Catherine W. Idzerda" />

Early news about late blight

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Catherine W. Idzerda
Monday, May 24, 2010
— Last year's tomato trauma left many local gardeners edgy and fearful.

Since there's nothing worse than edgy gardenersóthese people carry trowelsówe'd like to offer this basic primer on late blight.

Late blight? Is that when gardeners stay out late drinking and talking smack about each other's landscaping?

No, late blight is phytophthora infestans, a disease that can wipe out a tomato or potato crop in short order.

Last year, late blight swept through southern Wisconsin, freaking out home gardeners and sending the state's Department of Agriculture, Trade and Consumer Protection into red alert mode.

Organic farmers in Rock County lost endless rows of tomatoes and took a financial hit as a result. Home gardeners saw their dreams of fresh, sun-warmed tomatoes die on the vine.

Now, nearly a year later, gardeners are asking themselves:

Will the accursed late blight be back? Can it be prevented? And, most importantly, where did I put my trowel?

Here's what you need to know, courtesy of the UW Extension and the department of agriculture:

Q: What's the problem?

A: Late blight is a disease of tomatoes and potatoes caused by fungus that needs specific conditions to survive. Last year's cool, wet spring and summer were perfect for the disease's development.

Q: What does late blight look like?

A: UW-Madison plant pathologist Amanda Gevens described the progression of the disease in an Extension educational video that can be found at

Late blight usually starts as pale green to deep green lesions, or spots, on the leaves, Gevens explained. The lesions are sometimes described as "greasy" or "water soaked." As the spots dry out, they turn brown.

Sometimes, a white, mold-like substance can be found on the bottom of the leaves.

Lesions on the stems start out brown and change to black. Rot sets into the tomatoes at the stem end.

Q: Will it return this year?

A: "For a disease to form, the pathogen must be present, the environmental conditions must be right, and the plant must be susceptible," said Mike Maddox, UW Extension and Rotary Gardens educator. "If the conditions stay warm and dry this summer, our chance of infestation will be low.

"If you cleaned up your garden last year and left no infested plant debris in your compost pile, I suspect your chance for reinfection will be low."

Q: Where did it come from?

A: Gardening hell. The spores can be spread by the wind, can over winter on potatoes or on plants affected by the disease that haven't been disposed of properly.

Gevens noted that late blight can over winter in infected tubers and improperly composted plants.

Q: What can be done to prevent it?

A: Don't grow potatoes and tomatoes in the same place you grew them last year. Don't use potatoes from last year's crop for this year's crop. Do pull out any "volunteer" tomato seedlings. Grow tomato varieties that might be more resistant to fungal diseases.

Q: What else can you do to protect this year's plants?

A: Protective fungicides can be applied in advance of the disease and followed with a program of copper-based or other appropriate fungicides, Gevens said.

"If you want to use a preventative fungicide, as always, read the label to find what is appropriate for late blight and for your needs," Maddox said.

Check the Extension's late blight information for product recommendations.

Q: Anything else I should know?

A: To limit your gardening freakouts, it's crucial to understand the difference between late blight and other, common, nonletheal blights such as septoria leaf spot and early blight.

Early blight leaf blots have tan centers with concentric rings. Often the spots will have a yellow ring around them.

Septoria spots usually are 1/8 inch in diameter or smaller and have black spots in the center.

Last updated: 1:46 pm Thursday, December 13, 2012

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