Late blight infected tomato transplants found at home and garden center in Tompkins County, New York

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.  

Editor's note: This article was submitted by Mary Hausbeck, MSU Plant Pathology.

This article was originally published through Cornell University’s Cooperative Extension in Tompkins County on June 24, 2009.

Keith Perry, a colleague in Cornell’s Plant Pathology department, brought home some infected potted tomato plants from the Lowes store in Ithaca, New York (Tompkins Co.) on June 23. Plants are severely infected with late blight (Phytophthora infestans) with the symptoms indicative of the more virulent genotype of the pathogen. Plants at Lowes were provided by Bonnie Plants out of Alabama. There is a good likelihood that other late blight-infected plants could be on the plant shelves at other local sources (Home Depot, Walmart, Kmart, etc.) or already transplanted to home gardens. We are in the process of contacting the stores to get the infected plants destroyed.

At this point, we should assume that late blight has had the opportunity to spread throughout the county (especially with our favorable weather on Saturday and Sunday), and we assume that all tomatoes in our area are now at risk. Although this immediate threat would be to home gardeners, we do have considerable tomatoes grown commercially in the area and to our east, whether in the open field or in high tunnels. Plants provided by Bonnie Plants are widely distributed throughout the United States, so we can assume that late blight will be a threat in most areas where these plants have been distributed. All tomato varieties (conventional and heirloom) are susceptible, but some varieties, not currently available (like Mountain Magic), do carry genetic resistance for late blight.

If symptoms are found on any plants in the home garden, it would be prudent to destroy the plants immediately. Place the entire plant in a plastic bag and dispose of them into the garbage. If plants do not show signs of infection after thorough examination, then there still may be time to apply protectant sprays immediately and repeating if weather conditions warrant (warm temperature with rain or heavy dews). Characteristic symptoms are illustrated at: and elsewhere on the Veg MD web site.

The fungicide chlorothalonil is used commercially and is effective when used before the disease appears. Look for the common name chlorothalonil on home garden products from major suppliers like Bonide, Ortho and others. At this time we do not know if the genotype will also infect potato, but since late blight has been reported on potato in other states to the south of us (in New York), all potato growers (homeowners and commercial concerns) should be on a week schedule with fungicide sprays.

If you are growing either tomatoes or potatoes organically, then your options for control are limited. Copper sprays offer some help, but need to be applied preventatively and often. In New York state, two copper products that are listed by OMRI and are registered in New York for organic use are Basic Copper 53 and Nu-Cop in several formulations. Homeowners may also choose to apply these or other copper products, but control will not be as successful as when chlorothalonil is used regularly.

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