Cool, wet weather causes tomato and potato blight |

Cool, wet weather causes tomato and potato blight

LINCOLN, Neb. – Cool, wet and foggy conditions this last week across parts of Nebraska are responsible for an unusual late-season tomato and potato disease that can quickly kill the plants.

Late blight is striking potato and tomato plants, said Amy Ziems, University of Nebraska-Lincoln Plant and Pest Diagnostic Clinic coordinator.

“We have confirmed the blight in the Fremont area and in the Omaha metro area,” Ziems said. “It is being found because of the cool and wet conditions we’ve been having with temperatures at the 70 and 80 degree mark.”

Blight can kill the plant’s leaves after two to three days and cause the tomato fruit and potato tubers to rot after they’ve been picked. Signs of late blight include a dark green water-silk lesion. As it continues to grow, the edge turns brown and the lesion will dry up. Under really humid and foggy conditions, white spores will be present underneath the leaf. On the fruit, it causes at first a dark, greasy looking lesion that within two days will completely expand a soft rot to the whole fruit so it can’t be eaten.

This is similar for potatoes.

“If you have infected potato plants, it can go down into the tuber, and a lot of times cause a soft rot while in storage,” she said. “If you harvest potatoes in the next month or two, they may look fine, but when you go back and get the potato out of your storage area in December or January, it will be completely rotten.”

This is the same pathogen that caused the 1850s potato famine in Ireland, causing millions of people to immigrate from Ireland to the United States to avoid starvation. Ziems said there has been a sizable outbreak in the northeast United States, which started in June. It hit commercial organic tomato production especially hard. Organic tomato prices have skyrocketed in the northeast and many organic tomato producers had to go out of organic production.

Fungicides are available to treat the disease, but they must be applied immediately after symptoms are spotted. Be sure to follow label instructions. The fruit will still be safe to eat after applying the fungicide.

Gardeners with infected plants this year should remove them immediately and throw them away.

“Don’t put them in the compost pile, throw them in the trash,” Ziems said. “The disease can overwinter if a compost pile doesn’t get warm enough.”

For more information about late blight, consult the Sept. 3 episode of “Backyard Farmer” available at

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