Occurrence of the A2 mating type of Phytophthora infestans on potato in Japan.

Abstract

Of 207 isolates of P. infestans tested, 65 were A1, 141 were A2 and 1 was self fertile, producing abundant oospores in single culture. Both A1 and A2 mating types were detected among samples from many locations in Japan, but A2 predominated in all districts except Tohoku. All A1 isolates except 2 were isolated from potato cultivars lacking R genes, while A2 isolates were obtained from many cultivars both with and without R genes. Most of the A2 isolates formed oospores in old single cultures. The possibility is suggested that oospores are formed in Japanese potato fields.