Browsing by Author "Denman, S."

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    Characterization of South African isolates of Phytophthora infestans
    (American Phytopathalogical Society, 2001) McLeod, A.; Denman, S.; Sadie, A.; Denner, F. D. N.
    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 Although phenylamides were withdrawn from the southern coastal region in December 1996, screening tests carried out in 1998 indicated that resistance levels remained high (≥83%).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.
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    Fungicide sensitivity of phaeomoniella chlamydospora, the causal organism of petri grapevine decline
    (South African Society for Enology and Viticulture, 2000) Groenewald, M.; Denman, S.; Crous, P. W.
    Twelve fungicides, benomyl, chlorothalonil, fenarimol, fosetyl-Al, iprodione, kresoxim-methyl, mancozeb, metalaxyl, prochloraz manganese chloride, quintozene, tebuconazole and thiram were screened in vitro for their ability to inhibit mycelial growth of Phaeomoniella chlamydospora, the causal organism of Petri grapevine decline. Isolates of Pa. chlamydospora were obtained from different geographical areas in the Western Cape province. The effective concentration at which 50% of mycelial growth was inhibited (EC50) was calculated for each fungicide. Benomyl, fenarimol, kresoxim-methyl, prochloraz manganese chloride and tebuconazole were the most effective in inhibiting mycelial growth of Pa. chlamydospora with ECso values ranging from 0.01 to 0.05 μg/mL. Data obtained in this study represent the base-line sensitivity of local isolates to these fungicides, which is important for monitoring the development of pathogen resistance to fungicides.
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    In vitro screening of fungicides against phomopsis viticola and diaporthe perjuncta
    (South African Society for Enology and Viticulture, 2000) Mostert, L.; Denman, S.; Crous, P. W.
    Phomopsis viticola is the cause of Phomopsis cane and leaf spot disease of grapevines, while Diaporthe perjuncta is associated with bud mortality. The efficacy of nine fungicides (azoxystrobin, flusilazole, folpet, fosetyl-Al + mancozeb, kresoxim-methyl, mancozeb, penconazole, spiroxamine and trifloxystrobin) against isolates of P. viticola and D. perjuncta was determined in vitro using the mycelial growth test. Additionally, azoxystrobin, folpet, kresoxim-methyl, mancozeb, propineb and trifloxystrobin were tested for their ability to inhibit spore germination in vitro. Ten isolates of P. viticola and three of D. perjuncta were used in the mycelium inhibition tests, and five isolates of P. viticola in the spore germination tests. The effective concentration at which mycelial growth was inhibited by 50% and at which 50% of the spores (EC50 value) were inhibited from germinating was calculated for each isolate/fungicide combination. In the mycelium growth test flusilazole, penconazole and trifloxystrobin gave better inhibition at lower concentrations than folpet and fosetyl-Al + mancozeb. No significant differences in the mean ECso values were detected among azoxystrobin, flusilazole, kresoxim-methyl, penconazole, spiroxamine and trifloxystrobin. There were also no significant differences among the mean EC50 values of azoxystrobin, kresoximmethyl and mancozeb. Flusilazole and penconazole inhibited mycelial growth at the lowest mean EC50 values obtained. Kresoxim-methyl and trifloxystrobin inhibited spore germination at lower concentrations than folpet or mancozeb. Folpet required the highest concentration to inhibit 50% germination and was significantly different from mancozeb and propineb. There were also no significant differences among the mean EC50 values of mancozeb, propineb and azoxystrobin. The mean EC50 values of the strobilurin fungicides were not significantly different from one another. These results indicate that the strobilurin fungicides inhibited mycelial growth and spore germination of P. viticola. Trials need to be conducted to verify these findings under field conditions.