Septoria leaf spot on tomatoes Septoria lycopersici
IDENTIFICATION — Right after the tomatoes begin forming in early summer, septoria leaf spot starts showing up on the leaves closest to the ground. These irregular spots start out yellow, age to brown and then turn black as they begin producing spores. Spots range in size from 1/16 to 1/4 in. in diameter.
FAVORITE CONDITIONS — Septoria thrives in warm, wet weather as wind and rain spread spores that settle in plant debris and soil. So the first problems you’ll see are on the lower half of the plant — rain splashes the spores onto the leaves. As the disease moves up the plant, leaves turn yellow and fall off prematurely. Eventually, the plant is defoliated. Without leaves to shelter them, the tomatoes develop sunscald and may rot.
CONTROL — To get rid of septoria leaf spot, clean up plant debris this fall. Then spores won’t have a place to spend the winter. Then next year during the growing season, try not to water overhead and make sure you get rid of weeds, especially horsenettle, jimsonweed and nightshade. (They are hosts for this fungal pest.) If this is a recurring problem, rotate your tomato crop every year, also avoiding other related plants, such as peppers, potatoes and eggplant. For a really large problem, use a fungicide such as Liqui-Cop®, a liquid copper spray, according to label directions.