Citrus is a valuable fruit crop in world trade. Citrus Black Spot (CBS), caused by Guignardia citricarpa Kiely, is a fungal disease of citrus. It occurs in many citrus producing countries including parts of Australia and South Africa, but it does not occur in the countries of the European Union (EU) or the United States of America (USA). To prevent the introduction of CBS, the EU and the USA have phytosanitary regulations that restrict the import of citrus fruit from areas where CBS is found. This study uses two bioclimatic modelling approaches — CLIMEX and response surface modelling — to predict which areas have climates suitable for CBS to establish. The work focuses on the citrus growing areas of South Africa and Europe, but other parts of the world are also considered. As CBS is dependent on citrus, geographical areas of global citrus production are also mapped, and models are used to predict which areas of South Africa have climates suitable for citrus cultivation under current and future climates. The potential impacts of climate change on CBS distribution in South Africa are also estimated. Results indicate that under current and future climates many areas in South Africa where citrus is not currently grown have a climate suitable for citrus cultivation, but most of these areas are also climatically favourable for CBS. Of the current citrus producing areas in South Africa, only the Northern and Western Cape Provinces are predicted to be unsuitable for CBS. Under climate change scenarios, some citrus production areas of Western Cape are predicted to become suitable for CBS, but the greater part of the Northern Cape will remain climatically unsuitable for the establishment of CBS in future. The climates of several CBS-free citrus producing areas around the world, such as Mexico, and Florida and Texas (USA) are suitable for CBS. However, European climate is unfavourable for CBS establishment, and provided importing countries comply to minimum standards, phytosanitary restrictions on the import of fruit from CBS infected areas may be unnecessary. This study is the first of its kind in citriculture, and in South Africa it is one of the few studies that investigates the effects of climate change on the potential distribution of a plant pathogen. Bioclimatic modelling was found to be a very useful means to combine complex data in order to make predictions relevant to Pest Risk Assessments. / Thesis (PhD (Environmental Management))--University of Pretoria, 2007. / Microbiology and Plant Pathology / unrestricted