Brandywine yellowing leaves

[markd42]

Hello all, Last year I grew two (2) Brandywine Sudduths Strain and I easily got 50 beautiful tomatoes that were without question the best I ever tasted. THe leaves started turning yellow at some point midseason, but the plant still survived well enough to provide the aforementioned bounty. This year, I have eighteen (18!!) plants situated in rows three apart, with 30 inches between each plant (see attacehd). THey are doing quite well and have set their first fruits. However behold in the pic, the lowest leaves are starting to yellow, this time with also some damage to the leaves as well. Please tell me what this is and how to remedy.

THanks in advance,
Mark