Occurrence and genetic and pathogenic characterisation of Pyrenophora teres f. teres, P. teres f. maculata and their hybrids

Occurrence and genetic and pathogenic characterisation of
Pyrenophora teres f. teres, P. teres f. maculata and
their hybrids

Pyrenophora teres is the causal agent of net blotches, which are important foliar diseases of barley (Hordeum vulgare) worldwide. Pyrenophora teres has two forms, P. teres f. teres (Ptt) and P. teres f. maculata (Ptm), which causes the net and spot forms respectively based on the symptoms that they induce on barley. The life cycles of both forms are almost identical in which they can undergo both sexual and asexual reproduction. Sexual reproduction contributes to genetic and pathogenic evolution and provides challenges for development of commercial resistant cultivars. It is necessary to understand the genetic and pathogenic diversity of P. teres to effectively breed commercial barley cultivars with resistance. This will improve productivity for barley growers and industry and help reduce reliance on chemical fungicides.

The two forms of P. teres consist of different virulence genes. Sexual recombination between two forms of P. teres provides a potential threat to breeding for resistance as it may result in combination of virulences from the two forms. Hybrids have been reported occasionally in the field. This study was conducted to investigate the frequency of Ptt x Ptm hybrid development in the field where both forms of P. teres co-exist. For this purpose, three field trial sites were established for three successive years and Ptt and Ptm of opposite mating types were inoculated to facilitate hybridisation. In addition, samples collected from barley growing regions in Australia during 1976–2015 were analysed. To identify hybrids between the two forms, twelve Ptt and Ptm specific markers were developed using a whole genome comparative approach. These markers provided a more efficient and robust method than previously developed markers for identification of hybrids. No Ptt x Ptm hybrids were identified in the collected samples. This indicates that sexual recombination between the two forms of P. teres was rare, even where conditions were conducive. Further, pre- and postmating barriers which may be responsible for reproductive isolation between Ptt and Ptm were reviewed.

A thorough knowledge of the degree of genetic variation in the pathogen is important for successful deployment of resistance genes in barley breeding. Genetic changes occurring in Ptt populations during a three year period was investigated using diversity array technology markers. In the field new genotypes were identified together with the original inoculated isolates in the first year of the cropping season. This indicated clonal reproduction and migration event. In two subsequent years, high number of recombinant genotypes were identified which indicated that gene flow combined with sexual recombination plays a prominent role in rapidly changing the structure of Ptt populations.

Barley grass (H. leporinum) is an alternative host to P. teres and can harbour virulent pathotypes of P. teres which can infect barley cultivars. Genetic diversity between P. teres collected from barley grass and barley showed that P. teres from the respective host were distinct, which suggested genetic isolation. Pathogenicity assay result showed that virulent P. teres from barley grass were avirulent on barley. This indicated that barley grass contributes little to the genetics of the P. teres population and negligible risk to commercial barley cultivars although the two hosts are often grown in close proximity.

In summary, this study investigated several areas of research to better understand and manage net blotch resistance in future. Further research to investigate mating barriers responsible for reproductive isolation between two forms of P. teres is important to understand the evolution of the pathogen. In addition, identification of virulent/avirulent genes is required to understand virulence mechanism of the pathogen and its interaction with the barley host.