SEGREGATION OF AVIRULENCE VIRULENCE ON 3 RICE CULTIVARS IN 16 CROSSES OF MAGNAPORTHE-GRISEA

Two isolates, 6-20 and Guy 11, both of which are pathogenic on rice, were crossed; the segregation for avirulence/virulence on three cultivars is given for 15 subsequent crosses. Both parents and all progenies were pathogenic on cultivar S201. Certain progenies were selected for intercrossing in each generation, and some progenies were backcrossed to Guy 11. Crosses between some isolates, both virulent on a particular cultivar, produced avirulent progenies. Crosses of isolates, one avirulent and the other virulent on a particular cultivar, usually segregated for avirulence/virulence. Crosses of two isolates, both avirulent on a particular cultivar, usually gave all avirulent progenies. The infection type ranges of parent isolates did not necessarily indicate the infection types of the progenies. The recovery of avirulent progenies in crosses between two virulent isolates suggests that one of the parent isolates classified as virulent contained an avirulence gene(s) whose expression was suppressed. The genes controlling avirulence and virulence on each of the three rice cultivars are different from one another. A postulated genotype of each of the parent isolates is presented.