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Citrus health management
Prevention is the only truly effective means of reducing the losses caused by most citrus diseases (Timmer & Duncan, 1999). Disease control is usually specific to a particular disease, but there are some general concepts that are applicable to disease control in general. Most importantly, disease-free material should always be used in citrus cultivation (Davies & Albrigo, 1994). Pathogen-free and healthy sources of bud-wood are maintained for distribution to nurserymen and growers. A reliable source of disease-free planting material is essential to the success of any citrus industry as dissemination of diseased trees may have catastrophic effects.
For this reason, citrus producing countries have stringent nursery regulations (Davies & Albrigo, 1994). In most countries where citrus is produced there are also restrictions on the import of citrus fruit and propagating material from areas where particular diseases occur.
Diseases may also be avoided by planting rootstocks and scions tolerant of or resistant to local diseases (Davies & Albrigo, 1994). However, the choice of cultivar is usually dictated by consumer demand and growers often plant disease susceptible cultivars even though more resistant cultivars are available (Timmer & Duncan, 1999). Chemical control of diseases can be costly and labour intensive. Despite the potential adverse environmental impacts they might cause, copper and copper-based products are still widely used to prevent citrus diseases (Davies & Albrigo, 1994), but concerns about chemical residues are beginning to restrict the market access of fruit treated with chemicals. Moreover, there is an increasing public expectation that chemical inputs to the environment should be minimised. Therefore research is being focussed on alternatives to chemicals for the prevention and control of citrus diseases (Obagwu & Korsten, 2003).
Origin and history
The arrival of citrus fruit in Southern Africa is documented in the journal of Jan van Riebeeck, the first governor of the Dutch colony in Cape Town (Reuther et al., 1967). On the 11th of June 1654, citrus plants arrived from the island of St. Helena, where citrus material had previously been established by Dutch merchants trading with the Orient (Oberholzer, 1969). As pioneer settlers moved inland, they took citrus seed with them, but for over two centuries citrus production was only on a small scale and localised. With the discovery of diamonds and gold in the late 1800s, European immigrants flocked into the country. This created an increased demand for agricultural produce, including citrus. Initial plantings were small, as fruit was destined for local consumption (Oberholzer, 1969), but these plantings would eventually lead to the development of the South African export industry (Ray & Walheim, 1980).
The industry today
Citrus represents one of South Africa’s most important agro-commodities. The total area under citrus cultivation is estimated at about 57 000 ha (Mabiletsa, 2003a) and citrus yields in mature orchards average about 40 to 60 tons per hectare (Mabiletsa, 2003b). Oranges are the most important citrus grown, with Valencias being the most important cultivar (Mabiletsa, 2003b). Lemons and grapefruit are also produced, but to a lesser extent (von Broembsen, 1986). Citrus is grown in almost every province in South Africa. Main areas of production are found in Limpopo Province in the areas surrounding Tzaneen, Letsitele and Letaba; in Mpumalanga around Nelspruit, Hectorspruit, Groblersdal and Marble Hall; in North Western Province around Rustenburg; in Kwazulu-Natal around Muden; in the Western Cape Province around Clanwilliam (Kotzé, 2004, Personal Communication), Citrusdal, Somerset West and Grabouw (Kelly, 1995); and in the Eastern Cape Province around Uitenhage, the Kat River and the Sundays River Valley (Mabiletsa, 2003b; Oberholzer, 1969; Reuther et al., 1967; Urquhart, 1999). Smaller areas of citrus cultivation can be found in the Vaalharts and Warrenton areas (Mabiletsa, 2003a) and other parts of the Northern Cape (le Roux, 2004; Urquhart, 1999). Citrus is cultivated in a variety of different climatic regions which allows a range of cultivars and varieties of fruit to be produced across South Africa (Mather, 1999). The Western Cape and Eastern Cape are considered to be cooler citrus growing areas and production is focussed on lemons (Veldman & Barry, 1996), Navel oranges and soft citrus (mandarins). In these two regions farm sizes are smaller than in Mpumalanga, Limpopo and KwaZulu-Natal, where the climate is better suited to the cultivation of grapefruit and Valencia oranges. In terms of volume, Mpumalanga and Limpopo provinces produce the greatest amounts of citrus (Mather, 2003). Citrus cultivation requires access to water and the majority of farms operate with capitalintensive irrigation equipment. Fruit farming is labour intensive, particularly during harvesting season. More than 100,000 farm workers are permanently employed with an additional unknown number of seasonal workers also employed (Mather, 1999). The other main cost associated with citrus production is agrochemicals (London & Myers, 1995).
Export of citrus from South Africa
The introduction of refrigerated shipping facilities led to the first ever successful export of 3000 standard cases of citrus fruit to the U.K. in 1906. This was a great stimulus for citrus production in South Africa (Oberholzer, 1969). Since then export of citrus fruit has increased steadily. By the 1960s, South Africa was exporting over half of all southern hemisphere fresh citrus and was ranked amongst the top five fresh citrus exporters in the world (Mather, 2003). Currently, South Africa is the world’s third largest exporter of fresh citrus fruit after Spain and the USA (Citrus Growers Association, 2004; FAO, 2002a), and up to 70% of the citrus produced is exported annually, with more than 50 million cartons sold world-wide. This earns South Africa around R2 billion in foreign exchange (Mabiletsa, 2003b). South African citrus exports provide overseas markets with a steady supply of citrus as fruit from different geographical areas in this country matures at different times. In Limpopo Province and Mpumalanga, fruit ripens earlier than the citrus varieties in the relatively cooler Western and Eastern Cape regions (Mather & Greenberg, 2003). The combination and assortment of fruit available and South Africa’s counter-season advantage in being able to supply fruit to countries in the northern hemisphere during its summer is central to the South African Citrus Industry’s marketing strategy (Mather, 1999).
Chapter 1 — Background and Aims
1.1 Citrus
1.2 Citrus Black Spot
1.3 Phytosanitary barriers to trade and Pest Risk Assessments
1.4 Bioclimatic modelling of plant disease distribution
1.5 The impact of climate change on plant pathogens
1.6 Thesis aims
1.7 References
Chapter 2 — A review of citrus and of global citrus production
2.1 Abstract
2.2 Origin and history
2.3 Uses of fruit
2.4 Taxonomy and commercially important groups
2.5 Climatic and geographic factors that influence cultivation
2.6 Other factors that influence cultivation
2.7 Citrus health management
2.8 The South African Citrus Industry
2.9 Citrus production around the world
2.10 Tables of the citrus producing countries of the world
2.11 References
Chapter 3 — A review of Citrus Black Spot
3.1 Abstract
3.2 Origin, history and distribution
3.3 Symptoms
3.4 Factors that influence symptom development and severity on fruit
3.5 Nature of the causal organism and biotypes
3.6 Epidemiology
3.7 Host plants
3.8 Control
3.9 Economic importance
3.10 Phytosanitary barriers to trade & inspection and detection methods
3.11 References
Chapter 4 — Analysis of the suitability of European climate for establishment of Citrus Black Spot disease caused by Guignardia citricarpa Kiely
4.1 Abstract
4.2 Introduction
4.3 Methodology
4.4 Results
4.5 Discussion
4.6 References
Chapter 5 — The potential global geographical distribution of Citrus Black Spot caused by Guignardia citricarpa (Kiely), with emphasis on the likelihood of disease establishment in the European Union
5.1 Abstract
5.2 Introduction
5.3 Methodology
5.4 Results
5.5 Discussion
5.6 References
Chapter 6 — The effect of climate change on the distribution of Citrus Black Spot in South Africa
6.1 Abstract
6.2 Introduction
6.3 Methodology
6.4 Results and discussion
6.5 References
Chapter 7 — Modelling the potential range of citrus production in South Africa: a response surface approach
7.1 Abstract
7.2 Introduction
7.3 Methodology
7.4 Results
7.5 Discussion
7.6 References
Chapter 8 — Modelling the range of Guignardia citricarpa Kiely, the causal agent of Citrus Black Spot, in South Africa: a response surface approach
8.1 Abstract
8.2 Introduction
8.3 Methodology
8.4 Results
8.5 Discussion
8.6 References
Chapter 9 — General discussion and conclusions
