The Homeowners Column

The Homeowners Column

Be on the look out for a potentially devastating tomato/potato disease – late blight

Photo of Sandra Mason

Sandra Mason
State Master Gardener Coordinator

This summer has been a plant pathologist's dream. Not a "lying on the beach" dream but more like internists huddled around an ailing patient. They finally see what they've only read about. A learning experience unless you happen to be the patient in the middle of the huddle.

Last week's column I discussed common tomato diseases – septoria and early blight. Both start as spots on lower leaves which can quickly yellow and then die but often remain attached. Septoria causes small water-soaked spots which become circular to angular with dark margins and grayish white centers. Early blight causes small brown leaf spots with a target-like series of concentric rings in each lesion. We see these fungal diseases to some degree every year, but this year diseases have been especially rampant.

However there is another disease we need to be wary of – late blight. It's not really a new disease but most years it doesn't make it to Illinois. This year the Mid-Atlantic and eastern states have been battling an outbreak of late bight which can be devastating to tomatoes and potatoes both commercially and in home gardens.

So why does this disease have potentially devastating effects? Remember from your history books that pivotal event called the Irish Potato Famine in the 1840's? Entire potato crops rotted in fields and storage bins. This is the same fungus - Phytopthora infestans. It is exceptionally contagious in tomatoes and potatoes.

Late blight does not overwinter here, however it can "blow in" during the season from other states where it does overwinter. In addition the late blight fungus can be moved through diseased potato tubers and tomato transplants. In the Northeast the late blight outbreak was traced back to diseased tomato transplants sold through several mass retailers.

With our cool, wet conditions this season late blight and other less common diseases may emerge in a larger than normal area. Late blight appears to be moving westward on weather patterns and just recently a late blight-infected tomato was confirmed in Northern Illinois through the UI Plant Clinic.

Late blight can also infect eggplant, petunia, ground cherries and numerous other plants in the potato-tomato family (Solanaceae). Common early symptoms on transplants are dark brown lesions on stems or leaves and sometimes white mold will develop. Seedlings quickly die.

If infection occurs on more mature plants, late blight symptoms on tomato plants include small (at least nickel sized) to large irregularly shaped, rapidly enlarging, water-soaked, pale green to greenish black lesions which usually start at the margins or tips of the leaves. In dry weather, these lesions turn dark brown, dry, and wither. A pale green "halo" often surrounds affected leaf areas. As spots enlarge entire leaflets are killed. Lesions can expand rapidly and result in extensive to complete defoliation within 2 weeks. Severely affected plants may appear as if damaged by frost. Symptoms are similar on potato plants.

Infection of both green and ripe tomatoes starts near the stem-end or the side of the fruit. It soon spreads over the entire fruit. Infected areas are dark green, brown, or brownish black and "greasy," with a rather firm but slightly wrinkled surface. A soft wet rot may develop.

A good rule is to always buy healthy transplants - no leaf or stem spots. In addition buy locally grown transplants when possible. A few tomato and potato varieties show resistance to late blight but not many. To manage late blight with fungicides requires treating before symptoms appear.

For more information about late blight - or or stop by our office for a copy.

If you are really confused about vegetable diseases, take a sample to your local UI Extension office or to the UI Plant Clinic1401 W. St. Mary's Road Urbana, IL 61802 PH: 217-333-0519.

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