Late Blight Confirmed in a Western Maryland Greenhouse

Posted: May 18, 2016

On May 17, 2016, late blight was confirmed on tomato transplants in a greenhouse in western Maryland.
Characteristic symptoms of late blight on the upper surface of a tomato leaf.

Characteristic symptoms of late blight on the upper surface of a tomato leaf.

Tomato transplant samples were sent to Cornell for genotyping and determined to be US23. The past several years, US23 has been the predominant genotype along the east coast. The grower has destroyed the symptomatic plants and is now applying fungicides. The original source has not yet been determined but these plants were grown from seed and destined for the home garden market.

This is a good time to remind everyone that tomato and potato plants are susceptible to late blight, caused by the pathogen Phytophthora infestans, at any growth stage from a seedling to a mature plant. If observed in a greenhouse roguing out not only the symptomatic plants and flats but also roguing out all the surrounding non-symptomatic flats is important. Many of these plants are also likely infected but have not developed symptoms yet and may not for several days or weeks depending on the environmental conditions.

To identify late blight, look for lesions that are irregular in shape and initially water-soaked and pale-green before turning more gray-brown in color. Under humid conditions, the lesions on the underside of the leaves will sporulate giving them a white fuzzy appearance.

Although late blight did not run rampant across Pennsylvania last year, the relatively mild winter and current cool and wet conditions are favorable for late blight infected potato volunteers to be a potential source of the pathogen. This is the first report of late blight north of South Carolina this season. For more information on late blight disease outbreaks and management recommendations throughout the season check back to our website as well as the website.