Home gardeners: Kill diseased tomatoes and potatoes to prevent late blight next year

Editor’s note: The following information is being shared in partnership with the Michigan Potato Industry Commission. This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Late blight came in with a vengeance last summer. It caused severe tomato and potato losses, hitting home gardeners and organic farmers and larger commercial growers.

Action right now can help prevent a repeat next year, says Michigan State University plant pathologist Dr. Willie Kirk. If you have any damaged tomato or potato plants or rotting tomatoes or potato tubers, make sure they’re dead by next spring. The easy way is to let the winter cold kill them. Pull up the plants and lay them on the ground, and spread the remains of the bad tomatoes or potatoes on the surface.

Freezing temperatures should deal a killing blow both to the vegetation and to the nasty organism that causes late blight. This organism is a fungus-like water mold that attacks plants in the nightshade family including tomatoes and potatoes. Its most famous feat in history was causing the Irish potato famine in the 1840s.

The late-blight organism is still around in slightly different strains but as vicious as ever. Once established, it sweeps through fields, converting lush vegetation to skeletal remains in little more than a week.

Kirk notes that commercial tomato and potato growers can control the spread of the disease through carefully timed and applied fungicides. He estimates that Michigan potato growers alone spent an additional 2.5 million dollars this past growing season to defend their crops against the onslaught

Organic growers were especially vulnerable because they do not use synthetic pesticides on their crops. Especially hard hit this past summer were many Amish farmers.

Home gardeners also suffered throughout the Lower Peninsula. Many did not recognize what the symptoms meant until too late, and their control attempts may have been hit-or-miss with substances unsuited to stamp out the disease organism.

The abnormally wet summer contributed to the above-average incidence of late blight. The organism is well equipped to proliferate, developing spores so tiny they cannot be seen with the naked eye. They can drift on air currents across several counties or even farther. Since Michigan has so many large potato and tomato fields, the air-borne spores found an abundance of moist sites for settling out and destroying crops.

“Home gardeners may be disappointed at their losses, but organic growers were devastated. This is their livelihood,” Kirk says. “We don’t want the same thing to happen next year.”

He emphasizes that everybody from the hobby grower to the commercial producer is involved in the issue. Spores from an urban or suburban backyard garden can float out into the countryside and infect the plants of farmers.

“It’s important to make sure the organism gets killed this winter,” Kirk says. “Leftover potatoes should not be put in piles because interior tubers may resist freezing, and if they carry the organism, they allow a foothold for disease next year. Spread them out on the ground so they’ll freeze. Do the same for tomatoes and for uprooted potato and tomato plants.”

Another point to remember about potatoes is that if infected tubers remain underground, they may not freeze during the winter. As you pull out potato plants this fall, make sure any tubers attached come up so they also can be exposed to killing cold temperatures.

For more information, check Dr. Kirk’s website, www.lateblight.org.

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