1989 Volume 55 Issue 5 Pages 615-620
The A2 mating type of Phytophthora infestans has been found in Japan. Oospores, within oogonia bearing amphigynous antheridia, were formed in a great abundance when compatible isolates were mated. When 207 isolates were tested for their mating types, 65 were A1, 141 were A2 and one was self-fertile in single culture. Both A1 and A2 isolates were detected among samples from many locations in Japan, but the A2 type was found to be predominant in all districts except Tohoku district. All A1 isolates except two were isolated from potato cultivars lacking R genes, while A2 isolates were isolated from a large number of cultivars having or lacking R genes. Most of the A2 isolates formed oospores in single aged cultures. There is a possibility that oospores have been formed in the fields of Japan.