The Q&A Archives: Tomatoes have black spots

Question: I grew beautiful tomato plants last year but when I picked the fruits, they had black spots on them and rotten areas underneath the spots. What causes this disease? David Jackson Georgetown, MA

Answer: Cool, damp weather made 1992 a banner year for foliar disease on tomatoes in our area. The organism on your tomatoes was probably bacterial spot, says Ed Marrotte, consumer horticulturist at the University of Connecticut in Storrs. Bacterial spot overwinters in plant debris in the soil and reinfects leaves and young fruit by splashing up when it rains, You'll see some black, circular, water soaked spots on the leaves, but bacterial spot mostly affects the fruit, says Marrotte. The green fruit will have small, black specks that are hardly noticeable until the fruit ripens and the specks enlarge to become black, scabby spots, he says. If these 1/4 inch diameter craters crack, fungal rot organisms can also invade. There are steps you can take to control the disease. Mulch the tomatoes to avoid rain splashing soil up onto the plants. Keep the foliage dry by spacing the plants at least three feet apart and staking or caging them. And eliminate related weeds, such as nightshade, that can host the disease. If the infection is severe, you can apply a fixed copper spray, which gives better coverage than copper dust, at the first sign of disease and repeat spraying every seven to 10 days if it's rainy, says Marrotte.

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