Masters Degrees (Plant Pathology)

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Browsing Masters Degrees (Plant Pathology) by Subject "Apple replant pathogens -- Quantification techniques"

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    Quantification of apple replant pathogens from roots, and their occurrence in irrigation water and nursery trees
    (Stellenbosch : Stellenbosch University, 2016-03) Moein, Shahrooz; McLeod, Adele; Mazzola, Mark; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: Apple replant disease (ARD), an economically important disease of apple, occurs when young apple trees are planted on soil previously cultivated to apple or closely related species. Soil is considered as the main inoculum source of pathogens causing ARD, which include some Rhizoctonia spp., Cylindrocarpon-like spp., parasitic nematodes (Pratylenchus spp.) and oomycetes (Pythium, Phytopythium and Phytophthora). However, additional inoculum sources might be nursery trees and irrigation water. Investigations into inoculum sources, and effective disease management strategies, require knowledge on relationships between pathogen quantification techniques and the extent of plant damage. The relationship between quantification techniques [percent root infection, and pathogen DNA biomass (absolute and relative)] and disease in apple seedlings were investigated for the ARD pathogens Pythium sylvaticum, Pythium irregulare, Pythium ultimum, Phytopythium vexans and Phytophthora cactorum in glasshouse trials. Quantification data from natural P. irregulare nursery infections were also investigated. In glasshouse trials, the percent root infection and pathogen DNA biomass quantities (absolute and relative) were good predictors of apple seedling growth reductions for P. sylvaticum, Pht. vexans and P. ultimum, when targeting the fine feeder root system. Significant and high correlations were also found for these pathogens between pathogen DNA biomass and percent infected roots. This, however, was not true for Ph. cactorum and P. irregulare. For P. irregulare only a low, but significant correlation was found between percent root infection and seedling stunting. In the nursery where P. irregulare was investigated, good and significant correlations were found between percent root infection and pathogen DNA biomass (absolute and relative). Apple nursery trees and irrigation water (only oomycetes) were investigated as ARD inoculum sources over two seasons in the Western Cape province of South Africa. Nursery trees were a large inoculum source of P. irregulare, Cylindrocarpon-like spp. and Pratylenchus spp., and to a lesser extent of P. ultimum. Pythium sylvaticum was rarely detected, whereas Ph. cactorum and Pht. vexans were absent in nursery trees. The nurseries differed in the occurrence of trees infected with specific pathogens, but none were free of ARD pathogens. Almost all trees were infected with P. irregulare (95%) and Cylindrocarpon-like species (100%). Pythium ultimum was present in 60% of nurseries and 41% of trees. An average of 35% of trees were infested with Pratylenchus spp., with some trees (6-22%) having unacceptably high infestation levels. Irrigation water analyses in dams, and at the exit point of irrigation lines over a 5 month period in each of two seasons revealed the presence of a few ARD pathogens. Only P. irregulare was considered as important and was detected in each of the sampled months in 31 to 82% of the orchard water samples, and the pathogen also occurred in dam water. Pythium ultimum, Phytopythium litorale and Pythium spp. complex B2A were rarely detected in orchard water. The study demonstrated that percentage root infections and pathogen DNA biomass quantity (absolute and relative) can provide a good prediction of apple seedling growth reduction for P. sylvaticum, Pht. vexans and P. ultimum, but not Ph. cactorum and P. irregulare. Oomycete pathogen DNA biomass in roots can be quantified equally well using relative or absolute qPCR amplification. Knowledge was also gained on the colonization pattern of the root system by different oomycete pathogens. Apple nursery and irrigation water of apple orchards were shown to be inoculum sources of apple replant pathogens by using two approaches, conventional isolation and qPCR assay. qPCR significantly improved detection of ARD pathogens relative to conventional isolations. For nurseries, rootstock management strategies will be required. The biological significance of ARD oomycete pathogens in irrigation water requires further investigation.