An isolate of Magnaporthe grisea was collected from a blast lesion on oat in Brazil. Sequence analysis of the rDNA-ITS-2 region and DNA fingerprinting with repetitive elements revealed that the Avena isolate belongs to the "crop isolate group" and is similar to Triticum isolates. At high temperature (28degreesC), the Avena isolate caused severe disease symptoms on primary leaves of oat and wheat. When the temperature was decreased to 20degreesC, wheat leaves expressed resistance to the Avena isolate. Cytologically, this temperature-dependent resistance was associated with an increase in the incidences of papilla formation and a hypersensitive reaction. Pathogenicity tests with various plant species at 20degreesC revealed that the Avena isolate is exclusively parasitic on oat. To elucidate genetic mechanisms of this species-specific parasitism, the Avena isolate was crossed with a Triticum isolate and resulting F-1 progenies were subjected to pathogenicity tests on oat seedlings. In the F-1 population, avirulent and virulent cultures segregated in a 1:1 ratio, suggesting that the specific parasitism on oat is controlled by a single gene. This locus was designated as Pat1.