In 2009, late blight devastated many tomato and potato crops throughout New York State. Late blight is predicted to affect our vegetable crops again this year, since we had such a warm winter followed by a wet and humid spring. Discarded potatoes and volunteers that had late blight last year may not have been killed during our mild winter weather and could soon exhibit signs of the disease while already infecting other plants.
It is important that we all are vigilant in our garden scouting program this year to reduce the likelihood of infection. "This is a community disease because it is so contagious and destructive. We all need to work together to manage it," said Meg McGrath, Ph.D., plant pathologist at Cornell University's Long Island Horticultural Research and Extension Center. "An outbreak cannot be left unmanaged, no matter how small, due to the potential impact on others growing these crops."
What Does Late Blight Look Like?
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When scouting for the disease-look for brown spots, or lesions, on the stems, with white fungal growth developing. If the stem is OK but you start to see at least nickel-size olive-green or brown spots on leaves with some white fungal "fuzzy" growth on the underside of leaves in the early morning or after rain, your plant has most likely been struck by the disease. Sometimes the spot is surrounded by yellow or looks water-soaked. Another clue is late blight tends to develop randomly on the plant, and not only on the bottom leaves. Spotting late blight early isn't always easy; it resembles other tomato plant problems like drought stress, early blight, septoria leaf spot, and gray mold, making it hard for gardeners to make a positive ID.
What Can You Do To Manage Late Blight?
1. Select resistant tomato varieties (e.g. Defiant, Mt. Magic, Mt. Merit, Matt's Wild Cherry, Plum Regal, Legends, etc.) and certified seed potato. Note: while "certified" means less chance of infection, no seed is guaranteed to be pathogen-free.
2. Apply fungicide weekly. Understand symptoms cannot be cured plus this disease can develop rapidly; so late blight is harder to manage when fungicides are applied after symptoms are seen. For those who prefer not to use any fungicides, understand that late blight cannot be left unmanaged because of the potential impact on others' plants (commercial and home gardeners); destroy affected plants as soon as the disease is identified. Cooper is a good choice for organically produced plants; chlorothalonil is the most effective conventional fungicide ingredient available to gardeners. Before using any fungicide, read the entire label, and understand the safety information.
3. Learn symptoms and inspect plants at least weekly after planting. Take samples of any plant with symptoms you suspect are late blight to your local extension office right away for diagnosis or submit suspect plants or tubers promptly to the diagnostic clinic: 334 Plant Science Building Ithaca, NY 14850, 607-255-7860 - plantclinic.cornell.edu.
4. If late blight is confirmed, promptly:
bag or destroy affected plant tissue right way,
do not compost plants with late blight,
notify neighbors, and
continue applying fungicides weekly if plants are kept.
5. Visit usablight.org/ to learn more about late blight, its symptoms and management (including resistant varieties), and for current information on late light outbreaks.
"If we all work together we can prevent a major epidemic from developing," McGrath says. "It absolutely can be controlled if everyone is on top if it."
For an informational brochure on Late Blight, please stop by the Frank Bratt Ag Center Monday-Friday 8 a.m. to 4:30 p.m. or go online to usablight.org/node/50.
Resources Used:
"2012: End of Organic Tomatoes" by Leah Zerbe, Rodale News
"Effectively growing Tomatoes & Potatoes requires understanding and responding to Late Blight" brochure USAblight.org
The mission of the Chautauqua County Master Gardener Program is to educate and serve the community, utilizing university and research-based horticultural information. Volunteers are from the community who have successfully completed 50 plus hours of Cornell approved training and volunteer a minimum of 50 hours per year.
For more information on the Master Gardener Program, please contact:
Betsy Burgeson, Master Gardener Coordinator, 664-9502, ext. 204, or email Emh92@cornell.edu. "Like" the Chautauqua County Master Gardeners on Facebook for gardening news and information.