Q&A: Tomatoes That Fail to Ripen
Q: Last fall I tried to ripen some green tomatoes indoors, but they developed sunken spots and later rotted. Why did this happen?
— Robert F., Plymouth, N.H.
A: The problem is anthracnose, says Otho Wells, vegetable specialist at the University of New Hampshire in Durham. Anthracnose fungal spores overwinter on plant debris. They're released during wet weather and spread by wind and splashing water onto tomato plants. Although the disease can infect tomato leaves, it's primarily a problem on the fruits. The disease isn't visible on green fruits, but once they begin to ripen, sunken, 1/2-inch-diameter black spots develop. Secondary rot organisms follow the initial infection and the tomatoes can't be saved, says Wells. Anthracnose can occur all season long but is mostly a problem during wet weather and when plants have a severe early blight infection.
To control the disease, try to prevent the spores from landing on developing fruit by planting tomatoes in black plastic mulch or under grow tunnels. Other prevention measures such as thoroughly cleaning up any plant debris in fall and rotating tomatoes to another part of the garden will help. If the disease is very severe, you can spray chlorothalonil or captan fungicide weekly during wet weather.