Magnaporthe grisea is a fungal pathogen with two mating types, MAT1-1 and MAT1-2, that forms a specialized cell necessary for pathogenesis, the appressorium, Saccharomyces cerevisiae alpha-factor pheromone blocked appressorium formation in a mating type-specific manner and protected plants from infection by MAT1-2 strains. Experiments with alpha-factor analogs suggest that the observed activity is due to a specific interaction of alpha-factor with an M. grisea receptor. Culture filtrates of a MAT1-1 strain contained an activity that inhibited appressorium formation of mating type MAT1-2 strains. These findings provide evidence that a pheromone response pathway exists in M. grisea that can be exploited for plant protection.