The influence of competition, aggressiveness and host resistance on selection of Phytophthora infestans populations in the US and Northern Ireland

Abstract

The majority of commercially available potato cultivars are highly susceptible to foliar late blight caused by the oomycete Phytophthora infestans. Although new resistant cultivars with good agronomic characteristics are now becoming commercially available, the impact of cultivar on selection in the surrounding late blight population is poorly understood. In 2003, 2004 and 2005 field trials were planted in Michigan and N. Ireland using cultivars with differing ratings for field resistance to foliar late blight. Epidemics were established using multiple local genotypes of P. infestans, through the use of inoculated spreader rows of a susceptible cultivar which surrounded test plots. Once plots had reached approximately 1% infection, lesions were collected and isolated for characterisation and genotypic assignment. There was a significant effect of cultivar on the distribution of genotypes within the N. Ireland field trials. All genotypes infected the most susceptible cultivar but extreme genotypic selection was observed within the other more resistant cultivars. This trend was not observed in the US field trials where one genotype (US-8) dominated infection of all cultivars. The US-8 was significantly more aggressive than other US genotypes in detached leaflet aggressiveness tests. In contrast, there was little correlation observed between competitive fitness in the field and aggressiveness on detached leaflets in the N. Ireland study.