Induced Rice Resistance to Blast Varies as a Function of Magnaporthe grisea Avirulence Genes.

Incompatibility reactions between rice and the blast fungus Magnaporthe grisea produce various degrees of lesions, from large brown flecks to small, nearly invisible lesions. We previously identified four avirulence genes (AvrPia, AvrPii, AvrPit, and Avr-Hattan3) in M. grisea isolates by genetic analysis of progeny from crosses between isolates with differing pathogenicity. Using progeny known to contain a specific avirulence gene, we demonstrated that the type of resistance lesion produced in rice by an avirulent isolate and the degree of leaf blast suppression by preinoculation with that isolate were determined by the combination of avirulence and resistance genes in the isolate and the cultivar. The degree of leaf blast suppression by preinoculation with an avirulent isolate increased with larger resistance lesions. When two genes were involved in an isolate's avirulence, lesions appeared that resembled those expected for the gene that produced the smaller lesion. The degree of leaf blast suppression by the isolate with two avirulence genes was comparable with that induced by the isolate with the avirulence gene that produced the smaller effect. The ability of specific resistance gene combinations that effectively suppress blast disease is discussed for each avirulence gene.