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Late Blight Coming On Early

Late Blight Coming On Early

Scout for this plague of tomatoes and potatoes; Address quickly.

University of Vermont Extension Plant Pathologist Ann Hazelrigg is advising commercial growers and gardeners to be on the lookout for late blight in tomatoes and potatoes. Scouting is the key to avoiding the same level of devastation that occurred in 2009.

The fungus-like organism that wreaked havoc with tomatoes and potatoes in 2009 is back again this summer in Vermont.

BAD SIGN: Tomatoes with brown spots on middle to upper leaves may be infected by late blight. Since it may be confused with heat stress symptoms, have an Extension expert confirm the symptoms. Photo courtesy of Cornell University

The Phytophthora infestans organism causing late blight of tomatoes and potatoes was identified in the University of Vermont Plant Diagnostic Clinic on Wednesday. And there have been other confirmations in other eastern states. The disease also can infect tomatillos, peppers, eggplant, petunias and other members of the potato family.

Symptoms begin as nickel-sized water-soaked spots on leaves. These spots don't 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. If there are morning fogs or lots of dew, the spots will spread rapidly throughout the plant in a matter of days, says Hazelrigg.

Stems and fruit also can be infected. Infected tomato fruit develop large brown areas. If plants are infected, 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. 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.

For large 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 doesn't survive on dead issue.

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 ensure that there's no living potato foliage as that'll 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 isn't 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 protect only 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.

For more information about late blight, including pictures of the disease and other diseases that can look like late blight, go to You also can link to a webinar on the topic of late blight at this web site.

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