Abstract
Isolates of Phytophthora infestans were collected from potato and tomato in Japan from 1987 to 1993, and their mating type (A1 or A2) was determined. In 772 isolates collected from potato during 1987 to 1993 in Hokkaido, the frequency of A2 mating type strains increased from 54% in 1987 to 97% in 1993. In eastern Hokkaido A2 strains were already present at a high frequency in 1987 and 1988, and since 1989, A1 strains have not been detected. In western Hokkaido the A1 strains were common in 1987 but decreased in frequency after 1990. The geographical distribution of the A2 strains seems to have spread westwards in Hokkaido. Of 539 isolates collected from potato during 1988 to 1990 outside Hokkaido, the frequencies of A2 mating type isolates were 72%, 96%, and 87% in 1988, 1989, and 1990, respectively. The A2 strains were already highly predominant in western Japan in 1988, while the A1 strains were common in Kanto and Tohoku districts, the eastern part of Honshu. However the frequency of A1 strains decreased even in Kanto and Tohoku districts in 1989 and 1990. The geographical distribution of the A2 strains seems to have spread northeastwards outside Hokkaido. There were some fields where A1 and A2 isolates were found together in western Hokkaido and northern Tohoku in 1990, but after 1990 such fields became very rare in Hokkaido. The high predominance of the A2 strains suggests that the probability of oospore production by mating is low in Japan. Seven out of eleven tomato isolates collected were of A1 mating type and four were of A2, indicating a significantly higher proportion of the A1 strains from tomato than from potato.
