Long-Term Studies of Puccinia coronata f. sp. avenae in Australia Reveal High Pathogenic Diversity, Regional Virulence Differences, Evidence of Clonality and Rapid Emergence of Virulence Matching Deployed Host Resistance

Long-term, detailed Australia-wide studies of pathogenic variability in Puccinia coronata f. sp. avenae (Pca) were conducted from 1 April 1998 to 31 March 2024, along with analysing crown rust resistance in oat cultivars grown to examine its potential role in shaping Pca populations. A total of 2846 identifications of 172 pathotypes were made from 1559 rust samples. Six regional populations (Queensland [Qld], northern NSW [nNSW], southern NSW [sNSW], Victoria and Tasmania [V&T], South Australia [SA], Western Australia [WA]) became more variable over time, and variability decreased along an east-to-west gradient. Evidence was obtained for migration of at least five pathotypes between eastern and western cereal belts, indicating limited gene flow between them predominantly in a west-to-east direction. Two pathotypes were isolated throughout the study in most eastern regions, providing evidence of long-term persistence of pathogen clones. Two major shifts in the composition of regional populations were documented. The first involved rapid adaptation by Pca to Avena sterilis-derived resistance genes following their deployment in principally grazing oat cultivars in the subtropical region spanning nNSW and Qld. The second shift involved large changes between regional populations in Qld, nNSW, sNSW and WA beginning around 2011, and in V&T and SA around 2015, which were associated with the emergence, spread and increase of pathotypes typified by virulence for Pc64. Because Pca spreads rapidly throughout Australia and moves freely between grazing, hay and milling oat crops, reducing the impact of this damaging disease will require the deployment of durable resistance in oats of all end-uses.