A: Based on the look of the spots and the fact that the older leaves are showing these signs first, I’d say early blight. This fungus overwinters in the soil and can splash up onto those lower leaves when the plants are watered. Mulch and an organic fungicide will slow it down, but affected leaves will brown and die. The best way to avoid this next year is to rotate tomatoes (and all vegetables in the nightshade family) to a different bed, since the fungus can’t be removed from the soil.