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Be on the lookout for symptoms of tomato late blight

TomatoesBy Daniel Robison

Late blight, a fungal disease that infects tomatoes, usually shows up in Oregon gardens as weather turns wet and humid, and it’s dispersed by the wind and rain.

This devastating disease kills tomato and potato plants, as well as peppers and eggplant, and usually does not arrive until mid-August or September.

According to Ross Penhallegon, Oregon State University Extension horticulturist in Lane County, the disease is holding off because of the current warm weather. Once it turns wet and cool, start looking for late blight, Penhallegon advises.

Gardeners can take steps to keep late blight from spreading, and prevention is the best cure. Penhallegon recommends watering tomatoes around the base and not from above to avoid prolonged wetting of leaves. Make sure to give plants space. Stake and prune to keep air circulating and plants dry.

The fungus that causes late blight is the same one that caused the Irish potato famine of the 1840s. It can be brought to gardens with purchased tomato starts, or it can come from volunteer potato or transplanted tomato plants, or even from spores that blow in from infected gardens, Penhallegon said.

“Fortunately, there is an OSU variety that is resistant to the blight: Legend,” said Penhallegon. More information on the variety can be found at

You'll recognize the disease on tomato plants that have irregular, greenish water-soaked spots on the leaves and stems. Under cool, moist conditions, the spots rapidly enlarge to form purplish black lesions. The lesions girdle the stems and leaves, killing the foliage.

Penhallegon's advice is to destroy volunteer tomato and potato plants as well as plants that are obviously diseased. Put them in a plastic bag and into the trash. Do not compost them.

Good garden sanitization is critical to combat late blight. Clean your gardening and pruning tools with alcohol or a 10-percent bleach solution. Do not prune your tomatoes without sanitizing the equipment.

"Several years ago an entire greenhouse was contaminated when the pruners were not sanitized, spreading the disease to the rest of the greenhouse plants," Penhallegon said.

For more information, call or drop by your local county office of the OSU Extension Service.

Image above: Photo by Lynn Ketchum. An ounce of prevention is worth a pound of cure for late blight in tomatoes, say OSU gardening experts.


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