A total of 134 isolates of Phytophthora infestans were collected from potato and 42 from tomato fields in Switzerland and France in 1996 and, and compared with isolates from other countries. The structure of the populations was analysed phenotypically and genotypically, and associated to geographical, seasonal and host origin. Phenotype characteristics of the isolates included mating type; sensitivity to phenylamide fungicides; virulence on potato differentials; and pathogenic fitness. Genotypes were assessed for mitochondrial DNA haplotype with restriction fragment length polymorphism-polymerase chain reaction (RFLP-PCR) as well as amplified fragment length polymorphism (AFLP) and simple sequence repeats or microsatellites (SSR). The majority (96%) of isolates originating from potato were the A1 mating type, whereas half the isolates collected from tomato were A2 mating type. Isolates with sensitive, intermediate and resistant responses to the phenylamide fungicide metalaxyl-M were detected in the populations. Isolates from potato represented races with highly complex virulence spectra, whereas those from tomato belonged to simple races. The pathogenic fitness of isolates was highest on the host of origin, and was significantly reduced for isolates from potato on tomato. One of the four haplotypes, Ia, dominated the population (93% of isolates). Among isolates collected from potato, 15 different SSR genotypes were detected of which two, A-03 and A-06, dominated the population. From tomato, 11 SSR genotypes were found of which four, A-03, B-03, D-03 and F-01, formed the major proportion of the population. Every ninth and fourth isolate from potato and tomato, respectively, represented a different SSR genotype. Four genotypes were isolated from both hosts, whereas 11 and seven genotypes, respectively, originated exclusively from either potato or tomato. The SSR genotype D-02, represented by the 'old' Ib haplotype, was still detected in a few isolates in the current population, and in older reference isolates from different countries. The SSR genotype was not associated with mating type or sensitivity to phenylamide fungicides. A total of 40 AFLP genotypes were distinguished among the isolates, every second isolate representing another genotype. The diverse phenotypic and genotypic structure of the current field populations in Europe suggests that they may have evolved from local processes including sexual recombination, host preference and selection rather than through long-distance migration.
Nineteen crosses were carried out in vitro between seven and nine isolates of Phytophthora infestans originating from potato and tomato, respectively. Oospores were produced abundantly in all but two crosses, but oospore germination was generally low (a few per cent) and depended on the combination of parental isolates. The highest fertility in F 1 progeny was observed when at least one parental isolate originated from tomato, and the lowest in crosses of isolates from potato; half the crosses were not fertile. Forty-three F 1 and 51 F 2 single-oospore progeny of a selected cross, along with the 16 field isolates, were analysed phenotypically and with molecular markers. Phenotypic characterization included mating type; sensitivity to the phenylamide fungicide metalaxyl-M; specific virulence on potato R-gene differentials; and aggressiveness (infected leaf area) on potato and tomato leaf discs (host preference). Isolates and progeny were also assessed for mitochondrial DNA haplotype with RFLP-PCR (restriction fragment length polymorphism-polymerase chain reaction), and characterized with AFLP (amplified fragment length polymorphism) and SSR (simple sequence repeats, microsatellites). Parental isolates were sensitive and resistant to metalaxyl-M, whereas all F 1 were intermediate phenotypes. In the F 2 progeny, the majority of isolates (43 of 51) were intermediate in sensitivity and four each were sensitive and resistant to metalaxyl-M, respectively. In both F 1 and F 2 progeny, four isolates emerged through selfing. The A1 : A2 ratio was 25 : 18 in F 1 and 24 : 21 (plus six self-fertiles) among the F 2 progeny. Many F 1 progeny isolates were highly aggressive on both hosts, but 15 and 23% of isolates preferred tomato and potato, respectively. Among F 2 progeny, few highly aggressive isolates were recorded and a quarter of isolates lost pathogenicity almost completely. Isolates preferring tomato increased, and those preferring potato significantly decreased in the F 2 progeny. Inheritance of mitochondrial haplotype in F 1 progeny was uniparental and mostly (25 of 27) from one parent only. Six and four different SSR genotypes were identified in F 1 and F 2 progeny, respectively, of which two were identical to the parents. The two microsatellite loci, 4B and 4G, segregated in the ratios 15 : 22 : 2 and 22 : 17 in F 1 and 24 : 17 and 26 : 15 in F 2 progeny, respectively, while the majority of AFLP markers segregated in either a 1 : 0, 1 : 1, 3 : 1 or 1 : 2: 1 ratio. There was no obvious association between AFLP and SSR genotypes, nor between genotypic and phenotypic traits.
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