A plant health management system for aphididae on lettuce under variable shadehouse conditions in the central Free State, South Africa

Thesis (M. Tech) --Central University of Technology, Free State, 2008Aphids (Hemiptera: Aphididae) are amongst the most destructive insects in agricultural crop production systems. This reputation stems from their complex life cycles which are mostly linked to a parthenogenetic mode of reproduction, allowing them to reach immense population sizes within a short period of time. They are also notorious as important and efficient vectors of several plant viral diseases. Their short fecund life cycles allow them to be pests on crops with a short growth period, e.g. lettuce (Lactuca sativa L.). It is common practice to provide this crop with some degree of protection from environmental extremes on the South African Highveld. Shadehouses are popular in this regard, but aphids are small enough to find their way into these structures, and their presence on lettuce is discouraged due to phytosanitary issues. In addition, the excessive use of insecticides is criticized due to the negative influence on human health, and because aphids can rapidly develop resistance. This necessitates the use of alternative control options in order to suppress aphid numbers. Biological control is popular in this regard and the use of predatory ladybirds (Coleoptera: Coccinellidae) is a popular choice. This study investigated the aphid and coccinellid species complex encountered under varying shadehouse conditions on cultivated head lettuce in the central Free State Province (South Africa). Their seasonality was also examined, along with variations in their population size throughout a one-year period. Finally, the impact of varying aphid populations on some physical characteristics of head lettuce was examined, and recommendations for aphid control (using naturally occurring coccinellid predators) were made. Two shadehouse structures were evaluated during this study. One was fully covered with shade netting and designed to exclude the pugnacious ant, Anoplolepis custodiens (Hymenoptera: Formicidae), while the other was partially covered with shade netting (on the roof area) allowing access to the ants. Six cycles of head lettuce were planted and sampled four times during each cycle. These were scheduled to monitor the seedling, vegetative and heading stage of lettuce. Four important aphid species were recorded on the lettuce, namely Acyrthosiphon lactucae, Nasonovia ribisnigri, Myzus persicae and Macrosiphum euphorbiae. Both structures harboured similar aphid and coccinellid species, but their population dynamics differed. A. lactucae dominated in the absence of A. custodiens in the fully covered structure (whole study), while N. ribisnigri dominated in the partially covered structure in the presence of these ants during the warmer months (December – January). M. euphorbiae replaced this species as the dominant species in the absence of A. custodiens (April – September). M. persicae occured during the winter (May – August) in the fully covered structure. Promising coccinellid predators were Hippodamia variegata and Scymnus sp. 1, and to a lesser extent, Exochomus flavipes and Cheilomenes lunata. However, the fully covered structure hampered the entrance of the larger adult coccinellid species, resulting in their lower occurrence. Aphid and coccinellid activity peaked during the summer months (October – January), and the fully covered structure attained the highest aphid infestation levels and coccinellid larval numbers during this time. On the other hand, aphid numbers were higher in the partially covered structure during the cooler months of the year (April – July) and this structure also harboured more adult coccinellids. In most cases, aphid infestation levels did not affect the amount of leaves formed. However, symptomatic damage in terms of head weight reduction did occur under severe infestation levels. Specific environmental conditions within a shadehouse structure concurrently contributed to this reduction, with less favourable conditions accelerating this condition. Results from this study have shown that even though the type of shadehouse structure does not influence the insect species complex found on lettuce, it does have an influence on detrimental and beneficial insect population dynamics. Aphid species infesting lettuce have been identified, along with coccinellid predators that could potentially be used in their control. Both types of structures had advantages and disadvantages, and therefore, decisions concerning shadehouses should not be focused on which type of structure to use, but rather which type of structure to use during different seasons of the year

Development of an automated robot vision component handling system

Thesis (M. Tech. (Engineering: Electrical)) -- Central University of technology, Free State, 2013In the industry, automation is used to optimize production, improve product quality and increase profitability. By properly implementing automation systems, the risk of injury to workers can be minimized. Robots are used in many low-level tasks to perform repetitive, undesirable or dangerous work. Robots can perform a task with higher precision and accuracy to lower errors and waste of material. Machine Vision makes use of cameras, lighting and software to do visual inspections that a human would normally do. Machine Vision is useful in application where repeatability, high speed and accuracy are important. This study concentrates on the development of a dedicated robot vision system to automatically place components exiting from a conveyor system onto Automatic Guided Vehicles (AGV). A personal computer (PC) controls the automated system. Software modules were developed to do image processing for the Machine Vision system as well as software to control a Cartesian robot. These modules were integrated to work in a real-time system. The vision system is used to determine the parts‟ position and orientation. The orientation data are used to rotate a gripper and the position data are used by the Cartesian robot to position the gripper over the part. Hardware for the control of the gripper, pneumatics and safety systems were developed. The automated system‟s hardware was integrated by the use of the different communication protocols, namely DeviceNet (Cartesian robot), RS-232 (gripper) and Firewire (camera)

Bestimmung der Rotorlage in aktiven Magnetlagern durch Messung magnetischer Streuflüsse

In dieser Arbeit wird die Möglichkeit untersucht, durch die Messung magnetischer Streuflüsse und unter Berücksichtigung der durch die Steuerströme hervorgerufenen Durchflutung, auf die Position des Rotors im Magnetlager zu schließen. Die Streuflüsse werden in der Regel vernachlässigt, stehen aber im unmittelbaren Zusammenhang zur Luftspaltlänge, wie theoretische Betrachtungen zeigen. Anhand von analytischen und numerischen Modellen, welche durch Messungen verifiziert werden, ist eine Linearisierung und Kompensation des Einflusses der Durchflutung möglich. Auf dieser Basis wird ein Messsystem entwickelt, mit dem die streuflussbasierte Positionsregelung eines Testlagers realisiert wird. Hierfür kommen Hall-Sensoren zum Einsatz, die auf Leiterplatten sitzen, welche anstelle der konventionellen Nutverschlüsse in das Magnetlager eingebracht werden. Aufgrund der direkten Nähe der Sensoren zu den Lagerspulen und der gepulsten Steuerströme weisen die Messsignale jedoch ein erhebliches Rauschen auf. Um dem entgegenzuwirken, kommt ein Kalman-Filter zum Einsatz, mit dem eine deutliche Verbesserung der Signalqualität erreicht werden kann.:Verzeichnis der Formelzeichen, Indizes und Abkürzungen vii 1 Einleitung 1 1.1 Exkurs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.2 Systematik magnetischer Lager . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.3 Sensoren für Magnetlager . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.4 Sensorlose Magnetlagerung . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1.5 Motivation und Struktur der Arbeit . . . . . . . . . . . . . . . . . . . . . . 14 1.5.1 Motivation und Zielstellung . . . . . . . . . . . . . . . . . . . . . . . 14 1.5.2 Struktur der Arbeit . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 1.6 Zusammenfassung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 2 Theoretische Betrachtungen zu magnetischen Streuflüssen 17 2.1 Magnetische Streuflüsse in Magnetlagern . . . . . . . . . . . . . . . . . . . . 17 2.1.1 Heteropolarlager . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 2.1.2 Homopolarlager . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 2.1.3 Dreischenkliges Magnetlager . . . . . . . . . . . . . . . . . . . . . . . 20 2.2 Verallgemeinertes Reluktanzmodell . . . . . . . . . . . . . . . . . . . . . . . 21 2.3 Zusammenhang zwischen Luftspaltlänge und Streuflussdichte . . . . . . . . 28 2.3.1 Intrapolarer Streufluss . . . . . . . . . . . . . . . . . . . . . . . . . . 28 2.3.2 Interpolarer Streufluss . . . . . . . . . . . . . . . . . . . . . . . . . . 31 2.4 Betrachtung der magnetischen Streuflüsse mit Hilfe numerischer Rechnungen 33 2.5 Zusammenfassung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 3 Magnetische Streuflüsse im realen Magnetlager 39 3.1 Auswahl eines geeigneten Lagertyps und möglicher Messpositionen . . . . . 39 3.1.1 Streuflüsse bei Rotorverschiebung entlang der x- und y-Achse . . . . 41 3.1.2 Streuflüsse bei Rotorverschiebung entlang der a- und b-Achse . . . . 43 3.1.3 Änderung der Streuflüsse bei Querverschiebung des Rotors . . . . . 45 3.2 Nutzbarkeit der intra- und interpolaren Streuflüsse als Lagemesssystem . . 48 3.3 Vergleich gemessener und berechneter Streuflusswerte . . . . . . . . . . . . 52 3.4 Zusammenfassung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 4 Realisierung des Messsystems 57 4.1 Erstellung von Kennfeldern . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 4.2 Versuchsaufbau . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 v Inhaltsverzeichnis 4.3 Messsystem zur Messung der magnetischen Streuflussdichte . . . . . . . . . 60 4.3.1 Auswahl geeigneter Bauelemente . . . . . . . . . . . . . . . . . . . . 62 4.3.2 Sensordesign . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 4.3.3 Kalibrierung der Sensoren . . . . . . . . . . . . . . . . . . . . . . . . 66 4.4 Statische und dynamische Eigenschaften des streuflussbasierten Messsystems 69 4.5 Zusammenfassung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 5 Betrachtungen zur Verbesserung der Signalqualität 75 5.1 Modellbildung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 5.1.1 Übertragungsverhalten der Messsysteme . . . . . . . . . . . . . . . . 76 5.1.2 Elektromagnetisches Modell . . . . . . . . . . . . . . . . . . . . . . . 80 5.1.3 Mechanisches Modell . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 5.1.4 Modellierung variabler Induktivitäten . . . . . . . . . . . . . . . . . 93 5.1.5 Stromrichter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 5.2 Kalman-Filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 5.3 Ergebnisse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 5.4 Zusammenfassung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 6 Zusammenfassung und Ausblick 115 6.1 Zusammenfassung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 6.2 Ausblick . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 6.2.1 Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 6.2.2 Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 A Mathematische Überlegungen zu Streuflussfunktionen 121 A.1 Grenzwerte für den intrapolaren Streufluss . . . . . . . . . . . . . . . . . . . 121 A.2 Anstieg intrapolare Streuflussfunktion . . . . . . . . . . . . . . . . . . . . . 122 A.3 Maximum des interpolaren Streuflusses . . . . . . . . . . . . . . . . . . . . . 123 B Tabellen 127 B.1 Gemessene Streuflüsse an verschiedenen Rotorpositionen und unterschiedlichen resultierenden Steuerströmen . . . . . . . . . . . . . . . . . . . . . . . 127 B.2 Ströme und Positionen nach Streuflussmesswerten sortiert . . . . . . . . . . 128 C Schaltpläne, technische Zeichnungen und Blockschaltbilder 129 C.1 Schaltplan Streuflusssensor . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 C.2 Kalibrierschaltung des Messkonverters . . . . . . . . . . . . . . . . . . . . . 130 C.3 Beispielgeometrie . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 C.4 Magnetlagerrotor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 C.5 Blockschaltbild des Modells eines Stromrichters . . . . . . . . . . . . . . . . 131 Literaturverzeichnis 133 Thesen 141In this work, the possibility of inferring the position of the rotor in magnetic bearings by measuring magnetic leakage fluxes is investigated. These are usually neglected, but are directly related to the air gap length, as theoretical considerations show. In addition, the magnetic flux caused by the control currents must be taken into account. By means of analytical and numerical models, which are verified by measurements, a linearization and compensation of the influence of the magnetic flux is possible. Based on this, a measurement system is developed to realize a flux leakage-based position control of a test bearing. For this purpose, Hall-sensors are used, which are located on printed circuit boards that are inserted into the magnetic bearing instead of the conventional slot locks. However, due to the direct proximity of the sensors to the bearing coils and the pulsed control currents, the measurement signals exhibit considerable noise. To counteract this, a Kalman-filter is used to achieve a significant improvement in signal quality.:Verzeichnis der Formelzeichen, Indizes und Abkürzungen vii 1 Einleitung 1 1.1 Exkurs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.2 Systematik magnetischer Lager . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.3 Sensoren für Magnetlager . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.4 Sensorlose Magnetlagerung . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1.5 Motivation und Struktur der Arbeit . . . . . . . . . . . . . . . . . . . . . . 14 1.5.1 Motivation und Zielstellung . . . . . . . . . . . . . . . . . . . . . . . 14 1.5.2 Struktur der Arbeit . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 1.6 Zusammenfassung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 2 Theoretische Betrachtungen zu magnetischen Streuflüssen 17 2.1 Magnetische Streuflüsse in Magnetlagern . . . . . . . . . . . . . . . . . . . . 17 2.1.1 Heteropolarlager . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 2.1.2 Homopolarlager . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 2.1.3 Dreischenkliges Magnetlager . . . . . . . . . . . . . . . . . . . . . . . 20 2.2 Verallgemeinertes Reluktanzmodell . . . . . . . . . . . . . . . . . . . . . . . 21 2.3 Zusammenhang zwischen Luftspaltlänge und Streuflussdichte . . . . . . . . 28 2.3.1 Intrapolarer Streufluss . . . . . . . . . . . . . . . . . . . . . . . . . . 28 2.3.2 Interpolarer Streufluss . . . . . . . . . . . . . . . . . . . . . . . . . . 31 2.4 Betrachtung der magnetischen Streuflüsse mit Hilfe numerischer Rechnungen 33 2.5 Zusammenfassung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 3 Magnetische Streuflüsse im realen Magnetlager 39 3.1 Auswahl eines geeigneten Lagertyps und möglicher Messpositionen . . . . . 39 3.1.1 Streuflüsse bei Rotorverschiebung entlang der x- und y-Achse . . . . 41 3.1.2 Streuflüsse bei Rotorverschiebung entlang der a- und b-Achse . . . . 43 3.1.3 Änderung der Streuflüsse bei Querverschiebung des Rotors . . . . . 45 3.2 Nutzbarkeit der intra- und interpolaren Streuflüsse als Lagemesssystem . . 48 3.3 Vergleich gemessener und berechneter Streuflusswerte . . . . . . . . . . . . 52 3.4 Zusammenfassung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 4 Realisierung des Messsystems 57 4.1 Erstellung von Kennfeldern . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 4.2 Versuchsaufbau . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 v Inhaltsverzeichnis 4.3 Messsystem zur Messung der magnetischen Streuflussdichte . . . . . . . . . 60 4.3.1 Auswahl geeigneter Bauelemente . . . . . . . . . . . . . . . . . . . . 62 4.3.2 Sensordesign . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 4.3.3 Kalibrierung der Sensoren . . . . . . . . . . . . . . . . . . . . . . . . 66 4.4 Statische und dynamische Eigenschaften des streuflussbasierten Messsystems 69 4.5 Zusammenfassung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 5 Betrachtungen zur Verbesserung der Signalqualität 75 5.1 Modellbildung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 5.1.1 Übertragungsverhalten der Messsysteme . . . . . . . . . . . . . . . . 76 5.1.2 Elektromagnetisches Modell . . . . . . . . . . . . . . . . . . . . . . . 80 5.1.3 Mechanisches Modell . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 5.1.4 Modellierung variabler Induktivitäten . . . . . . . . . . . . . . . . . 93 5.1.5 Stromrichter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 5.2 Kalman-Filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 5.3 Ergebnisse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 5.4 Zusammenfassung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 6 Zusammenfassung und Ausblick 115 6.1 Zusammenfassung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 6.2 Ausblick . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 6.2.1 Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 6.2.2 Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 A Mathematische Überlegungen zu Streuflussfunktionen 121 A.1 Grenzwerte für den intrapolaren Streufluss . . . . . . . . . . . . . . . . . . . 121 A.2 Anstieg intrapolare Streuflussfunktion . . . . . . . . . . . . . . . . . . . . . 122 A.3 Maximum des interpolaren Streuflusses . . . . . . . . . . . . . . . . . . . . . 123 B Tabellen 127 B.1 Gemessene Streuflüsse an verschiedenen Rotorpositionen und unterschiedlichen resultierenden Steuerströmen . . . . . . . . . . . . . . . . . . . . . . . 127 B.2 Ströme und Positionen nach Streuflussmesswerten sortiert . . . . . . . . . . 128 C Schaltpläne, technische Zeichnungen und Blockschaltbilder 129 C.1 Schaltplan Streuflusssensor . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 C.2 Kalibrierschaltung des Messkonverters . . . . . . . . . . . . . . . . . . . . . 130 C.3 Beispielgeometrie . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 C.4 Magnetlagerrotor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 C.5 Blockschaltbild des Modells eines Stromrichters . . . . . . . . . . . . . . . . 131 Literaturverzeichnis 133 Thesen 14

Qualitative and quantitative gas chromatography-mass spectrometry analysis of the principal toxic constituents of some South African plants in human biological fluid

In South Africa there are regular cases of human and animal poisoning where plants used in ethnic medicine are suspected to be the source of the poison. This project aims to develop methods for identifying and quantifying the principal toxins of four such plants in human urine. In this way a steppingstone for further research and development on this subject is produced. This project focuses on toxins from four plants that fall into two broad classes of compounds: cardiac glycosides (Acokanthera oppositifolia and Urginea sanguinea) and alkaloids (Boophane disticha and Gloriosa superba). The principal toxic compounds within the respective plants are: acovenoside A, scillaren A, buphanidrine and colchicine. In this study, buphanidrine was isolated from a dichloromethane-methanol plant extract of B. disticha, through the utilisation of multiple chromatographic techniques, and its structure confirmed by nuclear magnetic resonance spectroscopy and high-resolution mass spectrometry. Samples of known analyte concentrations were prepared by spiking blank urine samples with specific amounts of analyte stock solution. Extraction of colchicine and cardiac glycosides from urine was accomplished by solid-phase extraction. The extracted cardiac glycosides were hydrolysed in hydrochloric acid and the reaction products extracted via liquid-liquid extraction. Following extraction, the products of the hydrolysis reaction were silylated for gas chromatography-mass spectrometry analysis. Extracts from urine samples containing colchicine were prepared without a hydrolysis process. Urine samples spiked with buphanidrine were prepared by buffering to alkaline pH, and subsequently conducting a liquid-liquid extraction. The organic phase extracts for both alkaloids were concentrated by evaporation and reconstitution in a small volume of an organic solvent. Both alkaloids were analysed without derivatisation following up-concentration. Response models were generated by duplicate analyses of three batches of each analyte in a de-ionised aqueous solution and urine, respectively. The applicability of the linear response models to the measured data was evaluated for each respective analyte. The relative standard deviation was evaluated to establish the variance in the linear models. The response models, developed for colchicine, acovenoside A and scillaren A, were problematic and further investigated to determine the contributing factors. In the case of colchicine and acovenoside A, the utilisation of log-linear response models was less problematic in describing the trends in the data. In analysing the trends in the processed data, the limits of detection and quantification were calculated statistically for buphanidrine, acovenoside A and colchicine. These were then compared to limits observed in the qualitative analysis of the analytes based on ion ratios. For each analyte five characteristic ion ratios were selected, considering the molecular ion signal area, relative to that of the base peak ion for each analyte. Applying multivariate Gaussian statistical analysis techniques, the correlation of ion ratios and their dependence upon analyte concentration was established. This proved insightful regarding the mechanism of analyte fragmentation. Suggested further work, following this project, should look at factors concerning the optimisation of sample preparation methods, especially the purification via solid-phase extraction. The project provided an opportunity for the investigation of trends in ion ratios, to determine the fundamental limits of identification.Dissertation (MSc (Chemistry))--University of Pretoria, 2021.ChemistryMSc (Chemistry)Unrestricte

Factors influencing the occurrence of premature and excessive leaf abscission in the avocado (Persea americana Mill.) cultivar 'RYAN' and possible preventative measures

Premature and excessive leaf abscission during flowering time in the late avocado (Persea americana Mill.) cultivar ‘Ryan’ is a considerable problem for avocado growers. They are especially concerned that premature and excessive leaf abscission will have a negative effect on yield. No previous investigations have been performed where premature and excessive leaf abscission in avocado has been studied in detail. This study therefore aimed to investigate the pattern of premature and excessive leaf abscission in ‘Ryan’, and compare it with two other important commercial cultivars, ‘Fuerte’ and ‘Hass’, which do not display this phenomenon. Time course studies of leaf abscission in the orchard were performed during 2006 and 2007 to determine the pattern of leaf abscission on ‘Ryan’, Fuerte’ and ‘Hass’. This also included anatomical studies to determine the time of leaf abscission zone formation. Possible stress factors, which accelerate leaf abscission were also investigated, namely unfavourable climatic conditions (temperature, solar radiation, rainfall, relative humidity and evapotranspiration), nutrient imbalances, excessive flowering and leaf area. The possible impact leaf abscission may have on yield was then assessed by determining levels of reserve carbohydrates in the bark of the tree. In addition, practical solutions, i.e. the application of fertilizers, plant growth regulators (PGRs) and kaolin, were investigated in order to reduce or eliminate premature and excessive leaf abscission. This study was carried out over the period 2005 until 2007, with experiments being modified on an annual basis as information was gathered on the phenomenon. Experiments began in 2005 with a study on the pattern of leaf abscission in ‘Ryan’, which revealed an increase in leaf abscission just prior to flowering. However, this increase was not significant. During 2006, the leaf abscission pattern for ‘Ryan’ was compared with the leaf abscission patterns of ‘Fuerte’ and ‘Hass’. Leaf abscission for ‘Ryan’ was significantly higher than for ‘Fuerte’ and ‘Hass’ during 2006. During 2006 ‘Ryan’ displayed two periods of high leaf abscission, namely the spring flush between bud dormancy and bud swell, and a drastic increase in spring and summer flush leaf abscission between inflorescence development and full bloom. These periods of increased leaf abscission were absent during the 2007 season. In addition, ‘Fuerte’ and ‘Hass’ did not display these peaks of high leaf abscission, with leaf abscission occurring in these cultivars at higher rates from full bloom onwards. Premature and excessive leaf abscission is therefore not an annual event in ‘Ryan’ and is in all likelihood influenced by external factors. Anatomical studies did not reveal any results in terms of initiation of leaf abscission, with only the protective layer of the abscission zone being visible after leaf yellowing occurred. During 2006, two peaks of extremely low temperatures (<4°C) occurred just prior to the acceleration of leaf abscission. During the second period of low temperatures, the solar radiation:temperature-ratio was also considerably higher. These periods of low temperatures were absent during 2007, indicating that cold and light stress could be contributing to premature and excessive leaf abscission in ‘Ryan’ in 2006. In addition, ‘Ryan’ flowered excessively during 2006, which could have been triggered by low temperature stress just prior to flower initiation. A significant correlation was found between excessive flowering and excessive leaf abscission in ‘Ryan’ during 2006. The occurrence of reduced flowering in ‘Fuerte’ and ‘Hass’ may possibly be due to these two cultivars being more tolerant to stress, and it is possible that ‘Ryan’ is genetically more prone to excessive flowering than ‘Fuerte’ and ‘Hass’. Excessive flowering could accelerate leaf abscission by causing an unusually high demand for water, nutrients and carbohydrates, resulting in the acceleration of leaf abscission. No significant relationship between nutrient levels and excessive leaf abscission was found for either 2006 or 2007. In addition, no significant correlation could be found between leaf abscission on a branch and the total leaf area of that branch during the 2007 season. During 2007, leaf abscission was low and it is possible that a significant correlation could be found in a season with excessive leaf abscission. During 2005, chemical applications to reduce leaf abscission did not yield any significant improvement in leaf retention. In fact, the 50 g/tree Solubor® and 50 g/tree Solubor® in combination with 2 kg/tree dolomitic lime had a significant negative effect on fruit set, possibly because too high concentrations were applied too close to fruit set. Chemical applications during 2006 were therefore made at bud dormancy and bud swell, as it was found that leaf abscission occurred before flowering time. However, no effect was observed on leaf retention or fruit set. During 2007, emphasis was placed on treatments that might reduce stress, as it became evident that stress could be responsible for premature and excessive leaf abscission in ‘Ryan’. Most treatments showed a slight positive effect on leaf retention, but no significant results were obtained possibly because that particular season was a season of low leaf abscission. Further research on application of stress-reducing treatments is therefore recommended. Best farm management practices including optimal fertilization and irrigation is therefore vital to prevent stress, accelerating leaf abscission. CopyrightDissertation (MSc)--University of Pretoria, 2009.Plant Scienceunrestricte

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