In 2002–03, 597 Phytophthora infestans isolates were collected from potato and tomato crops in nine provinces of Vietnam. A representative set of isolates were characterised for mating type, metalaxyl sensitivity, virulence, mitochondrial DNA (mtDNA) haplotype and RG57-fingerprint. All of the 294 tested isolates were of mating type A1. Metalaxyl-resistant isolates were found on both the potato and tomato hosts, and dominated the population in Lam Dong province (south of Vietnam). Nine known virulence genes were found among 27 tested isolates, although virulence to resistance gene R5 was not found. Among the 95 isolates tested for mtDNA haplotype, all were Ib type. Of these, all 34 isolates fingerprinted showed US-1-like genotype. Mating type, mtDNA haplotype and RG57-fingerprint data from this study indicate that the P. infestans population on tomato and potato from Vietnam still consist of the ‘old’ population.
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