Spot form net blotch (SFNB) is a major foliar disease of barley in Australia that can cause significant losses to grain yield and quality. The causal organism, Pyrenophora teres f. maculata (Ptm), is a fungal pathogen that can reproduce sexually and produce new virulent pathotypes that can overcome resistance in barley varieties. This poses challenges to the deployment of durable resistance and creates the potential for severe outbreaks to occur. Continuous monitoring is essential to inform the industry about emerging virulence and to support the development of resistant varieties. To facilitate this, a differential set comprising several barley genotypes (~25 barley lines) with putative novel resistance or susceptibility to SFNB was established. Monoconidial cultures (n = 194) of Ptm were isolated from infected leaf samples collected from barley fields across Australia from 2020 to 2024 and screened against the differential set under controlled environmental conditions. Disease reactions were assessed for each isolate, and the responses of 14 commonly used barley lines over five years were analysed to track pathogen diversity and its evolution in the country.
Pathogenic diversity within the Ptm populations across Australia was abundant. The susceptible control, Kombar, consistently indicated isolate virulence or avirulence. Most genotypes in the differential set exhibited some level of susceptibility, except for Keel, which demonstrated a resistant reaction to all tested isolates. Keel remains a key resistance source to SFNB within the Australian barley breeding program, and these findings suggest it continues to be effective. Genotypes Yangsimai 3 and Navigator, share a similar genetic profile to Keel, displayed susceptibility to 29 isolates, indicating that virulence is present in the Ptm population. Of concern was the increased disease severity reaction observed in the latter years toward Compass and Skiff. These varieties serve as resistance sources for multiple Australian barley varieties, and their increasing susceptibility indicates increased virulence in the Ptm population. The results did not show any association between virulence and geographic distribution, likely due to the long-range dispersal of spores. However, seasonal variation was evident, with higher virulence detected during wetter seasons. Several isolates were identified with novel virulence, including Ptm19-197, which had virulence toward most barley genotypes, highlighting Ptm’s potential to evolve novel virulence that could threaten current resistance sources utilized in breeding programs. This isolate will be added to future barley germplasm screenings to continue to identify resistant breeding material.