Doctoral Degrees (Plant Pathology)

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Browsing Doctoral Degrees (Plant Pathology) by Subject "Apple industry -- South Africa"

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    Management of postharvest diseases of apples using essential oils
    (Stellenbosch : Stellenbosch University, 2015-12) Mbili, Nokwazi Carol; Lennox, Cheryl L.; Vries, F. A.; Opara, U. L.; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: Grey mould, blue mould and bull’s eye rot, caused by Botrytis cinerea, Penicillium expansum and Neofabraea alba, respectively, are postharvest diseases which cause significant losses to the apple industry in South Africa and other parts of the world. Current control methods of the diseases raises some concerns, such as development of fungicide resistance, residues harmful to humans, and increasing public health restrictions on the use of certain fungicides. These public concerns over synthetic pesticides in foods and the environment has spurred interest among researchers and the global public to find effective and safe non-fungicide means of controlling postharvest pathogens. The aim of this study was therefore to evaluate the effect of lemongrass and citrus essential oils in combination with cold storage regimes to control B. cinerea, P. expansum and N. alba, both in vitro and in vivo. Volatile composition of essential oils of lemon (Citrus limon), lime (Citrus aurantifolia) and lemongrass (Cymbopogan citratus) were analysed using gas chromatography-mass spectrometry (GC-MS). A total of 44 volatile compounds were detected and identified in C. limon, with limonene (58.52%) and γ-terpinene (19.80%) as the major compounds; C. aurantifolia showed 20 components with limonene (79.00%) and γ-terpinene (11.22%) as the abundant compounds; while C. citratus revealed 26 components with geranial (48.14%) and neral (38.32%) as the major compounds. Direct contact and vapour phase methods were used to test the in vitro antifungal activity of citrus and lemongrass oils against B. cinerea, P. expansum and N. alba. Direct contact phase of lemongrass oil, mixtures of lemon + lemongrass and lime + lemongrass oils exhibited the strongest toxicity and completely inhibited the mycelial growth and spore germination of all three tested pathogens, at concentrations of 1.50-100.00% at 20°C (95% relative humidity), regular atmosphere (-0.5°C, air), and controlled atmosphere (5%O2 + 10% CO2, -0.5°C). Lemongrass oil, mixtures of lemon + lemongrass and lime + lemongrass oils were fungicidal against the three postharvest pathogens both in direct contact and vapour phase of 1.50% and 0.13%, respectively, at 20°C, regular atmosphere and controlled atmosphere. Lemon and lime oils also showed antifungal activity, but were much less effective than lemongrass essential oil. In the vapour phase method, all essential oil treatments significantly (p < 0.05) controlled all three pathogens at concentrations of 0.02-1.00% compared to control treatments. Mycelial growth of fungal pathogens was inhibited by the tested essential oils in a dose-dependent manner. The mechanisms of antifungal action of lemongrass oil against B. cinerea was investigated. Light microscopy and scanning electron microscopy observations indicated that B. cinerea hyphae exposed to lemongrass oil undergo morphological damage such as vesiculation, cytoplasmic disruption and collapsed hyphae. Glucose-induced reduction in external pH of B. cinerea was inhibited by lemongrass oil in a time and concentration dependent manner. Botrytis cinerea spores treated with lemongrass oil showed strong propidium iodide fluorescence in the cytosol. Lemongrass oil significantly altered the plasma membrane, the release of cell constituents, and the total lipid content of B. cinerea. These observations indicate that the antifungal activity of lemongrass oil can be attributed to the disruption of the cell membrane integrity and membrane permeability. Application of lemongrass and citrus essential oils through fumigation, thermal fogging and dipping were tested in vivo for their potential to inhibit postharvest decay caused by B. cinerea, P. expansum and N. alba on ‘Granny Smith’, ‘Golden Delicious’ and ‘Pink Lady’ apples. Treated fruit were stored at controlled atmosphere (‘Granny Smith’ and‘Pink Lady’: 1.5% O2 + 1% CO2,-0.5°C and ‘Golden Delicious’: 1.5% O2 + 2.5% CO2,-0.5°C) for 28 days followed by 7 days at 20°C. After storage, lesion diameter was measured and expressed as percentage inhibition relative to control treatment. Application of citrus and lemongrass essential oils through fumigation, dipping and thermal fogging showed some potential to inhibit B. cinerea, P. expansum and N. alba in vivo. Postharvest control of B. cinerea, P. expansum and N. alba by essential oils through dipping and thermal fogging in combination with controlled atmosphere indicated that a mixture of lime + lemongrass oil was the most effective, followed by a mixture lemon + lemongrass oil, lemongrass oil, lime oil and lemon oil. The effects of citrus and lemongrass essential oil treatments on quality parameters of apples was evaluated after controlled atmosphere storage. Citrus and lemongrass essential oils tested had no significant effect on fruit firmness of ‘Granny Smith’ and ‘Pink Lady’ apples. Significant differences were observed in total soluble solids, titratible acidity, pH and total soluble solids: titratible acidity ratio of both cultivars treated with essential oils compared to control treatments. Citrus and lemongrass essential oil exerted a positive influence in postharvest and quality of ‘Granny Smith’ and ‘Pink Lady’ apples. The results presented in this thesis highlight the potential of essential oils in combination with cold storage regimes as an alternative control strategy against grey mould, blue mould and bull’s eye rot in the apple industry. This suggest the application of essential oils by incorporating the oil into wax or coating treatments that are being used in the packing-line and the possibility of fumigation and thermal fogging during the storage period.
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    Responses of Venturia inaequalis to sanitation and regional climate differences in South Africa
    (Stellenbosch : Stellenbosch University, 2014-04) Von Diest, Saskia Gudrun; Lennox, Cheryl L.; MacHardy, William E.; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: The apple industry in South Africa currently relies entirely on chemical fungicides to control apple scab, caused by Venturia inaequalis. In this dissertation, alterative management strategies against V. inaequalis were tested for the first time in South Africa. New information on the behaviour of the sexual winter phase of V. inaequalis in different climatic conditions was found and sources of asexual inoculum overwintering in apple orchards were identified. The effect of leaf shredding on fruit and leaf scab incidence and severity was tested against a non-shredded, non-sprayed negative control, a positive control that followed a commercial fungicide programme and a combined treatment of a commercial fungicide programme with leaf shredding, from 2010 to 2013. Reductions in fruit and leaf scab incidence and severity in the leaf shredding treatment were significantly lower compared to the negative control. Quantitative real-time polymerase chain reaction (qPCR) of airborne ascospores trapped using volumetric spore traps was used to measure the reduction in airborne ascospores in the shredded plots, and confirmed the efficacy of shredding found by comparing scab incidence and severity on fruit and leaves. Shredding twice during leaf-drop increased the efficacy of the treatment. Results indicate that leaf shredding should be integrated into scab management strategies in future. However, practical considerations unique to South African orchards, e.g. timing of leaf shredding relative to leaf-drop and orchard layouts, need to be addressed. Pseudothecial densities (PD, number of pseudothecia per fertile lesion) and ascal densities (AD, number of asci per pseudothecium) were compared between in Koue Bokkeveld (KB), a cold winter region, and Elgin (EL), a warm winter region experiencing climate warming, in 2012 and 2013. Scabbed leaves were detached during leaf-drop and overwintered in their region of origin and in the other region. The PD in leaves collected in KB and overwintered in KB was significantly higher than for leaves collected in EL and overwintered in EL, and leaves collected in KB and overwintered in EL. These results agreed with what was expected, as temperature during pseudothecial formation (i.e. the first four weeks after leaf-drop) was significantly lower in KB than in EL. However, the PD for leaves collected in EL and overwintered in EL did not differ significantly from EL leaves overwintered in KB. AD values in all treatments did not differ significantly from one another. Results suggest that factors other than temperature may be involved in controlling PD, e.g. the EL population may include strains not present in the KB population, with higher optimal temperatures for pseudothecial formation. Apple buds and pygmy apples were collected and tested for presence, number and viability of conidia in 2010, 2011 and 2012. Pygmy apples are small, late season fruit that remain attached to the tree throughout winter, especially in regions with warmer winters where trees do not experience sufficient chilling to complete dormancy. High conidial numbers were found on outer bud tissue and low numbers on inner bud tissue, but viable conidia were only found on inner bud tissue, using microscopy, and generally in orchards with high scab levels in the previous season. Molecular methods using PCR-RFLP and qPCR confirmed the presence of high amounts of V. inaequalis DNA in outer bud tissues, although calculated conidial amounts were higher than data obtained when using microscopy, which could indicate presence of mycelia not detected during microscopic examination. Higher numbers of conidia with higher percentage viability were found on pygmy apples, which are a more likely source of asexual inoculum in South African apple orchards than the low number of viable conidia on inner bud tissue.