2.2.80
OCCURRENCE OF NEW STRAINS OF A1 MATING TYPE AFTER DISPLACEMENT OF AN A1 STRAIN BY AN A2 STRAIN OF PHYTOPHTHORA INFESTANS IN JAPAN

M KATO, N SATO and S NAITO

Hokkaido National Agricultural Experiment Station, Sapporo 062-8555, Japan

Background and objectives
Phytophthora infestans, the causal agent of potato late blight, produces oospores by mating between A1 and A2 matings. The A2 mating strains of P. infestans have been found world-wide since 1980s. In Japan, an A2 strain was first discovered in 1987 in Hokkaido, the northernmost district of Japan, and outside Hokkaido in 1988, and was already predominant [1]. However, it remained unknown how the frequency and the distribution of the A2 strain would change: whether the A2 strain would displace the A1 strain completely, coexist with the A1 strain, or be displaced again by the A1 strain is important for estimating the probability of oospore production. The objective of this study is to elucidate the change in geographical distribution of the mating types in Japan, particularly in Hokkaido.

Materials and methods Diseased potato leaflets were collected from Hokkaido Island in 1987-97 and outside Hokkaido in 1988-90. P. infestans was isolated and each isolate was examined for mating type by pairing with mating type-known isolates. A subset of the isolates were examined for colony feature on potato tuber slices, growth on oatmeal agar, virulence to a cultivar with resistance gene Rl and allozyme pattern of glucose-6-phospate isomerase (Gpi) by cellulose acetate electrophoresis.

Results and conclusions

Outside Hokkaido the frequency of A2 mating was 72% in 1988, 96% in 1989 and 87% in 1990. Displacement of A1 by the A2 strain outside Hokkaido seemed to have spread northeastwards. In western Japan the A2 mating was highly predominant throughout the survey; however, in eastern Japan the A1 strain was found at a relatively high proportion in 1988 but became rare in 1990. In Hokkaido, the frequency of the A2 mating type increased from 62% in 1987 to more than 90% in 1991 to l995. Displacement of the A1 strain by the A2 strain seemed to have spread westwards in Hokkaido. In eastern Hokkaido the Al strain was found until 1988 but not from 1989 to 1993. In south-western Hokkaido the A1 strain was predominant in 1987, decreased to a small proportion in early 1990s, and was not detected in 1996 and 1997. An almost complete displacement of the A1 by the A2 strain suggests that the possibility of oospore formation was low. Mosa et al. [2] showed that each mating type of Japanese P. infestans has a distinct character, indicating that the probability of oospore formation was low during the rapid complete displacement. However, a high proportion of A1 strains was found in eastern Hokkaido in 1996 (18%) and 1997 (55%). These A1 strains differed from the A1 strain collected before 1996 in several characters. The old A1 strain essentially showed short aerial mycelia with abundant sporangia on tuber slices, poor growth on oatmeal agar, avirulence to R1 cultivar, and a 861100 Gpi band pattern . The new Al strains showed long aerial mycelia with few sporangia, virulence to Rl cultivar, and a 1001100 Gpi band pattern. There were two types of growth reaction on oatmeal agar in new Al strains. These results indicate occurrence of more than one new strains of Al mating type. In conclusion, the old A1 strain was displaced by the A2 strain during late 1980s and early 1990s and new Al strains occurred in 1996 and 1997. The possibility of oospore formation has become greater.

References
1. Mosa AA, Kato M, Sato N, Kobayashi K, Ogoshi A, 1989. Annals of the Phytopathological Society of Japan 55, 615-620.
2. Mosa AA, Kobayashi K, Ogoshi A, Kato M, Sato N, 1993. Plant Pathology 42, 26-34.