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Late Blight Detected in Blair, Mifflin, and Franklin Counties

Posted: June 6, 2012

Gardeners and commercial vegetable growers throughout Pennsylvania are once again forced to face one of the most notorious diseases in the world, late blight.

While local residents hope that they won’t have to re-live the Irish Potato Famine of the 1840’s, late blight has once again been detected in potato and tomato fields in Blair, Franklin, and Mifflin Counties. The pathogen that causes late blight, Phytophthora infestans infects tomatoes, potatoes, tomatillos, petunias, and other members of the nightshade family when temperatures are cool and when there is ample moisture to nurture their wind-blown spores.

Gardeners and commercial growers should examine susceptible plants daily for signs of infection. Many home gardeners noted that that this disease wiped out their plants in 3-5 days in 2009 after the detection of the first lesion on a tomato or potato leaf and we are receiving reports from growers that this strain appears to be killing unprotected potato fields in 2-3 days.  Late blight starts out as a greenish gray to black spot or lesion on the leaf surface. As the disease progresses, you will begin to see dark black to brown lesions develop on the stems of tomatoes and potatoes.  If the disease is allowed to progress, gray colored spores will develop on the undersides of the leaves. These spores when mature can be picked-up by animals and people, blown by the wind, and carried on tools or cultivation equipment to susceptible plants.

Home gardeners upon detection of this disease on tomato plants should cover the infected plant with a large black plastic trash bag and pull the plant up roots and all. The gardener should tie the trash bag up and allow it to sit out in the sun for several days to hopefully kill the spores and plant tissues. Once the tomato refuse has dried in the garbage bag, dispose of the bag with the tomato debris inside though your local trash service. Under no condition should gardeners make any effort to compost plants infected with late blight. Unfortunately, spores will continue to develop and mature on infected green tissues in the compost pile and could be blown by the wind to neighboring solanaceous crops. If a gardener does not have trash service, they should bury the infected plants under 12-18 inches of soil far away from the vegetable production area.

Gardeners who note late blight symptoms in their potatoes need to react very quickly to save their underground tubers. As soon as the first signs of infection are detected, mow the tops of the potato plants off at ground level and allow the plant tissues to dry thoroughly. Once the potato tissues have dried thoroughly for a couple of weeks you can begin digging the tubers. Unfortunately, infected potato tubers can look perfect after digging, but will begin to rot in storage. Inspect all potato tubers carefully after digging and discard all tubers that display any obvious black or brown lesions on the skin. Do not attempt to compost infected potato tubers or you may increase the risk of infection in your area in 2013. Bury infected tubers 12-18 inches below the soil or have them removed through your local trash service.

Gardeners who have tomato or potato plants which display no visible signs of infection can apply a properly labeled fungicide like chlorothalonil, mancozeb, or a fixed copper to plants on a 5-7 day schedule. Commercial growers should consider tank-mixing a protectant fungicide like chlorothalonil or mancozeb with a fungicide like Tanos, Curzate, Ranman, Forum (potato only), Revus Top, Gavel (contains mancozeb), Presidio (tomato only) or Previcur Flex.

For additional information on late blight and/or other commercial vegetable production problems please contact Extension Educator Tom Ford at (814) 472-7986.