Masters Degrees (Horticulture)
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- ItemEstablishing the water requirements and crop coefficients of a mature mango orchard under subtropical conditions(Stellenbosch : Stellenbosch University, 2024-03) George, Nel; Sebinasi, Dzikiti; Kleinert, Aleysia; Stellenbosch University. Faculty of Agrisciences. Dept. of Horticulture.ENGLISH ABSTRACT: The study was done in the Inkomati - Usuthu Water Management Area (IUWMA) in north - eastern South Africa. The region is a prime production area for a range of crops, mostly subtropical tree crops (banana, citrus, litchi, macadamia, mango etc.) and sugarcane, mainly under irrigation. Yet water resources in the catchment are almost fully allocated due to the rising competition between various water use sectors e.g. agriculture, industrial development, eco-tourism etc. The increasing frequency and severity of droughts due to climate change are exacerbating the problem, so there is a need for accurate water management practices to maximize crop production per drop of water. The goal of this study was to establish the maximum water requirements of mango orchards to improve the management of scarce water resources in the catchment. The focus was also on generating information on how water use related to the yield of mango orchards under local growing conditions. To support economic development in the catchment, the IUWMA plans to implement Water Allocation Reforms (WAR) to provide fair access to water to various water users in the catchment and to fulfil the requirements of the National Water Act of South Africa (of 1998). Implementation of these reforms requires accurate quantitative information on the water use of key irrigated crops like mango and other tree crops. In this study, we used state-of-the-art methods to measure the actual water use of mango orchards, collecting detailed data on the soil-plant-atmosphere interactions over close to two growing seasons. Data were collected at the leaf level using infrared gas analysers while whole tree transpiration rates were quantified using the heat ratio sap flow method. An open path eddy covariance system was used to measure the whole orchard evapotranspiration rates. To ensure that the results obtained from the specific study orchard can be transferred to mango orchards elsewhere, we used the measured data to test and compare the performance of two independent models of estimating orchard water use. Key findings from the study are that firstly, based on detailed leaf gas exchange measurements, mango trees have significant stomatal and non - stomatal limitations to net CO2 assimilation. The trees are predominantly isohydric in nature, closing their stomata under unfavourable environmental conditions, e.g. excessive atmospheric evaporative demand, soil water deficit, etc. This, in turn affects their transpiration rates. The observed maximum net CO2 assimilation rate for well-watered trees under clear sky conditions was less than 6.0 μmol/m²/s. This is substantially lower than that of other irrigated fruit tree crops reported in literature. Secondly, the daily maximum transpiration of a fully grown mango tree (leaf area index between 3.3 and 3.7) ranged between 40 and 62 litres of water per tree per day depending on canopy size and weather conditions. The measured annual total transpiration was about 601 mm (or 6 010 m³/ha/y), while the annual evapotranspiration predicted by the water use models ranged from 880 to 910 mm/y (or 8 800 to 9 100 m³/ha/y). The farm scheduled their irrigation using a crop coefficient of 1.0 during the entire irrigation season. However, this study showed that the actual crop coefficient was not constant; it varied between 0.75 and 0.93 depending on the crop growth stage. This highlighted an opportunity for significant water savings if the correct crop coefficients are used. Lastly, but also quite significantly, this study modified and improved an approach for deriving both the crop coefficients and basal crop coefficients for mango orchards using readily available data such as the average tree height and the fractional vegetation cover. This approach will allow mango growers to obtain accurate estimates of crop coefficients for their orchards depending on their specific conditions e.g. tree age, soil type, irrigation system, cover crop status, etc. These and other findings are summarized in a manuscript that is currently in the second round of review by the Scientia Horticulture journal.
- ItemInvestigation of the effect of cooling and delays as well as CO2 concentrations during storage on internal browning in 'Fuji' apples(Stellenbosch : Stellenbosch University, 2024-03) Kriegler, Anmari; Botes, Wilhelmina Jacoba; Jooste, Mariana; Crouch, Elke; Stellenbosch University. Faculty of Agrisciences. Dept. of Horticulture.ENGLISH ABSTRACT: High-quality apple fruit is expected from various markets throughout the year. The use of controlled atmosphere (CA) storage is implemented to increase the availability of high-quality apple fruit. Unfortunately, the storage conditions required during CA also cause the appearance of physiological defects such as internal browning (IB) that lead to several market and financial losses. Since CA storage conditions contribute to IB occurrence, the possible use of stepdown cooling treatments was investigated in the first objective to determine if it influences IB as well as the fruit quality of ‘Fuji’ apples. Harvest maturity played an important role in the development of physiological defects since bitter pit occurred when fruit were harvested closer to optimum maturity while IB occurred when fruit were harvested at a more post-optimum maturity. Ethylene influenced bitter pit development since the stepdown treatments had the highest ethylene concentration and bitter pit levels. The study concluded that fruit should be harvested at optimum maturity and be room cooled under regular atmosphere (RA) for 49 d at 0.5 °C immediately after harvest and should be stored for longer than 4 months in CA (1.5 kPa O2 and 0.5 kPa CO2) to prevent IB and bitter pit development. The second objective was to determine the effect of delayed cooling and delayed CA establishment on CO2 injury and fruit quality of ‘Fuji’ apples. Again, the importance of harvest maturity was emphasized since CO2 injury developed in 2022 when fruit were harvested at post-optimum maturity and had an overall effect on fruit quality. This study found that rapid cooling of fruit after harvest is important to maintain fruit quality and several fruit quality and biochemical parameters were affected by the delayed cooling treatments. The development of CO2 injury was significantly affected by the storage duration, especially after shelf-life. Several findings of the first objective were confirmed during this study. During the third objective the possibility of predicting CO2 injury after long-term CA storage (6 months) by subjecting fruit to a short-term exposure of 3 d to high CO2 concentrations after harvest was investigated. In 2022, when fruit were harvested at post-optimum maturity, CO2 injury developed during short- and long-term CO2 treatments. However, 0.5 kPa CO2 treatment resulted in the highest CO2 injury after each evaluation period. The harvest maturity and the CO2 concentration play important roles in the development of CO2 injury during long-term CA storage and had an impact on overall fruit quality. The study concluded that a short-term treatment involving high CO2 and low O2 stress after harvest is not an accurate method for predicting CO2 injury development during long-term CA storage. Furthermore, the results suggested that harvesting ‘Fuji’ apples at optimum maturity not only prevents CO2 injury, but also enables the use of lower CO2 concentrations while maintaining fruit quality.
- ItemThe stability of productivity and fruit quality traits of ‘Fuji’ apples on different Rootstocks(Stellenbosch : Stellenbosch University, 2024-03) Ngindi, Buhle; Siboza, Xolani ; Theron, Karen ; Stellenbosch University. Faculty of Agrisciences. Dept. of Horticulture.ENGLISH ABSTRACT: Rootstocks of apple trees are essential for productive and economical orchards. Rootstocks influence tree architecture, yield, and fruit quality. Most of the current rootstocks used in South African apple orchards lack precocity, are vigorous and not able to overcome some of the challenges faced in these orchards. Some of these challenges include poor soils, lack of winter chill as well as the prevalence of pests and diseases. Thus, productivity is negatively affected. The Geneva rootstocks are being bred to confer pest and disease resistance/tolerance, a range of tree vigour, but highly precocious. In this study, rootstock genotypes were planted in two separate but adjacent plantings, i.e., vigorous rootstocks and dwarfing rootstocks. The Fuji cultivar was evaluated on these genotypes in three different environments across South Africa – Oak Valley Estate and Breëvlei, both in Grabouw, Western Cape, and Helderwater, Eastern Cape. These environments differed in climatic conditions, soil conditions and management practices such as training systems used, irrigation and pruning strategies. The vigorous plantings consisted of MM.109, M.793, M.7 EMLA (control), G.778, G.228 and G.202. The dwarfing plantings comprised of M.9 EMLA, M.9 Nic29, M.7 EMLA (control), G.222, G.778/G.222, G.778/M.9 EMLA, MM.109/G.222, MM.109/M.9 EMLA, M.793/G.222 and M.793/M.9 EMLA. The influence of rootstocks on tree vigour, yield and yield efficiency, and fruit quality traits (mass, diameter, ground colour, red foreground colour, firmness, total soluble solids (TSS) and starch breakdown) was determined. The tendency of ‘Fuji’ apples planted on different rootstocks to bear alternately was investigated from the first year of harvest (2016/2017) to 2020. Rootstock productivity maybe influenced by the genotype (i.e., rootstock and scion), the environment and the interaction between the genotype and environment (GEI). The environment includes soil, climate and management factors. The influence of GEI on tree vigour and productivity was investigated. The presence of GEI complicates selection and requires that the stability and adaptability of genotypes be assessed to select for the most stable and adaptable rootstock genotypes. Stability refers to a genotypes’ ability to behave predictably against environmental fluctuations whilst adaptability refers a genotype’s ability to show high and constant yields in certain environments. For the vigorous rootstocks, GEI was significant (p < 0.001) for all yield traits and was non-significant for tree vigour. The most stable and high-yielding genotype was G.778 based on 2016/20 cumulative yield (149.00 kg/tree) and 2016/20 cumulative yield efficiency (2.39 kg/cm2 TCSA). For the dwarfing rootstocks, the GEI was significant for all yield traits and tree vigour. The most stable and high-yielding dwarfing genotype combinations based on 2016/20 cumulative yield and 2016/20 cumulative yield efficiency were G.778/G.222 and G.778/M.9 EMLA. ‘Fuji’ apples are prone to the development of internal browning during long-term storage. Since rootstocks may influence fruit maturity, the rootstock genotype may influence the occurrence and severity of internal browning. Thus, the influence of rootstocks on the occurrence of internal browning and postharvest fruit quality was investigated. Fruit were stored under controlled atmosphere conditions for 5 months at -0.5 °C, then subsequently moved to regular atmosphere conditions for 6 weeks, after which fruit were subjected to shelf-life conditions. A significant GEI (p < 0.001) on the occurrence of internal browning was obtained. As such, the environment also influenced the occurrence of internal browning. Fruit from trees on Geneva rootstocks (including those with interstems) had higher and more severe incidences of internal browning compared to the industry standards due to the more advanced maturity.
- ItemUnderstanding soft scald and bitter pit development in ‘Scifresh’ apples(Stellenbosch : Stellenbosch University, 2024-03) Ladegourdie, Jason; Crouch, Elke; Botes, Anel; Jooste, Mariana; Stellenbosch University. Faculty of Agrisciences. Dept. of Horticulture.ENGLISH ABSTRACT: The South African pome fruit industry annually contributes approximately R11.47 billion to the country's economy. Effective storage techniques are essential to maintain fruit quality and remain competitive. This study focused on two storage disorders, namely soft scald and bitter pit, both of which significantly impact the South African apple industry. Soft scald is becoming increasingly problematic. Although a great deal of research has been conducted on bitter pit, it remains a common issue. 'Scifresh', a relatively new apple cultivar, was used in this study and is particularly susceptible to soft scald and bitter pit. This study comprises of a literature review and four scientific papers, with a focus on soft scald and bitter pit development under South African conditions. The main aim of the study was to understand the factors that influence disorder susceptibility and propose strategies to minimize the risk. The first paper assessed the impact of various storage regimes on 'Scifresh' apple quality and investigates biochemical indicators such as reactive oxygen species, internal ethylene concentration, total phenolics, total antioxidant capacity, and apple volatiles. Harvest maturity and cooling rate after harvest were identified as factors in soft scald and bitter pit development. Stepwise cooling limited the incidence of soft scald but increased the risk of bitter pit. Ethylene reduction methods are suggested for further research to address bitter pit in regular atmosphere storage whilst stepwise cooling is used for soft scald reduction. The second paper evaluated the effects of regular and controlled atmosphere (CA) storage, storage times, and 1-methylcyclopropene (1-MCP) on post-storage quality. The results indicate that 1-MCP reduced bitter pit in fruit stored under regular atmosphere. Post-optimum harvested fruit treated with 1-MCP that was stepwise cooled and CA stored long-term (6-8 months), had an increased risk of bitter pit development. This was not a risk for 1-MCP treated fruit that was cooled immediately and CA stored long-term (6-8 months). The third paper investigated the effects of forced-air cooling (FAC) after packing on 'Scifresh' apple quality, with a special focus on potential chilling disorders during the re-cooling process. FAC after packaging did not affect fruit quality compared to passive cooling, when re-cooling remained below 10 °C. The fourth paper investigated the effect of different fruit harvest maturities on post-storage fruit quality. It was found that ‘Scifresh’ apples must be harvested at optimum maturity to mitigate the risks of soft scald and bitter pit. Blush colour cannot be used as a single harvest maturity index but rather in combination with other harvest maturity parameters. The recommended harvest maturity includes a narrower starch breakdown window of 30–40 %, TSS > 12 %, firmness > 8 kg with a 40 % blush colour and F4 colour intensity according to the ‘Scifresh’ colour swatch. In conclusion, the research provided valuable insights into the storage disorders, soft scald and bitter pit, offering recommendations for optimal storage protocols and harvest practices of ‘Scifresh’ apples.
- ItemInvestigating sustainable approaches to late maturity and fungal infection of organic ‘Medjool’ date palm (Phoenix dactylifera L.) fruit in the Western Cape, South Africa(Stellenbosch : Stellenbosch University, 2023-03) Peddie, Rude Jo-Anne; Lotze, Elmi; Brink, Casper; Stellenbosch University. Faculty of Agrisciences. Dept. of Horticulture.ENGLISH ABSTRACT: The commercial production of date palm in South Africa was established approximately 50 years ago in the Northern Cape with the largest orchard stretching over 100 hectares. The province has a hot and dry climate, similar to the regions of the Middle East and North Africa where date palms are traditionally produced. These conditions result in little to no issues regarding the growth and development of the fruit, the presence of pests and/or diseases during its cultivation. Date palm cultivation has since spread to climatically suitable regions in the Western Cape, which is one of the richest fruit-growing regions in the country. However, on one of the farms (approximately 30 hectares in the Hermon region), irregular ripening and high incidences of microbial spoilage were observed soon after cropping. Since the specific farm employs organic agriculture, a sustainable approach is required to address these two challenges to enable marketing of the crop. The aim of this study was to investigate the application of preharvest fruit bunch bagging as an environmentally sustainable approach to the acceleration of date fruit ripening, as well as the control of fungal infections in an organic date orchard in the Western Cape. During the 2022 season on the Kleinplasie organic date orchard near Hermon in the Western Cape, ‘Medjool’ date palm trees underwent three different non-perforated bagging treatments (no bag, blue low-density polyethylene bag, white high-density polyethylene abg) at two different phenologically important times (Khalal at 17 February 2022, Khimri at 31 March 2022) during the fruit ripening period. Preharvest analyses found that the technique, particularly the blue low-density polyethylene bags implemented later in the fruit ripening process, significantly increased fruit weight, size, and improved fruit colour at harvest. However, the microbial load was found to significantly increase at harvest, leading to higher rates of fungal infection, when compared to the other bagged treatments (p = 0.034). Postharvest analyses concluded that bunch bagging, particularly blue low-density polyethylene bags implemented earlier in the fruit ripening process, could have a positive effect on ripening of fruit under the appropriate storage conditions, and the organoleptic properties of the fruit was significantly different between treatments. According to the results of the study, it was recommended that implementing a blue low-density polyethylene bag later in the ripening process at Khalal until harvest could result in the accelerated ripening of date fruit, while applying a white high density polyethylene bag applied earlier at Khimri could aid in the control of fungal infection. This is due to the colour of the bagging material acting as a filter of photosynthetically active radiation, which refers to the range of wavelengths that aid in photosynthesis.