The fungus-like organism that wreaked havoc with tomatoes and potatoes last year is back again this summer in Vermont. Phytophthora infestans, the pathogen causing Late Blight of tomatoes and potatoes, was identified in the UVM Plant Diagnostic Clinic August 3, on tomato foliage from two different backyard gardens located in Lamoille County. It is assumed the disease originated from infected volunteer potatoes in the area.

Although the disease can infect tomatillos, peppers, eggplant, petunias and other members of the potato family, we have seen the disease in the past couple of years in Vermont only on tomato and potato. The symptoms of late blight start on tomato and potato leaves as nickel sized water soaked spots. These spots do not typically start at the bottom of the plant like the other fungal blights. Under moist conditions whitish gray fungal growth can be seen on the leaf undersides. If the weather is wet or if there are morning fogs or lots of dew, the spots will spread rapidly throughout the plant in a matter of days. Stems and fruit can also be infected with the disease. Infected tomato fruit develop large brown areas. If plants are infected, the unaffected fruit on plants can be safely eaten but should not be canned.

Late blight spores are easily carried long distances on the wind, so anyone growing tomatoes or potatoes should be watching their plants daily for signs of the disease, and act quickly to destroy them in order to limit spread of the disease to other growers. Late blight needs living plant tissue to survive, so infected tomato plants should be destroyed as soon as the disease is identified. In small plantings, cut the plants and put in trash bags and send to the landfill. For larger plantings and farms, the plants should be cut, gathered in piles and burned, or turned under so they can decompose. Once the tomato tissue breaks down, it poses no late blight threat to future plantings since the pathogen does not survive on dead tissue.

If potato vines become infected, cut the tops (vines) before the stems become heavily infected. In small plantings, bagging and land filling vines will reduce the chance of spread to other plantings. Wait to dig the tubers at least 2 or 3 weeks to insure there is no living potato foliage, as that will 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, limiting the number of cuts and bruises created at harvest, reducing places for spores to infect tubers. For larger plantings when it is not practical to remove vines, cut vines on a hot dry day so they will dry and die quickly to reduce chances of spread to other plantings.

Hot dry weather can slow the spread of the disease but with rainy weather or heavy dews, fungicides are needed for protection. Homeowners can apply a garden fungicide labeled for tomato or potato use that contains the active ingredient chlorothalonil. Organic growers can apply a copper fungicide labeled for these crops. These products can only be effective if used before the disease appears and they should be reapplied every 5-7 days if wet weather persists. Fungicides will only protect healthy tissue – infected leaves cannot be saved. Good coverage of all the foliage is critical, and repeat applications are needed to protect new growth from infection. Always read the pesticide label and follow the instructions carefully.

To find more information about late blight and see pictures of the disease and other diseases that can look like late blight go to http://www.hort.cornell.edu/lateblight. You can also link to a webinar on the topic of late blight at this site.

For submitting samples to confirm if you have late blight in your garden, go to the UVM Extension Master Gardener website at http://www.uvm.edu/mastergardener/.

Commercial growers should consult the New England Vegetable Management Guide for information for fungicides labeled for late blight control on various crops, see http://www.nevegetable.org/.

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