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Recent Genotypes of Phytophthora infestans in the Eastern United States Reveal Clonal Populations and Reappearance of Mefenoxam Sensitivity

September 2012 , Volume 96 , Number  9
Pages  1,323 - 1,330

Chia-Hui Hu, Department of Plant Pathology, North Carolina State University, Raleigh, 27695; Frances G. Perez, United States Department of Agriculture–Agricultural Research Service (USDA-ARS) PSI-GIFVL, BARC-West, Beltsville, MD, 20705; Ryan Donahoo, University of Florida, Florida IFAS-SWFREC, Immokalee, 34142; Adele McLeod, Department of Plant Pathology, University of Stellenbosch, Stellenbosch, South Africa; Kevin Myers, Department of Plant Pathology, Cornell University, Ithaca, NY; Kelly Ivors, Department of Plant Pathology, North Carolina State University; Gary Secor, Department of Plant Pathology, North Dakota State University, Fargo, 58108-6050; Pamela D. Roberts, University of Florida, Florida IFAS-SWFREC; Kenneth L. Deahl, USDA-ARS PSI-GIFVL, BARC-West; William E. Fry, Department of Plant Pathology, Cornell University; and Jean B. Ristaino, Department of Plant Pathology, North Carolina State University

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Accepted for publication 2 April 2012.

Isolates of Phytophthora infestans (n = 178) were collected in 2002 to 2009 from the eastern United States, Midwestern United States, and eastern Canada. Multilocus genotypes were defined using allozyme genotyping, and DNA fingerprinting with the RG-57 probe. Several previously described and three new mulitilocus genotypes were detected. The US-8 genotype was found commonly on commercial potato crops but not on tomato. US-20 was found on tomato in North Carolina from 2002 through 2007 and in Florida in 2005. US-21 was found on tomato in North Carolina in 2005 and Florida in 2006 and 2007. US-22 was detected on tomato in 2007 in Tennessee and New York and became widespread in 2009. US-22 was found in 12 states on tomato and potato and was spread on tomato transplants. This genotype accounted for about 60% of all the isolates genotyped. The US-23 genotype was found in Maryland, Virginia, Pennsylvania, and Delaware on both tomato and potato in 2009. The US-24 genotype was found only in North Dakota in 2009. A1 and A2 mating types were found in close proximity on potato and tomato crops in Pennsylvania and Virginia; therefore, the possibility of sexual reproduction should be monitored. Whereas most individuals of US-8 and US-20 were resistant to mefenoxam, US-21 appeared to be intermediately sensitive, and isolates of US-22, US-23, and US-24 were largely sensitive to mefenoxam. On the basis of sequence analysis of the ras gene, these latter three genotypes appear to have been derived from a common ancestor. Further field and laboratory studies are underway using simple sequence repeat genotyping to monitor current changes in the population structure of P. infestans causing late blight in North America.

© 2012 The American Phytopathological Society