarium wilt. Verticillium develops more slowly than fusarium wilt. Yellowing is less dramatic with verticillium and encircles the entire plant, while those infected with fusarium tend to show symptoms just on one side. Interior stem discoloration extends just 10-12 inches high in tomatoes infected with verticillium wilt, but all the way up the stem in those infected with fusarium wilt.