Tomato Late Blight Identified in Allegany County, Maryland

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland;

Late blight on tomato was detected last week in Allegany County, Maryland. In addition, there have been recent reports from several counties in Pennsylvania. Tomato growers in Delaware and on the eastern shore of Maryland should scout the crop for late blight. The pathogen (Phytophthora infestans) strain present in Maryland is US23, which affects both tomato and potato. In addition, US23 is sensitive to fungicides which contain the active ingredient mefenoxam, such as Ridomil Gold Copper. It is late in the growing season and many growers may decide that it is not economically justified to spray for late blight. However, for those growers that want to extend the harvest, the following additional products are effective: Curzate, Forum, Presidio, Previcur Flex, Ranman, Reason, Revus Top and Tanos. These products should be applied with a protectant fungicide such as chlorothalonil.

Organic growers who want to extend their tomato harvest are advised to apply a copper fungicide that is approved by your organic certifier.

Characteristics of strains of Phytophthora infestans recently detected in the United States

Lineage Mating type Host specialization Mefenoxam sensitivity
US8 A2 Potato I-R
US11 A1 Potato/tomato R
US22 A2 Potato/tomato S
US23 A1 Potato/tomato S
US24 A1 Potato S (some I)a

aMost individuals of US24 appear sensitive (S) to mefenoxam, but others appear intermediate (I) in their sensitivity based on an in vitro assay using mefenoxam-amended plates. US11 has been consistently resistant (R).

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