Genetic analysis of avirulence/virulence of an isolate of Magnaporthe grisea from a rice field in Texas

An isolate of Magnaporthe grisea, Tm4, from a rice field in Texas was crossed with a fertile laboratory strain, 70-6. The progenies showed segregation of avirulence/virulence on rice cvs. Newbonnet, Lemont, Lebonnet, Leah, and Katy. The avirulent/virulent segregation ratios were 29:6 on Newbonnet, Lemont, and Lebonnet; 28:7 on Leah; and 33:2 on Katy. There was cosegregation on the first three cultivars. Several avirulent progenies were backcrossed to virulent parent 70-6. Three generations of backcrossing avirulent progenies to 70-6 led to segregation ratios that suggested certain strains had only one avirulence gene. Strains avirulent only on cv. Katy or only on cvs. Newbonnet, Lemont, and Lebonnet were test crossed with virulent siblings. Strains that gave progeny ratios approximating 1 avirulent:1 virulent when crossed with virulent siblings were selected for further test crossing. Intercrosses between strains with possible single avirulence genes were made to determine whether these strains had the same or different avirulence genes. Many lines still segregated two genes for avirulence after three generations of backcrossing. This is based on the recovery of virulent progenies from crossing two avirulent siblings.