Browsing by Author "McLeod, A."

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    The 2009 late blight pandemic in the eastern United States - causes and results
    (American Phytopathological Society, 2013) Fry, W. E.; McGrath, M. T.; Seaman, A.; Zitter, T. A.; McLeod, A.; Danies, G.; Small, I. M.; Meyers, K. L.; Everts, K.; Gevens, A. J.
    The tomato late blight pandemic of 2009 made late blight into a household term in much of the eastern United States. Many home gardeners and many organic producers lost most if not all of their tomato crop, and their experiences were reported in the mainstream press. Some CSAs (Community Supported Agriculture) could not provide tomatoes to their members. In response, many questions emerged: How did it happen? What was unusual about this event compared to previous late blight epidemics? What is the current situation in 2012 and what can be done? It's easiest to answer these questions, and to understand the recent epidemics of late blight, if one knows a bit of the history of the disease and the biology of the causal agent, Phytophthora infestans.
  • Thumbnail Image
    Item
    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.
  • Thumbnail Image
    Item
    A protocol for molecular detection of Phaeomoniella chlamydospora in grapevine wood
    (Academy of Science for South Africa, 2005) Retief, E.; Damm, U.; Van Niekerk, J. M.; McLeod, A.; Fourie, P. H.
    PETRI DISEASE IS A SERIOUS DECLINE AND dieback disease of young grapevines. The principal causal organism, Phaeomoniella chlamydospora, is distributed mainly by infected propagation material. Pathogen detection and accurate diagnosis are currently based on fungal isolation in artificial growth media. The fungus is extremely slow-growing, however, and cultures are often overgrown by co-isolated fungi before it can be identified. To avoid this problem, we have developed and validated an efficient and cost-effective protocol for the molecular detection of Pa. chlamydospora in grapevine wood. This novel molecular technique, using a species-specific PCR, detected as little as 1 pg of genomic Pa. chlamydospora DNA. The protocol was validated with grafted grapevines from different nurseries, including grapevines that were first treated with hot water. The basal end of the rootstock was analysed for Pa. chlamydospora by means of both isolations in artificial medium and molecular detection. The latter was found to be considerably more sensitive than isolations, and detected Pa. chlamydospora in samples that recorded both positive and negative in isolations. The identity of PCR products obtained from a subset of samples that tested positive only for Pa. chlamydospora, based on molecular detection, was confirmed to be Pa. chlamydospora-specific through restriction digestion with AatII. The pathogen was not isolated from samples treated in hot water. However, as expected, Pa. chlamydospora DNA was detected in samples exposed to hot water at rates similar to those detected in material not immersed in hot water.