Characterization of South African isolates of Phytophthora infestans

Severe late blight epidemics in South Africa in 1995 and 1996 prompted an i nvestigation into the mating type, genotype, and metalaxyl sensitivity of p opulations of Phytophthora infestans. A country-wide survey was conducted f rom 1996 to 1998 in which isolates were collected from 101 potato fields (6 56 isolates) and 16 tomato fields (57 isolates). Six hundred and fifty-seve n isolates (600 potato and 57 tomato) were analyzed for mating type, while subsets of isolates were analyzed for genotype at the Glucose-6-phosphate i somerase locus (n = 148), DNA fingerprinting with probe RG-57 (n = 61) and mitochondrial DNA haplotype (n = 20). All isolates tested had the character istics typical of the pre-1980 population (A1 mating type, 86/100 Gpi genot ype, US-1 fingerprint pattern, and mtDNA haplotype I-b) previously found wo rldwide. Metalaxyl sensitivity testing of 656 potato isolates by the in vit ro leaf disk method showed that the frequency of highly resistant isolates (50% effective concentration [EC50] > 200 mug a.i./ml) in potato production regions increased from 35% in 1996 to 51% in 1997. The high frequency of r esistant 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 Although phenylamides were withdrawn from the southern coastal region in December 1996, screening tes ts carried out in 1998 indicated that resistance levels remained high (grea ter than or equal to 83%). Sensitive isolates (EC50 < 40 <mu>g a.i./ml) pre dominated in the remaining six potato production regions. Screening of 45 i solates collected from tomatoes indicated that no resistant strains were pr esent in the sample tested.