Continuous variation and specific interactions in the Pyrenophora teres f. teres –barley pathosystem in Uruguay

The nature of host–pathogen interactions between Pyrenophora teres f. teres and barley has been reported to be as well race-specific as race-non-specific with evidence that the definition of races is tenuous. Infection responses of 266 single-spore isolates of P. teres f. teres collected in Uruguay during regular surveys from 2005 to 2015 to 41 barley genotypes that had been determined on a 1–10 scale and thus allowed for qualitative and quantitative distinction in virulence and aggressiveness were subjected to cluster analysis. A total of four isolate clusters and seven barley clusters based on mean infection responses were determined. Infections reactions of the 266 isolates were continuous, and each isolate had a unique pathogenic spectrum. There were no effects of sampling year and location on the occurrence of similar isolates. Despite some very few and clear isolate-specific interactions, the interactions in this pathosystem can be described as predominately quantitative. For screening purposes, 1–2 isolates from the different clusters with broad aggressiveness to most or all of the respective barley genotype clusters will be the best choice.