The effect of a dominant phytophthora infestans genotype (13_A2) in Great Britain on host resistance to foliar late blight in commercial potato cultivars

As a result of recent changes in the Phytophthora infestans population in Great Britain, in particular the dominance of one genotype (13_A2) and the possible implications of this change on host resistance ratings, a coordinated programme of testing between SASA and JHI was initiated. Over 3 years of trials (2008-2010), 49 existing cultivars with published resistance ratings of 5 or greater on a 1-9 scale of increasing resistance, 42 new cultivars and a range of appropriate control cultivars were assessed using an isolate of the 13_A2 genotype of P. infestans. The mean resistance rating of 62% of the 49 existing cultivars was reduced by more than the maximum 5% LSD of 1.9. For some cultivars (e.g. Lady Balfour and Stirling), changes in resistance rating were specifically associated with the use of the 13_A2 genotype to inoculate the trials from 2008 onwards. The majority of the 42 new cultivars tested were susceptible to the 13_A2 genotype isolate, scoring 4 or less. However, cultivars with useful resistance were also identified with seven scoring 6 or 7. Published resistance ratings have been revised as a result of this study.