Masters Degrees (Plant Pathology)

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Browsing Masters Degrees (Plant Pathology) by browse.metadata.advisor "Fourie, Paul H."

Now showing 1 - 4 of 4
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    Fungicide resistance and control of citrus green mould
    (Stellenbosch : Stellenbosch University, 2014-04) Kellerman, Mareli; Fourie, Paul H.; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: Please refer to full text for abstract
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    Imazalil and wax coating application in citrus packhouses to control green mould and preserve fruit quality
    (Stellenbosch : Stellenbosch University, 2012-03) Njombolwana, Ncumisa Shelly; Fourie, Paul H.; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: Green mould is a major citrus postharvest disease caused by a wound pathogen called Penicillium digitatum. Imazalil (IMZ) is the most important postharvest fungicide that is currently registered against green mould and blue mould, given its good protective and curative control and sporulation inhibition. Imazalil can be applied in drench, dip, spray and wax coating applications. Wax coatings improve the quality of the fruit during shelf life by limiting moisture loss and providing a shiny appearance. The primary objective of this study was to investigate the application of imazalil in wax coatings in citrus packhouses to control green mould and preserve fruit quality. The first aim of this study was to evaluate the protective and curative control and sporulation inhibition of IMZ sensitive and resistant isolates of P. digitatum following single IMZ application in wax coating and IMZ application in aqueous dip as well as wax coating. For single application, Valencia orange fruit was curatively or protectively treated with a carnauba-based coating into which IMZ at 3000 μg.mL-1 was mixed. For curative treatments, fruit was treated after 24 hour's incubation. Coating was applied at 0.6, 1.2 and 1.8 L.ton-1 of fruit on a commercial coating applicator. Untreated control and fruit treated with coating only showed up to 80% infection for both sensitive and resistant isolates. Imazalil in coating proved to have better protective than curative action against the sensitive isolate, while control of the resistant isolate was poor. Imazalil residue levels increased with increasing coating load (0.85 to 1.75 μg.g-1). For double application, clementine, satsuma, navel and Valencia orange fruit were used. Curative and protective control following dip only treatment (IMZ sulphate at 500 μg.mL-1 for 45 s and 90 s) was compared with coating only treatment (IMZ at 3000 μg.mL-1 at 1.8 L.ton-1), and double application comprising dip (45 s in IMZ sulphate at 500 μg.mL-1) followed by 2000 μg.mL-1 IMZ in coating at 0.6, 1.2 and 1.8 L.ton-1. Double application resulted in improved residue loading (0.85 to 2.06 μg.g-1) compared to dip only treatments (≈ 0.49 μg.g-1). Coating only resulted in the highest residue levels (1.82 to 7.09 μg.g-1), often exceeding the maximum residue limit of 5 μg.g-1. In all treatments, poor curative and protective control of the resistant isolate (<46% and <55%, respectively) and generally no sporulation inhibition was observed. For the sensitive isolate, dip only treatments resulted in better curative control (≈ 77%) than protective control (≈ 38%). Double application showed increased protective control with increasing wax load (≈ 69%) as well as good curative control (≈ 83%). Wax coating only treatment resulted in poor curative control (≈ 26%), but good protective control (≈ 80%). Sporulation inhibition of the sensitive isolate was generally improved in treatments that included IMZ application in the wax coating. This study successfully showed the additive benefits of double application of IMZ in the wax coating following an aqueous dip application. However, despite improved control of the sensitive isolate, the resistant isolate could not be controlled. The second objective of the study was to evaluate green mould control and quality preservation following IMZ application using different coating and brush types. The inoculation and treatment procedure was similar to the single application of wax coating described above but only the sensitive isolate of P. digitatum was used. Carnauba- or polyethylene-based wax coatings, including IMZ at 3000 μg.mL-1, was applied at a dosage of 0.6, 1.2 and 1.8 L.ton-1 using commercial horsehair or synthetic brushes. Fruit was either incubated for 4 days prior to wound infection ratings or stored at -0.5oC for 26 days and 7 days shelf life. Imazalil residues on fruit increased with increasing coating loads on navel oranges (1.31 to 3.32 μg.g-1) and Valencia oranges (3.22 to 6.00 μg.g-1). Better protective (≈ 58%) than curative control (≈ 14%) was observed, with horsehair brushes resulting in lower sporulation levels (≈ 59%) than synthetic brushes (≈ 64%). Fruit weight loss and firmness loss ratios relative to uncoated control fruit were significantly higher in fruit treated with polyethylene coating compared with carnauba coating treated fruit. However, polyethylene coating resulted in shinier fruit before and after storage, whereas carnauba coating resulted in lower shine ratios. Gaseous (CO2) exchange ratios remained similar for both coatings, but higher polyethylene coating loads (1.8 L.ton-1) resulted in off-tastes similar to the uncoated control fruit and higher than the ratings for carnauba. Scanning electron micrographs revealed an amorphous crystallised natural wax layer with uncovered stomatal pores on the surface of uncoated fruit. The thickness of the applied coating layer increased with increasing coating load. Coating layers following application with synthetic brushes at 1.2 L.ton-1 appeared to be thicker than when applied using horsehair brushes. This study indicated the effects of brush type on the distribution and polishing of the wax coating on the surface of the fruit and also the importance of cautious application of coating as under or over application may lead to poor results in terms of bio-efficacy, residue loading and fruit quality.
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    The implementation and validation of reduced volume argrochemical applications in the south African citrus industry using novel technology
    (Stellenbosch : Stellenbosch University, 2019-03) Van Wyk, Tertia; Van Niekerk, Jan M.; Fourie, Paul H.; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: The South Africa citrus industry is the third largest exporter in the world and is considered one of the most important horticultural crops due to high economic export value. However, citrus trees are susceptible to a wide range of insect pests and fungal diseases. This places pressure on producers to deliver high quality fruit that adhere to strict export requirements. The largest and most important export market of citrus is the European Union (EU), which have a zero-tolerance approach towards Citrus black spot [Phyllostica citricarpa (van der Aa)] and false coddling moth [Thaumatotibia leucotreta (Meyrick)]. This leads to high spray volume applications that are seen as insufficient and not sustainable. The high input costs relating to water, labour and equipment as well as the environmental impact is a result of these high demands for 100% clean fruit. Furthermore, these high volumes are determined without taking canopy density into account, which contributed to high volumes being lost to run-off. The potential of reduced spray volumes has been investigated, however limited trials have been done on the feasibility, implementation and biological efficacy of these different spray volumes in a seasonal commercial spray program. Therefore, the aim of this study was firstly to evaluate the possible reduction of spray volumes in the South African citrus industry without compromising on the need to get 100% control of important pests and diseases. Secondly, to investigate the use of the novel technology (LiDAR) to characterize citrus tree canopy density. For the first objective spray trials were conducted in the Limpopo, Western and Eastern Cape provinces on commercial citrus producing farms. Reduced volumes (750 to 3000 L/ha) were compared with the farm’s standard spraying volume (4000 to 9000 L/ha) evaluating spray deposition parameters such as deposition quantity (FPC%), uniformity (CV%) and quality (ICD%). Furthermore, the pest and disease efficacy were also evaluated in terms of clean fruit. For the second objective trials were conducted on three commercial farms in the Western Cape to determine the effect of three different pruning categories on FPC%, CV% and ICD% in combination with two different spray application volumes (1500 L/ha as the reduced volume and 3000 L/ha as the standard volume). In an attempt to develop a non-destructive technique to measure canopy density use of LiDAR technology was investigated and compared with manual canopy measurements. From this study it was concluded that higher spray volumes result in better control of pests and diseases due to better deposition uniformity values. Furthermore, the importance of the penetration of spray mixtures into the canopy to achieve adequate control of pests and diseases is also essential. The manual manipulation of canopy density by pruning proved to be beneficial for spray deposition in creating more ‘spray-friendly’ canopies. The potential of LiDAR to be used as a calibration tool, was seen in this study, detecting differences in canopy densities. However, the LiDAR parameters were poorly correlated with manual measurements. It is suggested that the application be simplified in future studies for better correlation.
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    Quantification of spray coverage on grape bunch parts and the incidence of Botrytis cinerea
    (Stellenbosch : Stellenbosch University, 2005-04) Brink, Jan-Cor (Johannes Cornelius); Fourie, Paul H.; Holz, Gustav; Stellenbosch University. Faculty of Agrisciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: Various studies revealed that Botrytis cinerea, the causal pathogen of Botrytis bunch rot, is mostly associated with pedicels, rachises, laterals and berry bases, and not with berry skins as previously understood. Provided that sufficient coverage of inner bunch parts was achieved, laboratory studies have shown that fungicides can effectively reduce the amount of B. cinerea at the various positions in bunches, and prevent infection and symptom expression at all growth stages. The same efficacy was, however, not achieved with the same fungicides when using conventional spraying methods in vineyards. Poor disease control on fruit and leaves in vineyards is attributed to inappropriate timing of fungicide applications and/or insufficient coverage of susceptible tissue. Previously, spray coverage evaluations in South Africa were based on the use of water-sensitive cards. A variety of other methods have been used to assess spray coverage in vineyards, but none of these methods could assess spray deposits on a very small, three-dimensional area of interest such as the susceptible grape bunch parts. The methods were furthermore dependent on human objectivity, which lacks quantitative measuring and speed of measurement. Suitable technology to determine spray coverage on susceptible bunch parts is, therefore, not available. The aim of this study was to develop a protocol to visualise and quantify spray deposits in grape bunches, specifically on the inner bunch parts and to use the protocol to determine the effect of different levels of spray cover on artificially inoculated B. cinerea grape bunches, in order to facilitate future determination of minimum effective coverage levels for effective B. cinerea control. A spray coverage assessment protocol using fluorometry, photomicrography and digital image analyses was developed to measure spray coverage on susceptible grape bunch parts. Among several fluorescent pigments tested, a yellow fluorescent pigment (SARDI Fluorescent Pigment) from Australia was selected on the basis of its small particle size (2.45 - 4.90 μm). Bunches were sprayed at pea size and bunch closure with different volumes of a mixture of fenhexamid and the yellow fluorescent pigment. Sprayed parts from bunches were illuminated under black light (UV-A light in the 365 nm region) and visualised under a stereo microscope at 20 x magnification. Photos of the berry skin, pedicel and rachis were taken with a digital camera (Nikon DMX 1200). Image analysis of photos was done with Image- Pro Discovery version 4.5 for Windows (Media Cybernetics) software. The total area of deposited pigment in selected areas of interest (AOI) was calculated. The percentage area covered was subsequently calculated for each AOI. Good correlation was evident between the parameters, sum of objects and percentage area covered. Bunch parts at pea size generally had higher coverage values than at bunch closure. Spray applications earlier in the season would therefore result in higher and more effective spray coverage of the susceptible bunch parts. Similar deposition trends were observed on the inner bunch parts (pedicel and rachis). These were, however, significantly different from berry skins, which had significantly higher levels of spray deposits than the inner bunch parts. The variance component analysis indicated that the highest variance was observed for berries and bunches, and substantially less for image readings. For the same accuracy, means for percentage coverage values of at least 10 bunches per treatment (1 part per bunch and 3 readings per part) will be sufficient. In order to determine the biological efficacy of different levels of spray coverage on B. cinerea incidence on grape bunches, bunches were sprayed at pea size and bunch closure with different volumes of a mixture of fenhexamid and a yellow fluorescent pigment and the percentage fluorescent pigment coverage on pedicels was determine. Bunches were subsequently dusted with dry airborne conidia of B. cinerea in a settling tower and incubated for 24 h at high relative humidity (98%). Infection was determined by estimating the amount of B. cinerea infections occurring on sprayed bunch parts with isolations on to paraquat and Kerssies mediums. Linear regressions for the part x stage combinations of percentage B. cinerea incidence on different bunch parts were fitted on mean coverage levels. An increase in spray cover caused linear reductions in levels of B. cinerea on susceptible bunch parts. Higher B. cinerea incidences were recorded at pea size. Furthermore, higher B. cinerea incidences were found on paraquat medium for both stages, than on Kerrsies medium. The information gathered from this study will be used to facilitate future determination of minimum effective coverage levels for effective B. cinerea control in grape bunches. In these validation experiments, the results clearly showed that the protocol can be used to determine the effect of different levels of spray coverage on B. cinerea incidence and that an increase in spray coverage will decrease B. cinerea incidence. The information gathered from this study will be used to facilitate future determination of minimum effective coverage levels for effective B. cinerea control in grape bunches and subsequently be used as benchmarks to evaluate spray application in vineyards.