Effect of metalaxyl on mating type of Phytophthora infestans and P. parasitica.

Two single-oospore isolates (1 A1 and 1 A2) of P. infestans formed oospores in sectors when grown on medium containing metalaxyl. Single-zoospore cultures obtained from the oospore sector of either isolate consisted of both A1 and A2 mating type. All the single-zoospore cultures from the non-treated isolates had the same mating type as the parent isolate. Metalaxyl also caused mating type change in 3 A1 isolates of P. parasitica. The conversion of A1 to A2 mating type by metalaxyl in infected host tissues is postulated as a possible origin of A2 isolates of P. infestans in Europe.