Races of Magnaporthe oryzae in Australia and genes with resistance to these races revealed through host resistance screening in monogenic lines of Oryza sativa

Rice blast is the most serious disease threat to rice production worldwide. It is difficult to control due to the complex diversity and wide geographic distribution of the causal pathogen Magnaporthe oryzae. In Australia, rice blast occurs in northern Australia but remains exotic to the main south-eastern rice growing area; however, there is the potential for rice blast to threaten this area; in addition, rice production is currently expanding from south-eastern Australia into northern Australia, which makes rice blast a major concern and challenge to rice industry in Australia. Prior to this study, there was lack of information on the race status of M. oryzae present in Australia and on how to manage the disease through host resistance. The races of rice blast isolates collected in northern Australia was characterised based on the disease reactions of eight standard rice differentials used in an international race differential system. The following studies revealed genes conferring resistance to these races through investigating the responses of 25 monogenic rice lines with targeted resistance gene against different races. The rice blast isolates were characterised into five races: IA-1, IA-3, IA-63, IB-3 and IB-59. Genes Pi40, Piz-t, Pi9, Pi5(t) and Pi12(t) exhibited resistance to all the isolates belonging to five races. In addition, two genes showed complete resistance to multiple races, viz. Pi9 that showed complete resistance to races IA-1, IA-3, IA-63 and IB-3 and Pita2 that had complete resistance to races IA-3, IB-3 and IB-59. This study provides information about the races of M. oryzae in Australia. Genes identified conferring resistance to multiple races will not only streamline the identification via molecular markers of imported rice varieties with resistance to rice blast in Australia, but will also allow the Australian rice breeding program to develop new varieties with broad-spectrum resistance to rice blast and pyramid multi-gene resistance into Australian rice varieties.