Leaf brown on tomato
One possibility is early blight; we've included some info sheets on recognizing tomato problems, including blight:
Whoa cowboy/girl! Let this tomato lover add this info: Probably your 15 min watering is neither a 5/8" hose full stream on one plant nor a drip on one plant, but lots of issues arise regarding watering. While plants need adequate water, too much can be damaging just as too little. Lots of issues go into guessing how much water is appropriate, including soil type, plant type and size, sun and temps, mulch, etc. But pls remember:
1) place a tunafish or other can near plant to record how much water is reaching soil under plant;
2) 1/2" daily might be too much or adequate, but doubt it should exceed 1" - but check the soil and wait if still moist (and shallow watering discourages desirable deep roots);
3) water the soil, not the leaves - particularly since tomatoes are disease prone, and leaves will be damaged if sun shines on wet leaves;
4) water early in day so leaves have more opportunity to dry before nightfall, and mist or sun watering loses much to evaporation;
5) mulch to preserve moisture and reduce weeds and splash of disease spores onto leaves;
6) remove yellowed/diseased foliage, disinfecting knife or scissors between cuts;
7) do your "tending" (picking, suckering, tieing, etc.) when leaves are dry to reduce spreading disease.
Finally, realize that heirlooms, albeit tasty and romantic, are known to be more disease prone than modern hybrids. So consider hedging your bet next yr by planting several varieties, then compare plant results and taste to improve chances for the future. good luck from a tomato lover in MD.