Genetic structure of the Norwegian Pyrenophora teres population on spring barley

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Abstract Net blotch (NB), caused by Pyrenophora teres , is a major barley disease in Norway and worldwide. The two subspecies P. teres f. teres ( Ptt ) and P. teres f. maculata ( Ptm ) reproduce asexually via conidiospores and sexually via ascospores, the latter requiring opposite mating types. We analyzed the genetic diversity and structure of 339 Norwegian isolates and 61 globally collected isolates. Among the Norwegian collection, 325 (95.9%) were Ptt and 8 (2.4%) were Ptm , the latter restricted to Southeastern Norway. Two isolates are putative Ptt / Ptm hybrids. Bayesian clustering and principal coordinate analysis revealed one Ptm and two Ptt subgroups, with Ptt subgroups defined by host spike row-type and variety. Integration with two published global datasets showed that Norwegian row-type clustering is part of a broader global pattern of population structure driven by host vernalization requirement. No geographic differentiation was observed, indicating long-distance and inter-seasonal gene flow. Isolates collected 20 years earlier differed significantly from recent ones, though no change was detected over a recent five-year period. Both mating types occurred in near 1:1 ratios across regions. The high frequency of multilocus genotypes is consistent with sexual reproduction and with either passive dispersal through infected seed or rapid pathogen adaptation to specific host varieties. Genome-wide association analysis identified three markers associated with host variety on chromosomes 5 and 9, with the chromosome 5 locus co-localising with known virulence-associated regions. These findings highlight the high evolutionary potential of P. teres , which must be considered in breeding for durable resistance.