Wednesday, February 17, 2010

The Chilling Story of Late Blight

Hearing the words “late blight” will send chills down a farmer’s back these days. Late blight is a fungus pathogen that affects (and kills) tomatoes, potatoes and a few weeds that are also in the Solanaceae family. It usually affects crops only sporadically, but this year it is sweeping far and wide, and striking early in the season! Late blight is especially strong this year because of all the rain we have had. It flourishes in cold, wet conditions. It is currently all over the north eastern US. Penn State Extension has reports of late blight in 24 counties as of July 27th. Monday morning we found late blight on our tomatoes! While it spreads easily and reproduces quickly, we have caught it early. With diligent care and a lot of luck it is possible to contain it.

We have ripped out about 5 plants and sprayed with a fungicide that is approved under organic standards. Simply ripping out the plants is not enough because spores from one plant can spread to another by wind.

The Latin name for late blight is Phytophthora infestans, which means “plant destroyer.” It looks like a black, greasy spot on the leaf or stem, which appears brown when it dries. One lesion can produce thousands of tiny, white spores. It can also appear in either green or ripe fruits. Under the right conditions, spores can travel miles and still survive to affect a new area. Once a lesion occurs on a plant, it can not be controlled. The only management that can be taken is to remove the affected plants and try to limit the development of new lesions. A lesion on one plant can start producing spores in 4 to 6 days. While late blight is hitting harder than usual this year because of all the rain we have had, even a morning dew is can create an environment for spreading.

Late blight originated in the highlands of Mexico, and was first recorded in the US in the early 1840’s. It traveled across the Atlantic on some seed potatoes, and caused the Irish potato famine in the 1850’s! It was common in the US until the 1970’s when an extremely effective fungicide was created. However, in the late 1980’s resistant strains of blight were found in Mexico. They spread to the US and now there have been serious epidemics in most areas of the US for the last few years.


Steven B. Johnson, Ph.D.
Univeristy of Maine
“Update on Potato and Tomato Late Blight Control in Organic Production”

Beth K. Gugino
Penn State Vegetable Pathologist
Pennsylvania Weekly Vegetable Disease Update, July 27, 2009

What is Late Blight?
Cornell University, New York State Integrated Pest Management,

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