Citrus black spot (CBS) caused by Guignardia citricarpa is responsible for economic losses in Southern African countries such as South Africa, Mozambique, Swaziland, Zimbabwe and Namibia. Black spot is considered to be a phytosanitary disease for the European Union and the United States of America markets. Exporters to these countries incur losses throughout the supply chain due to phytosanitary restrictions. For these reasons, the occurrence and management practices of CBS and its impact on growers in Southern Africa were investigated through a survey using a questionnaire. In the study, it was found that when CBS was present it was primarily managed by using chemicals and general orchard sanitation. In addition, growers in some of the surveyed countries or production regions follow spraying programs that are based on disease forecasting models and this practice has proven very effective in managing the disease. Furthermore, furfural, a sugarcane waste product was assessed for its efficacy in controlling G. citricarpa. The efficacy of the product as a contact or a fumigant was demonstrated in vitro and in vivo on fresh leaves, leaf litter and fruit lesions as well as in soil. A molecular study, using a Polymerase Chain Reaction protocol was conducted to assess the survival of the pathogen in the soil after exposure to furfural. The product however only proved efficient under natural conditions. The non-target effect of furfural on the soil micro-flora was also assessed. The product proved suitable for soil applications as it is not phytotoxic and has minimal non-target effects on bacterial populations. Furfural proved to control G. citricarpa, by breaking the life cycle, thus reducing the disease incidence. The application of furfural on a larger scale (irrigation or spraying) will therefore improve the control of CBS in developing countries. / Dissertation (MInstAgrar)--University of Pretoria, 2008. / Microbiology and Plant Pathology / unrestricted

This study examines the public debt, public debt service and economic growth nexus in Zambia, Zimbabwe and South Africa using time-series data from 1970 to 2017. This research provides empirical evidence to contribute, firstly, to the ongoing public policy debate regarding the dynamic relationship between public debt, public debt service and economic growth, and their causal relationship; and secondly, to the relative impact of domestic and foreign public debt on economic growth in the selected study countries. For this purpose, four empirical models were utilised and estimated using the Autoregressive Distributed Lag (ARDL) bounds to cointegration and the error correction ARDL-based causality test. Model 1 explored the impact of aggregate public debt on economic growth, while Model 2 investigated the relative impact of domestic and foreign public debt on economic growth. Model 3 examined the impact of public debt service on economic growth, whereas the causality between aggregate public debt and economic growth, and between public debt service and economic growth is tested in Model 4a and Model 4b, respectively. Results show that in Model 1, aggregate public debt has a positive impact on economic growth in Zambia but is negative in Zimbabwe and South Africa. In Model 2, domestic public debt negatively impacts economic growth in Zambia and Zimbabwe and positive impact in South Africa. In addition, foreign public debt has a positive impact on economic growth in Zambia and negative impact in Zimbabwe and South Africa. The results from Model 3 largely support a negative relationship between public debt service and economic growth in Zambia and Zimbabwe, and an insignificant relationship in South Africa. The causality results for Model 4a indicate that it is economic growth that drives public debt in all the study countries. Finally, no causal relationship between public debt service and economic growth was confirmed in all the study countries (Model 4b). / Economics / D. Phil. (Economics)