2008 Annual Report 1a.Objectives (from AD-416) Monitor and conduct genetic analysis of strains of Phytophthora infestans in the United States using molecular detection methods for pathogen identification and fungicide resistance in field populations received from potato growers, extension agents and scientists. 1b.Approach (from AD-416) Determine if a dramatic strain replacement has occurred using a multi-year characterization of the P. infestans populations. We will analyze pathogenic/aggressiveness tests of isolates to determine if one lineage represents the primary pathogen population of that host, since the isolation of pathogen genotypes from a host is not an indication that they are primary pathogens of that host. Pathogens may at times infect alternative hosts, albeit weakly. Therefore, the primary purpose of pathogenicity tests is to detect host specificity among isolates of the same clonal lineage from different hosts. The detached-leaflet technique used with a balanced incomplete block design, which allows precise assessment of fitness parameters of a substantial number of isolates within a period of weeks, should also prove useful. We will also isolate and characterize P. infestans from alternate and novel hosts i.e., Petunia, woody, and black night shade. 3.Progress Report Late blight, caused by the oomycete pathogen Phytophthora infestans, has become an increasingly important problem to agriculture in the United States and many other countries in the past decade. More aggressive, fungicide-resistant and host-specialized isolates have appeared attacking potato and tomato crops. In the United States, the pathogen has been reported as being more genetically diverse on tomato than on potato. Since the migrations of P. infestans from its presumed center of origin, Mexico, beginning in the late 1970s, major population displacements and genetic changes in populations outside Mexico have occurred. The presence of these new genotypes resulted in renewed research on the pathogen since it might now reproduce sexually due to the introduction of the A2 mating type, because new populations demonstrated increased aggressiveness. The collation of molecular marker data with information on mating type (thus likely sexual reproduction) and fungicide resistance is a fundamental aim of the project. This data has allowed us to determine the mechanisms and tempo of genetic change within populations. This consideration of the impact of new lineages on the epidemiology of late blight in the United States indicates that there is now greater diversity within the tomato population of P. infestans than prior to the 1990s. The new lineages differ from old lineage(s) in some, but not all, epidemiologically important aspects. The occurrence of several novel lineages implies that there will be some diversity in the epidemiology of the disease. The implications to pathologists are that the differences need to be identified, quantified and incorporated into disease management recommendations. This information will benefit other scientists, plant breeders, and various producers. Progress was monitored by e-mail correspondence.