Almost certainly early blight. Look for spots on the affected leaves with a
bullseye look. Your choices are few: remove the yellowed branches and
treat with a fungicide recommended by your local garden center (if you use
chemicals). Keep the foilage dry by watering only at the base, thus
discouraging additional fungus growth. As I understand it, the fungicide
works better as a preventative measure than a corrective treatment, but it
may keep it from getting worse and it will help prevent later fungal
problems like late blight. Heirlooms may also be more sensitive to fungal
disease-- my own limited experience says unless you have ideal growing
conditions to stay away from heirlooms until you've got lots of experience
with tomato cultivation, or at least grow them with VFN hybrids to maximize
your chances of getting a harvest at all. Heirlooms taste great but are
very sensitive to disease and growing conditions and can be a real headache
if everything is not ideal.
Good luck.
++++==================================================+++
Paul & Kathy Lasakow
Lasakow@exis.net
Norfolk, Virginia
"One-fifth of the People are against everything all of the time".
-Robert Kennedy