The Australian Ascochyta rabiei population is genetically narrow with only one mating type gene detected to date, potentially precluding substantial evolution through recombination. However, a large diversity in aggressiveness exists. In an effort to better understand the risk from selective adaptation to currently used resistance sources and chemical control strategies, the population was examined in detail. For this, a total of 617 isolates were quasi-hierarchically sampled between 2013-2015 across all major Australian chickpea growing regions and commonly grown cultivars. Although a large number of haplotypes were identified (103) through short sequence repeat (SSR) genotyping, overall low population diversity (H=0.064) and genotypic diversity (D=0.58) was detected. More than 63% of the isolates assessed were of a single dominant haplotype (ARH01). Disease screening on a differential host set, including three commonly deployed resistance sources, revealed distinct aggressiveness among the isolates, with 17% of all isolates identified as highly aggressive. Almost 75% of these were of the ARH01 haplotype. A similar pattern was observed at the host level, with 46% of all isolates collected from the commonly grown cultivar Genesis090 (classified as “resistant” during the term of collection) identified as highly aggressive. Of these, 63% belonged to the ARH01 haplotype.
Isolates from ARH01 haplotype pose a significant risk to the Australian chickpea industry, being not only widely adapted to the diverse agro-geographical environments of the Australian chickpea growing regions, but also containing a disproportionately large number of aggressive isolates, indicating fitness to survive and replicate on the best resistance sources in the Australian germplasm. The highest risk isolates are provided to the national chickpea breeding program for screening breeding materials and to validate markers for resistance selection, this assists breeders to select for sustainable resistance and farmers to adopt best disease management practice necessary to maintain good crop yields and quality.