Opposite mating type genuine haploid endospores of Protomyces inundatus do not cause infection to the host plant Apium nodiflorum Usually infection is caused by sexually incompatible diploid endospores derived by fusion of opposite mating type haploid endospores However, observations document that haploid strains could give rise to diploids, by self diploidization, and these sexually compatible diploids cause infection to the host plant. It is suggested that ability to infect is dependent an ploidy of the culture rather than on the presence of both opposite mating type genes.