Characterization of South African Isolates of Phytophthora infestans

Abstract

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 Although phenylamides were withdrawn from the southern coastal region in December 1996, screening tests carried out in 1998 indicated that resistance levels remained high (≥83%).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.

Mating Type, Virulence, Aggressiveness and Metalaxyl Resistance of Isolates of Phytophthora Infestans in Poland

“…Complex races prevailed both in our study and in the previous ones (Zarzycka et al 2002;Sujkowski et al 1996). Complex races rarely possessing virulence factor 9 were also detected in other parts of the world (Hermansen et al 2000;McLeod et al 2001;Elansky et al 2001;Cohen 2002).…”

“…The A2 mating type is still absent in some parts of the world e.g. in South Africa (McLeod et al 2001) and in some parts of Russia, like Sakhalin or Omsk (Elansky et al 2001). In other countries, the percentage of A2 varies from a few percent in England and Wales (Day et al 2004), 15% in Finland, 25% in Norway (Hermansen et al 2000, 50% in Toluca Valley, Mexico (Grünwald et al 2001) to even 100% in some parts of Russia: Khabarovsk and Birobijahn (Elansky et al 2001) or in Uruguay (Deahl et al 2003).…”

Caracterização de isolados de Phytophthora infestans do Distrito Federal e de Goiás

“…Em outras regiões do Brasil, fora do Distrito Federal e Goiás, verificou-se que as duas populações do patógeno continham isolados resistentes, intermediários e sensíveis ao metalaxyl (Reis et al, 2005). Este fato também foi observado em Filipinas (Koh et al, 1994), África do Sul (McLeod et al, 2001) e Equador (Adler et al, 2004), países tropicais ou subtropicais onde a população do patógeno também é clonal.…”

Resistance to Phytophthora infestans in Solanum tuberosum landraces in Southern Chile

“…In Southern Chile, highly aggressive blight attacks on potato crops with losses greater than 50% have been observed during several years. This may be explained by changes in the populations of the fungus towards more virulent forms due to excessive applications of metalaxyl (Pérez et al, 2001;Páez et al, 2005) or more favourable environmental conditions (McLeod et al, 2001). Acuña et al (2007) determined that the populations of P. infestans in Chile are only of the A1 (asexual) mating type; no sexual (A2) mating types have been reported in Chile yet.…”