Why do my healthy-looking tomato plants develop yellow leaves and black spots on the bottom of the plant? Then black splotches appear on stems. Only a few tomatoes ripen before the plant dies. —Linda Baggett, Asheville, NC
It sounds like you’re describing early blight, a common tomato disease caused by the fungus Alternaria solani. Early blight also infects potatoes, peppers, and eggplants—all members of the nightshade family, as are tomatoes.
Though it can occur any time, early blight usually appears shortly after fruit set, when the first tiny tomatoes begin to enlarge. Small brown spots form on the lower leaves, often with concentric rings around them. They spread to the stem, resulting in stunted growth and little, if any, fruit production. Ripening tomatoes may also become infected.
Avoid early blight by choosing resistant cultivars such as ‘Mountain Fresh’, ‘Big Beef’, and ‘Plum Dandy’. Rotate tomatoes with unrelated vegetables in your garden so they’re not planted where other nightshade-family vegetables have grown within three or four years. Plant them far enough apart to ensure good air circulation; staking also helps. Mulch the soil so soil-borne fungal organisms won’t splash onto the lower leaves.
Remove infected leaves immediately. Destroy plants that have most of their leaves diseased, and keep the area well weeded. Pay special attention to tomato-related weeds such as black nightshade, husk tomato, and Jerusalem cherry, as they can serve as hosts to the fungus. Consider spraying regularly with a fungicide to help prevent early blight. Check with your cooperative extension service or a good local garden center for recommendations in your area.