Pathogenic variation of Pyrenophora teres f. teres in Australia

Abstract

Pyrenophora teres f. teres (Ptt) is the causal agent of net form of net blotch (NFNB) – a major foliar disease of barley (Hordeum vulgare) crops worldwide. Deployment of genetic resistance in cultivars is the preferred method of control, but requires knowledge of the pathogenic variation of Ptt to be effective as spatial and temporal variation is common. In this study, 123 Ptt isolates collected from five states across Australia were examined for pathogenic variation using a set of 31 barley genotypes, composed of 11 international genotypes and 20 Australian cultivars. Barley seedlings were inoculated with spore suspensions from monoconidial isolate cultures and scored for infection response. Phenotypes were used to perform hierarchical cluster analysis for barley genotypes and Ptt isolates. Cluster analysis identified seven line groups, each containing barley genotypes that displayed similar responses to the Ptt isolates. Isolates clustered into four distinct isolate groups shown to harbour differential virulence to four key genotypes: Maritime, Prior, Skiff and Tallon. Isolates with virulence to any one of these genotypes accounted for 96.7% of the samples. Differential virulence was observed on a range of genotypes within each isolate group. The composition of isolate groups in eastern Australia was distinct from Western Australia, whereas all isolate groups were detected in southern Australia. Results suggest that cultivation of regionally adapted barley cultivars has led to regional evolution of Ptt, where the pathogen acquires virulence specific for resistance factors deployed in local cultivars. Detection of Ptt modern isolates that were highly virulent to historic cultivars indicates the long-term survival of virulence gene combinations in the pathogen population.