Early Blight Or Uneven Watering

There could be two causes of yellowing of bottom leaves on a tomato. The first, and most easily remedied, is uneven watering. If you let a tomato plant go too dry in between soakings the plant will drop its lower leaves to conserve water. Soaking the ground for a longer period of time less often, and not watering by hand but using a sprinkler or soaker hose, is the way to handle that situation. Mulch around the plant will also help keep a more even level of moisture. Try not to splash water on the leaves in the evening as this is a prescription for most fungal problems.

Early blight is a fungal disease that causes the lower leaves to yellow, and frequently become spotted with dark spots as well. Remove the leaves that you see are damaged and begin spraying with an organic fungicide. If you spray weekly from now until frost you should be able to continue to harvest. As at your local garden center for an organic fungicide that is labeled for early blight on tomatoes.