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A total of 401 isolates of Phytophthora infestans were collected from eight Asian regions (Korea, India, Taiwan, Indonesia, Thailand, Nepal, China and Japan) between 1992 and 2000 -318 from potato and 83 from tomato. The isolates were analysed for mating type, metalaxyl resistance, RG57 fingerprinting, mitochondrial DNA (mtDNA) haplotype and the polymorphism of three allozyme loci, i.e. glucose-6-phosphate isomerase ( Gpi ), peptidase ( Pep ) and malic enzyme ( Me ). The isolates were multilocus-genotyped based on RFLP (RG57) fingerprint, dilocus allozyme genotype, mtDNA haplotype and mating type. Twenty multilocus genotypes were identified among 125 isolates. Of these genotypes, 14 had not been previously reported. Some of the multilocus genotypes were common to isolates from several geographical regions, suggesting migration. The metalaxyl-resistant isolates belonged to the multilocus genotypes JP-1, JP-2, and JP-3. Multilocus genotypes coexisting in a single field were found in following regions: Thailand (1994), central China (1996), Nepal (1997 and Japan (1998 and2000). The possible origins of certain genotypes are discussed, including the possibility of sexual recombination within the P. infestans populations in Nepal and perhaps Thailand.

The mating type, allozyme genotypes of malic enzyme, glucose-6-phosphate isomerase and peptidase loci, and metalaxyl sensitivity of 336 Phytophthora infestans isolates collected in Korea, India, Taiwan, Indonesia, Thailand, Nepal, and China between 1992 and 1997 were determined. Only A1 mating type isolates were detected in India and Taiwan, and only A2 isolates in Korea and Indonesia, while both mating type isolates were detected in Thailand, Nepal, and China. The isolates from Korea, India, Taiwan, and Indonesia had a single allozyme genotype respectively, while the isolates from Thailand, Nepal, and China had two or three allozyme genotypes. Isolates that were highly resistant to metalaxyl were found in Korea, Indonesia, and China. All of the isolates from Taiwan and India, and some of the isolates from Thailand, Nepal, and northern and southern areas of China, belonged to the old population of P. infestans, which was predominant worldwide until the appearance of A2 isolates outside central Mexico. Some of the isolates from Thailand and Nepal were from new populations that have been discovered since the appearance of A2 isolates outside central Mexico. All of the isolates from Korea and Indonesia and most of the isolates from China belonged to two specific populations, which have been rarely detected elsewhere. The occurrence of these separate populations in Asia suggests that they followed a different migration pathway, or that they are better adapted to Asian conditions than to those elsewhere in the world.

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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.

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