BURLINGTON — The University of Vermont (UVM) Plant Diagnostic Clinic has identified its first case of late blight this year. The fungus-like organism was discovered on July 7 on potato foliage on an organic farm in Hinesburg.
Although the disease can infect tomatillos, eggplants, peppers, petunias and other members of the potato family, in Vermont it has only been found on tomatoes and potatoes in recent years. Damage from Phytophthora infestans, the pathogen causing late blight in these crops, was especially widespread in 2009.
The disease appeared in Vermont about five weeks later last year, but recent storms have most likely blown the spores in from western New York or New Jersey. Vigilance and quick action on the part of home gardeners and growers will help control its spread and protect commercial farmers’ tomato and potato crops.
“Late blight spores are easily carried long distances on the wind, so anyone growing tomatoes or potatoes should inspect their plants daily for signs of the disease,” says Ann Hazelrigg, UVM Extension plant pathologist. “The pathogen needs living plant tissue to survive, so once infected plants are destroyed, the spores will die and not spread the disease.”
Growers who don’t plan to aggressively combat the disease with fungicides should destroy plants immediately to limit the spread of the airborne spores to neighboring farms or gardens.
Late blight starts as nickel-sized, water-soaked spots on tomato and potato leaves. Unlike other fungal blights, the spots typically don’t start at the bottom of the plant but are seen first on the upper leaves.
Under moist conditions whitish-gray fungal growth can be found on the leaf undersides. Wet weather or morning fog or heavy dew will cause spots to spread rapidly throughout the plant in a matter of days.
“Stems and fruit also can be infected with the disease,” Hazelrigg notes. “Infected tomato fruit develops large brown areas. If plants are infected, the unaffected fruit on plants can be safely eaten but should not be canned.”
In small plantings, cut the plants and put in trash bags and send to the landfill. Do not compost the plants. For larger plantings and farm fields, cut the plants, gather in piles and burn or turn under so they can completely decompose. Once the plant tissue breaks down, it poses no late blight threat to future plantings since the pathogen cannot survive on dead tissue.
If potato vines become infected, cut the tops (vines) before the stems become heavily infected. In small plantings, bagging and putting vines in the landfill will reduce the chance of spread to other plantings.
Wait at least two or three weeks before digging the tubers to ensure that the potato foliage has died to limit the number of spores on the soil surface when the tubers are dug. It also allows time for the tuber skins to toughen up underground, minimizing the number of cuts and bruises created at harvest and reducing places for spores to infect tubers.
For larger plantings, if not practical to remove vines, cut vines on a hot, dry day. They will dry and die quickly, reducing the chance of spread to other plantings.
“Hot, dry weather can slow the spread of the disease,” Hazelrigg says, “but with rainy weather or heavy dews, use fungicides for protection. These products will only be effective if used before the disease appears. Reapply every five to seven days if wet weather persists.”
For home gardens, apply a garden fungicide labeled for tomato or potato use that contains the active ingredient chlorothalonil. Organic growers should use a copper fungicide labeled for these crops.
Fungicides will only protect healthy tissue, so infected leaves cannot be saved. Good coverage of all foliage is critical. Repeat applications are needed to protect new growth from infection. Be sure to always follow instructions on the pesticide label carefully.
To submit samples to confirm late blight in your garden, go to the UVM Extension Master Gardener website (www.uvm.edu/mastergardener) for instructions and images of the disease. Commercial growers should send samples to the UVM Plant Diagnostic Clinic (http://pss.uvm.edu/pd/pdc) and consult the New England Vegetable Management Guide (www.nevegetable.org) for information on fungicides labeled for late blight control on various crops.