Septoria Leaf Spot
Disease · Commonly affects: Tomato, Potato, Eggplant, Parsley, Celery
Septoria leaf spot is a common fungal disease caused mainly by Septoria lycopersici, and it is one of the most frequent foliage problems home gardeners see on tomatoes. It rarely kills a plant outright, but heavy leaf loss weakens the plant and cuts into your harvest, so catching it early matters.
How to identify it
Look for many small, roughly circular spots about one-eighth inch across, each with a dark brown edge and a lighter tan or gray center. A hand lens often reveals tiny black dots (spore-producing structures called pycnidia) scattered in the center of older spots, which helps tell it apart from early blight. The disease almost always starts on the lowest, oldest leaves first and works its way up the plant during warm, wet, humid weather.
Damage
As spots multiply, infected leaves turn yellow, then brown, and drop off, usually moving up from the bottom of the plant. Severe defoliation robs the plant of the leaf surface it needs to feed developing fruit, which lowers yield and leaves exposed fruit more prone to sunscald.
Prevention
Rotate tomatoes and other nightshades to a different bed each year, ideally waiting two to three years before replanting the same spot, since the fungus overwinters on old plant debris. Give plants plenty of room and stake or cage them so leaves dry quickly, and clean up fallen leaves and spent plants at season's end rather than leaving them in the garden. Start with certified disease-free transplants, choose vigorous varieties when available, and keep the soil healthy with a layer of mulch that blocks spores from splashing up from the ground.
How to manage it
At the first sign of spotting, pick off and remove the affected lower leaves and carry them out of the garden rather than composting them, which slows the fungus from spreading upward. Water at the base of the plant with a soaker hose or watering can instead of overhead, and water in the morning so any wet foliage dries fast; avoid handling or pruning plants while they are wet, since that spreads spores from leaf to leaf. Sanitize pruners between plants and keep a mulch layer in place to reduce soil splash.
General guidance for US home gardens, focused on prevention and non-chemical control. For a confirmed diagnosis and options specific to your area, contact your local Cooperative Extension office. Page updated Aug 4, 2026.