Genetic Analysis of Virulence in the Rice Blast FungusMagnaporthe grisea

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Abstract
Crosses between rice isolates of Magnaporthe grisea were made to determine the genetic control of virulence to rice. A cross between a hermaphroditic rice isolate, Guy 11, and a laboratory strain 2539, yielded more than 50% viable ascospores. Random spore and tetrad analysis showed that two loci, Pos1 (pathogenicity on Orva sativa) and Pos2, were involved in conditioning virulence to rice lines 51583 and Sha-tiao-tsao. A buff mutation was epistatic to virulence. Fertility and ascospore viability were low in crosses between field rice isolates. In a cross between two rice isolates. Guy 11 and CH 104-3, only 10% of the ascospores was biable, but all progeny were virulent to 51583 and Sha-tiao-tsao. From this cross, we identified two different loci, Pos3 and Pos4, controlling virulence on rice lines K59 and Kinandang Patong, respectively. Fertility and virulence were maintained in progeny recovered from F2 and F1 generations. Joint segregation analysis showed an excess of parental types with respect to virulence. This might reflect genetic linkage of virulence genes or segregation of other genetic factors epistatic to virulence.