Late blight (Phytophthora infestans) is the cause of the fungal growth on this tomato plant.

by Ann L. Hazelrigg

The fungus-like organism that wreaked havoc with tomatoes and potatoes in 2009 is back again this summer in Vermont.  Phytophthora infestans, the pathogen causing late blight of tomatoes and potatoes, was identified in the University of Vermont Plant Diagnostic Clinic Aug. 3, on tomato foliage from a garden in Jericho. 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, in the past couple of years the disease has only appeared on tomatoes and potatoes in Vermont. 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 also can 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 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 putting vines in the land fill will reduce the chance of spread to other plantings.

Wait to dig the tubers at least two or three weeks to insure that 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 and 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 should be reapplied every five to seven 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. 

For more information about late blight, including pictures of the disease and other diseases that can look like late blight, go to http://www.hort.cornell.edu/lateblight.

To submit samples to confirm if you have late blight in your garden, go to the UVM Extension Master Gardener web site at http://www.uvm.edu/mastergardener/.

Commercial growers should consult the New England Vegetable Management Guide for information on fungicides labeled for late blight control on various crops. The guide can be found online at http://www.nevegetable.org/.