Foliar aggressiveness of Northern Ireland isolates of Phytophthora infestans on detached leaflets of three potato cultivars

The aggressiveness of 20 Northern Ireland single-lesion isolates of Phytophthora infestans was compared following their inoculation onto detached leaflets of three potato cultivars chosen on the basis of their differing levels of race-nonspecific resistance to late blight: Bintje (highly susceptible); Cara (moderately resistant); and Stirling (more resistant). Five isolates from outside Northern Ireland were included for comparative purposes: two from the Republic of Ireland; two from the USA (representing the US-1 and US-8 clonal lineages); and one from Mexico. To control the variation between tests, a balanced incomplete block design was used, as opposed to either a complete block design or the method of inclusion of standard isolates, where such variation would have increased the error. Highly significant variation for disease parameters, including latent period, infection frequency, area under the lesion expansion curve (AULEC) and sporulation capacity, was found between isolates. These differences were much more marked on the cultivars exhibiting higher levels of race-nonspecific resistance. There was a significant interaction between isolate and cultivar for all parameters assessed and, overall, no one isolate was the most aggressive across all three potato cultivars. However, a group comprising seven of the 20 Northern Ireland isolates was consistently found to exhibit the highest levels of aggression towards all three cultivars for each of the disease parameters. These results demonstrate that significant variation for foliar aggressiveness exists within the Northern Ireland population of P. infestans , and indicate the importance of selecting appropriately aggressive isolates for evaluation of host resistance to late blight within breeding programmes.