Courtesy Purdue Extension/ Bethany Ratts

In 2009, late blight affected tomato and potato crop across the Northeast and Midwest, but the threat of the disease returning in 2010 is unlikely.

After late blight was such a burden in 2009, tomato growers in the Northeast and Midwest wonder if the potentially fatal disease will hit crops again this year. Experts at Purdue University say a repeat of last year’s outbreak is unlikely. 

Although late blight, which thrives in cool, damp conditions, should not return as aggressively as it did in 2009, tomato growers still should be aware of symptoms, says Daniel Egel, a plant pathologist at the Purdue cooperative extension.

“Late blight causes large brown lesions on tomato leaves and stems that under moist conditions are often ringed with the white fungus,” Egel says. “Symptoms caused by late blight may look like other common tomato diseases and, thus, may be easily missed if not sent for accurate diagnosis.”

Late blight, which is caused by a fungus called Phytophthora infestans, is spread by spores carried by wind, rain, or through seeds or transplants. It does not live in soil or dead plants. The late blight outbreak in 2009 is believed to have been caused by infected tomato transplants sold to homeowners at retail stores and spread to other plants before it could be stopped.

“These plants were grown in the South, where late blight is more likely to overwinter, and shipped north,” Egel says. “From homeowners, the disease jumped to commercial growers as well.”

Tomato growers, including those who grow in containers, who suspect the disease is affecting their plants can contact a county educator and send a sample to a plant and pest diagnostic laboratory. Call your local cooperative extension to see if it offers this service.

Send the infected part of the plant to be tested, says Egel. Late blight symptoms can show up on the leaves, stems or fruit.

“Roots of tomato plants won’t show symptoms, but potato tubers might,” he says.

In the case of confirmed late blight, pesticides containing the fungicide chlorothalonil can be used to help stop the spread of the disease to uninfected plants. Organic growers may find that copper products will slow the spread of late blight.

If you notice your tomatoes showing symptoms of late blight, take these steps to extinguish the late blight fungus, and keep you and your 2010 crop safe:

1. Do not eat the fruit. Plants infected with late blight can cause illness.
2. Do not compost plants infected with late blight.
3. Destroy this year’s volunteer tomatoes if your crop was affected by late blight in 2009.
4. Do not use 2009 potato seeds when planting the 2010 crop.
5. Avoid tomatoes or potatoes that overwintered in greenhouses. These plants potentially could have harbored the late blight fungus.
6. Inspect tomato transplants before buying. Avoid buying those that show symptoms of disease.