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Characterization of South African Isolates of Phytophthora infestans

Severe late blight epidemics in South Africa in 1995 and 1996 prompted an investigation into the mating type, genotype, and metalaxyl sensitivity of populations of Phytophthora infestans. A country-wide survey was conducted from 1996 to 1998 in which isolates were collected from 101 potato fields (656 isolates) and 16 tomato fields (57 isolates). Six hundred and fifty-seven isolates (600 potato and 57 tomato) were analyzed for mating type, while subsets of isolates were analyzed for genotype at the Glucose-6-phosphate isomerase locus (n = 148), DNA fingerprinting with probe RG-57 (n = 61) and mitochondrial DNA haplotype (n = 20). All isolates tested had the characteristics typical of the pre-1980 population (A1 mating type, 86/100 Gpi genotype, US-1 fingerprint pattern, and mtDNA haplotype I-b) previously found worldwide. Metalaxyl sensitivity testing of 656 potato isolates by the in vitro leaf disk method showed that the frequency of highly resistant isolates (50% effective concentration [EC₅₀] > 200 μg a.i./ml) in potato production regions increased from 35% in 1996 to 51% in 1997. The high frequency of resistant isolates was confined to the southern coastal regions in 1996 and 1997, as well as the western Free State in 1997. Although phenylamides were withdrawn from the southern coastal region in December 1996, screening tests carried out in 1998 indicated that resistance levels remained high (≥83%). Sensitive isolates (EC₅₀ < 40 μg a.i./ml) predominated in the remaining six potato production regions. Screening of 45 isolates collected from tomatoes indicated that no resistant strains were present in the sample tested.