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Late Blight Caution

July 9, 2010

In 2009, many gardeners in Allegheny County lost their tomato harvest to late blight.  Late blight, caused by Phytophthora infestans, is the same fungal disease that wiped out potatoes in Ireland in the 1840’s leading to the potato famine.  Late blight affects other members of the Solanacea family, including eggplant and peppers, though not to the same devastating degree as tomatoes and potatoes.  

The disease is spread by the wind or by rain splash during wet summer days when temperatures are moderate, around 60 to 80oF.  Lower leaves will show grey to brown spots spreading to the entire leaf.  As the disease spreads, a white mold will develop on leaves, stems will show dark brown lesions and fruit will turn brown and rot on the vine.  Once the fungus is present, there is no means of control and infected plants need to be pulled out, placed in trash bags and sent to a landfill!!!

Prevention is the most effective control.  The fungus can survive Pennsylvania winters in living tissue.  Any potatoes left in the ground un-harvested should be destroyed as well as any potato or tomato volunteers.  Scout your plants at least twice a week to monitor for signs of the disease.  Resistant strands of potatoes and tomatoes should be planted.  Only tubers free of P. Infestans should be planted; ask your seed potato producer to be sure. 

Doug Oster, author and organic garden writer for the Pittsburgh Post Gazette, recommends spraying Serenade NOW (before any problems are seen) to prevent tomato late blight.  Serenade is an organically accepted biological control of late blight.  Sandy Feather of the Penn State Cooperative Extension discusses late blight and the use of Serenade in a recent article.  Information on other organic controls are provided by the National Sustainable Agriculture Information Service, ATTRA .

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