ATTRA - National Sustainable Agriculture Information Service
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Late Blight Resources from ATTRA

July 2009

Potatoes infected with late blight are purplish and shrunken on the outside, corky and rotted inside.
Photo by Scott Bauer © ARS 2004

What to do
Catching an outbreak in its earliest stages can reduce losses and increase options for control. Growers should check fields twice a week. It is important to look at leaves and stems under the canopy, as this is where the disease gets established first. Farmers who think they have late blight should contact their local Cooperative Extension Office for more information.

What late blight looks like
The first sign of infected tissue is a water-soaked appearance of the leaves, which, in dry weather, quickly turn dark brown and brittle. Infected areas may be surrounded by a halo of chlorotic, or yellowed, tissue. During moist weather, a white cottony growth will develop on the underside of the leaves. Infected stems and petioles will turn dark brown or black.

Symptoms first show up around low-lying areas, ponds or creeks, near center-pivot irrigation rigs and in places protected from wind. Early planted fields are likely to be affected first. The ideal conditions for an epidemic of late blight are when night temperatures are 50 to 60 degrees Fahrenheit, along with fog, heavy dew, rain or overhead irrigation, accompanied by daytime temperatures of 60 to 70 degrees.

For complete information on managing late blight, including the use of copper spays and compost teas, read our publication Organic Alternatives for Late Blight Control in Potatoes.

Sanitation is the first line of defense against late blight. Avoid piling and leaving culls. Culled potatoes should be disked, buried, composted or otherwise disposed of before the new crop emerges. The duration of leaf wetness is a critical factor in late blight infection. Sprinkler irrigation should be carefully scheduled or minimized, particularly late in the season when the closed potato canopy provides ideal conditions for late blight development.

For more information, check out these ATTRA resources or call:
1-800-346-9140 (English) or 1-800-411-3222 (Español).

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Last Updated July 15, 2009

ATTRA - National Sustainable Agriculture Information Service is managed by the National Center for Appropriate Technology (NCAT) and is funded under a grant from the United States Department of Agriculture's Rural Business-Cooperative Service. Visit the NCAT Web site for more information on our sustainable agriculture projects.

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