Masters Degrees (Plant Pathology)

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Browsing Masters Degrees (Plant Pathology) by browse.metadata.advisor "Fourie, P. W."

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    Further optimisation of in-line aqueous application of imazalil to control citrus green mould caused by Penicillium digitatum
    (Stellenbosch : Stellenbosch University, 2017-03) Savage, Catherine; Du Plooy, G. W.; Erasmus, A.; Lennox, C. L.; Fourie, P. W.; Stellenbosch University. Faculty of AgriScience. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: South Africa has a successful citrus export industry. A threat to fresh citrus fruit is the fungal pathogen Penicillium digitatum causing green mould. Imazalil (IMZ) is the most important fungicide to combat green mould. Solution pH and temperature, and exposure time of the fruit to the solution, are important when using the sulphate form of IMZ. Research has increased our understanding of IMZ use, but further variables need to be investigated, along with an alternative application method. The control of green mould infection and sporulation by IMZ were tested using a heated flooder. Solution variables included the effects of pH (3; 4; 5; 6), temperature (45; 55; 65°C), and concentration (250 or 500 μg.mL-1) in a time of 8 s. Residues increased with increasing pH, temperature and concentration. The majority average residues loaded were between 0.4 and 3.0 μg.g-1. Treatments at pH 6 loaded higher residues at 55 and 65°C, where the maximum residue limit (MRL) of 5 μg.g-1 was almost always exceeded. The flooder loaded adequate residues, offering good curative and protective control. Sporulation inhibition of green mould was also linked to residues, and complete inhibition was achieved at the higher residue levels. The flooder was an effective applicator of IMZ. The fungicide bath is the most common IMZ application method in South Africa. The ability of IMZ to control green mould was investigated in a cold bath of 10°C and compared to ambient temperature and 35°C baths. Solution temperature had no significant effect on IMZ’s ability to cure 24 hr old green mould infections with all temperatures providing control above 80%. Sporulation inhibition and residue loading increased as solution pH, temperature, and exposure time increased. Sporulation inhibition was < 50% in pH 3 baths, irrespective of temperature, complete inhibition was obtained at 35°C and pH 6, but the IMZ MRL was exceeded at longer exposure times (> 45 s). The survival of Rhizopus stolonifer was studied in vitro at various water temperatures (10°C to 65°C) for exposure times of 1 or 60 min, and after a pasteurisation step. Sub-treatments included the addition of IMZ fungicide or green mould spores, with IMZ seemingly having a significant effect on Rhizopus spore survival. The same was not true for solutions at temperatures below 35°C, however, temperatures of 45, 55 and 65°C, particularly after a 60 min exposure, caused a significant reduction in Rhizopus spore viability. Complete Rhizopus eradication was achieved with 65°C and the pasteurisation step. In order to control fungal contaminants in the fungicide bath, packhouses need to apply IMZ in heated solutions (circa 45°C) and/or pasteurize fungicide baths overnight. Imazalil residue levels on citrus can be increased by increasing solution pH, temperature, concentration or exposure time. Most treatments gave excellent infection control and only a low residue is necessary to cure or prevent a green mould infection. Residue levels were closely linked to the level of sporulation inhibition achieved. Both the flooder and dip tank offered good green mould control. Contaminants that build up in solution can be eradicated at high temperatures.
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    Quarantine status of selected fungal pathogens on Malus, Prunus and Vitis species
    (Stellenbosch : Stellenbosch University, 2006-04) Carstens, Elma; Crous, P. W.; Fourie, P. W.; Stellenbosch University. Faculty of Agrisciences. Dept. of Plant and Pathology.
    ENGLISH ABSTRACT: Invasions of alien species into non-native environments pose one of the largest, but least addressed international threats to biodiversity, both within natural ecosystems and agricultural settings. It is without exception ranked as the greatest environmental threat of the 21st century. Their introduction and spread have been identified as one of the six major categories of change that could potentially alter the world's biodiversity. The number and variety of species introduced make it clear that it is no exaggeration to state that biological invasions are breaking down biogeographic barriers that created and maintained the major floral and faunal regions of Earth. It is, however, difficult to conceive that a single indicator could measure the impact of an invader on a country due to the difficulty in measuring their environmental as well as their financial impacts. Another contributing factor to this problem is the scarcity of data available on the impact of the thousands of invaders in other countries. For the agricultural sectors, alien invasive species are likened to a two-edged sword: on the one hand they are used in the development of new plant varieties and products and on the other hand they have the potential to threaten agricultural production and as a consequence agricultural economic development. An important challenge for countries lies in the development of improved protocols for the prevention, identification and managing of potential invasive alien species. Plant health has always influenced international trade and the backbone for nondiscriminatory, fair, predictable and transparent international trade is the set of rules that national governments have agreed to follow with the foundation of the World Trade Organisation (WTO) in 1995. With the establishment of the WTO, the "WTO - Agreement on the Application of Sanitary and Phytosanitary measures (WTO-SPS)" also came into force. This agreement lays out the provisions, rights and obligations of countries in setting measures to protect human, animal and plant life and health. It also guides quarantine policy and decision-making, with the objective to prevent the use of quarantine measures by governments as disguised or unjustified trade barriers to protect their agricultural industries from import competition. As a signatory member of the WTO-SPS, South Africa has the right to implement appropriate measures to protect our plant health and environment. To set the appropriate levels for protection, South Africa must either apply to international standards or undertake a scientific based risk analysis process, to justify quarantine measures. These measures must also be the minimum necessary to protect plant health. A key element in negotiations for market access is the provision of a list of regulated pests, as well as a list of all pests associated with the crop within the exporting country, to the trading partners. In this study lists of fungal pathogens associated with Malus spp. and Vitis spp. worldwide, including South Africa, were compiled to enable the National Plant Protection Organisation in South Africa to comply with their responsibilities as a signatory member of the international regulatory bodies and to assist them to safeguard our country against harmful invasive species. It was concluded that to have access to accurate plant health status information, all researchers are summoned to validate new pest records and to submit voucher specimen to our National Collection. Geographical distribution records of pathogens and pests are the basis for phytosanitary decision-making and therefore it is imperative for countries to have access to accurate information regarding the geographical distribution of pathogens within their boundaries. For a pathogen to be classified as an A 1-pest, the pest should be of economic importance to the endangered area and not yet present there or present, but not widely distributed and being officially controlled. Many disputes have arisen during the past years concerning the classification for Monilinia fructicola and Neonectria galligena as A 1- regulated pests for South Africa, due to official records of the presence of these pathogens dating back as early as 1917. The situation was further complicated by recent reports from some European countries that M fructicola was detected on stone fruit imported from South Africa. These issues were resolved in this study by following a molecular approach and guidelines as stipulated by the international regulatory bodies. The absence of M fructicola as well as N galligena from South African stone and pome fruit orchards was confirmed. The regulated status of M fructicola and N galligena in South Africa is therefore scientifically justified.