Detection and occurrence of a new pathotype of Puccinia triticina with virulence for Lr24 in Australia

The leaf rust resistance gene Lr24 remained effective in Australia from at least 1983, when the first wheat cultivar with this gene was released, until 2000, when a virulent isolate of Puccinia triticina was detected. Results of comparative greenhouse studies were consistent with the hypothesis that the new virulent isolate developed from pathotype 104-1,2,3,(6),(7),11 by mutation to virulence for Lr24. The new pathotype was first detected in South Australia (October 2000), and was subsequently detected in southern New South Wales (November 2000), Victoria (March 2001), and Queensland (March 2001), suggesting that it originated in South Australia and then spread to other parts of the eastern Australian wheatbelt. Greenhouse tests of 28 Australian wheat cultivars possessing Lr24 revealed that all except Dennis, Giles, Petrie, and Sunsoft 98 were seedling susceptible to the new pathotype. Cultivars Giles, Petrie, and Sunsoft 98 were postulated to carry Lr13, whereas cv. Dennis carries either Lr17b or Lr13. Adult plant field tests of 20 cultivars with Lr24 conducted during 2001 confirmed the resistance of Giles, Petrie, and Sunsoft 98, whereas all other cultivars tested were either moderately resistant to moderately susceptible or susceptible to the new pathotype. Given that some of these cultivars appear to possess Lr34, and that the expression of this gene is influenced by temperature and other environmental factors, further field testing under different seasonal conditions will provide a more accurate indication of their response. Cultivars with Lr37 or Lr13 in combination with Lr1 or Lr2a remain effective to all known pathotypes of P. triticina in Australia. Several new sources of resistance to P. triticina that are effective to Australian pathotypes are currently being evaluated, along with additive adult plant resistances. These sources should provide a greater diversity of resistance to this pathogen in future Australian wheat cultivars.