Long term surveys of pathogen populations underpin sustained control of the rust diseases of wheat in Australia

Abstract

The wheat stem rust pathogen Puccinia graminis f. sp. tritici (Pgt) and wheat leaf rust pathogen P. triticina have been present in Australia at least since European colonisation. The stripe rust pathogen of wheat, P. striiformis f. sp. tritici (Pst), was first detected in 1979. Surveys of pathogenic variability in Pgt and P. triticina began in 1921/22, and of Pst in 1979, and have continued annually, uninterrupted ever since. These surveys involve the identification of races (pathotypes) in greenhouse assays, using wheat genotypes ("differentials") carrying different resistance genes. Virulence determinations have targeted principally all stage ("seedling") resistance genes, and only rarely adult plant resistance because of the technical difficulties of working with adult plants under controlled conditions. Data compiled since surveys for each pathogen began strongly implicate periodic introduction of exotic isolates, single-step mutation, and more rarely somatic hybridisation, as the major processes generating genetic diversity. The surveys have also provided clear evidence of migration of rust isolates throughout Australian cereal growing regions, with many examples of inoculum exchange between the eastern and western cereal belts, principally in a west to east direction. The long term surveys of wheat rust pathogens in Australia have provided both information and pathogen isolates that have underpinned rust control efforts, from gene discovery to post-release management of resistance resources. Increasingly, information on the pathogenicity of rust isolates is being complemented by estimates of genetic diversity, using selectively neutral markers to gain refined insight into the evolution and maintenance of virulence, migration pathways, and periodic long-distance migration events.