Estimation of Fitness Values for Virulence Genes in Collections of Rice Blast Fungus from Niigata Prefecture

Data on host-pathogen relationship of rice (Oryza sativa) and blast (Pyricularia oryzae) obtained in Niigata Prefecture, Japan were analyzed by application of a simulation model for following the change of pathogen genotype frequencies. The proportion of growing areas of rice varieties with various genotypes for blast resistance were obtained from statistics data and frequencies of blast-fungus genotypes for virulence were calculated from the data from 1976 to 1986 in six areas in Niigata Prefecture. The lowering of fitness values for each of avirulence genes, Av-a, Av-i, Av-k and Av-z, accompanying the mutation to virulence was estimated by simulating the change in the frequencies of each genotype of blast fungus on the frequencies of genotypes of rice plant. Lower fitness values of virulence genes, Av-a+ and Av-i+, were obtained in all areas in Niigata Prefecture. This explains the recent predominance of a pathogen genotype, Av-a Av-i Av-k Av-z (main race 001) in Niigata Prefecture, though predominance of the genotype, Av-a+ Av-i Av-k Av-z (race number 003) was previously obtained in the Kanto District with similar gene frequencies for host resistance to Niigata Prefecture. Reverse tendencies between two genes were observed: fitness values obtained were slightly higher for the Av-a+ gene and significantly lower for Av-i+ gene in mountainous areas in comparison with plain areas.