If this is a problem that occor after the palnt begins blooming and
setting tomatoes and the vine also shows signs of wilting, You most
likely have Fusarium Wilt. The only thing you can do about is use
resistant/ tolerant varieties. They will have a list of disease
resistence after thier names (like VFN) .
http://ohioline.osu.edu/hyg-fact/3000/3122.html
JD wrote:
Location - South Central Kentucky

This is a recurring problem every year. I'm not sure, but neighbors
claim it is some sort of blight. When the humidity of summer finally
sets in, my tomato plants start turning yellow from the bottom up. No
matter how well I keep them watered and mulched. I've sprayed
fungicides, insecticides, and tried most everything. Neihbors and I are
at our wits end. Leaves are beginning to curl up like when aphids are
on them and the leaves on the very bottom of the plant have turned
yellow. If nothing is done, in a few weeks my tomatos will have stopped
being tomato plants and would have become compost.

Any help would be appreciated. I hate spending all this time on vines
that were beautiful all spring and then fall to pieces in the summer
months.

JD.