The influence of competition and aggressiveness on selection of Phytophthora infestans populations from Michigan State

From 1994-1996 devastating epidemics of potato late blight (caused by Phytophthora infestans) spread across the U.S. The outbreaks have been linked to the emergence of a new clonal lineage named US-8, which is currently the most common genotype found in the U.S. In this investigation, US-8 isolates were compared with other U.S. genotypes in competitive field trials. The aggressiveness of U.S. isolates was also compared with European genotypes (from Ireland) on detached potato leaflets. In 2003 and 2004, field trials were planted in Michigan using cultivars with differing ratings for field resistance to foliar late blight. Epidemics were established using multiple local genotypes of P. infestans inoculated onto spreader rows of a susceptible cultivar surrounding the test plots. Once plots had reached approx. 1% infection, lesions were collected and isolated for characterization and genotype assignment: 99% of isolates from 2003 and 98% from 2004 belonged to the US-8 lineage. In detached leaflet testing, the US-8 genotype was significantly more aggressive (in terms of percentage infection, latent period and lesion growth rate) than isolates of any other U.S. lineage, but some Irish isolates proved equally aggressive. The results indicate that the aggressiveness of US-8 may account for its competitive success in the U.S. in recent years.