![]() ![]() Spraying must continue regularly, using a product containing chlorothalonil, a state restricted fungicide which requires certification to use. Commercial growers wishing to control late blight should begin spraying fungicide immediately, even before symptoms are spotted. Any gardeners who suspect they have tomato or potato plants infected with late blight should dig them up, place them in plastic bags, and dispose of them in the trash. This is a serious, destructive disease that can spread quite rapidly when conditions are right, infecting an entire field within days. ![]() Eventually the lesions turn black, leaves start to die, and then the entire plant dies. Under moist conditions, there is a white, fuzzy growth on the underside of the leaves where the lesions occur, but the absence of this growth does not rule out late blight. Symptoms of late blight include small olive green or brown lesions on the upper surface of the foliage or the stems. Spores can disperse from one to several miles from the point of origin, with the infection spreading most efficiently in conditions of high moisture and temperatures ranging from 60° to 80☏. Late blight, the disease that was responsible for the Irish potato famine in the mid-19th century, is caused by a fungus-like pathogen that spreads through splashing rain or wind currents. Because a few instances of late blight have now been detected on tomato plants in our state, the Massachusetts Department of Agricultural Resources (MDAR) is asking anyone growing tomatoes or potatoes to monitor them for signs of the disease, in order to prevent its further spread. The late blight has been identified on tomato transplants sold in big box stores and other garden centers under the brand name Bonnie Plants, and has also been found in a potato field in Pennsylvania. Pack up diseased plants in a plastic garbage bag and discard – don’t leave them in the field or garden or on a compost pile, where the pathogen can be released into the air and spread to other plants.Late blight, a destructive disease caused by Phytophthora infestans, is a pathogen of tomato and potato plants that has recently been found in several states in the Northeast, including Maine, New York and Pennsylvania. Home gardeners should consider spraying tomatoes and potatoes with a fungicide containing chlorothalanil, and should prune out diseased tissue. This is particularly important for organic tomato or potato growers. If the disease is mainly found in one or a few foci, it is a good idea to remove and destroy the diseased plants. Organic producers must rely on applications of approved copper-based products. So far, Ohio late blight strains have been sensitive to Ridomil, so that is another option (but may be risky). Note the Pre-Harvest Interval (PHI) for these fungicides ranges from 0 – 5 days. Under cool wet conditions when late blight is likely, or if late blight has been found on the farm, use one of the following, tank mixed with one of the above protectants: Curzate (3 day PHI), Gavel (5 day PHI), Presidio (2 day PHI), Previcur Flex (5 day PHI), Ranman (0 day PHI) or Tanos (3 day PHI) to the spray tank with a protectant fungicide. Good protectants are chlorothalanil (Bravo, Equus, Echo), mancozeb (Penncozeb, Manzate, Dithane), and to a lesser extent, copper-based products – use according to label instructions. ![]() If late blight has not been observed and weather conditions are generally dry and warm, use a protectant fungicide on a 7-10 day schedule, depending on how fast the plants are growing. This should continue as long as rainy conditions, high humidity and/or heavy dews are expected. Growers need to maintain an effective fungicide program on tomatoes and potatoes. We appreciate hearing from you if late blight is suspected so that we can confirm and alert others. Wayne Ellett Plant and Pest Diagnostic Clinic in Reynoldsburg. If late blight is suspected, it can be confirmed by bringing or sending a sample to the OSU Vegetable Pathology Lab in Wooster or the OSU C. I t is very important that both potatoes and tomatoes be scouted regularly (at least twice per week) for late blight.
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