An integrated multivariate approach to net blotch of barley: Virulence quantification, pathotyping and a breeding strategy for disease resistance

Knowledge of pathotype diversity and virulence in local populations of Pyrenophora teres is a prerequisite to screening for durable resistance to net blotch. The current study aimed to quantify the virulence level of Moroccan isolates, identify and designate existing pathotypes, and select resistant genotypes. We developed a method for virulence quantification of P. teres isolates based on a conversion of infection responses into frequencies for use in correspondence analysis. Coordinates of the first axis of this analysis had a virulence spectrum and ranked isolates from virulent to avirulent. Mixed model analysis was also devised for virulence quantification. Coordinates of the first dimension of correspondence analysis were linearly correlated to BLUPs (Best Linear Unbiased Predictors) of the mixed model. A genotype by genotype by environment model (GGE) coupled with cluster analysis differentiated P. teres isolates into ten and nine pathotypes for net- and spot-forms respectively. Populations of these two forms were dissimilar in terms of classes of virulence. For P. teres f. maculata, avirulent, moderately virulent and highly virulent isolates represented one-third of the population, whereas 90% of P. teres f. teres population was composed of avirulent to moderately avirulent isolates. Barley differential sets were subsequently reduced to two new sets that simplified pathotyping through a key code based on resistant or susceptible reactions. Dendrograms of cluster analysis based on GGE analysis depicted the stability of a genotype’s reactions across all isolates, and using only resistant cultivars as sources of resistance to control net blotch disease would, based on this analysis, fail to control all pathotypes. Therefore, we propose an alternative breeding strategy to control net blotch effectively.