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Background on Citrus
By 2004, the literature on Citrus was considered to be in chaos dispite having been described in written accounts emanating from 2500 years ago, in records from the peoples of China (Southeast Asia) and the subsequent large amouts of work carried out on the cash crop (Mabberley, 2004). Nicolosi, (2007), outlines early reports of Citrus species and their use in regions of China, indicating that some of the ancient citron varieties were considered to be lucky talismans, and thus named „the hand of Buddha‟. Davies and Albrigo, (1994), Saunt, (2000), Gmitter et al., (2009) and Liu et al., (2012), provide comprehensive literature discussing the history, distribution and uses of citrus fruits into the modern era. Singh and Rajam, (2009), provide some literature on optimal climatic conditions for citrus production, pests and diseases affecting the crop as well as tree improvement through biotechnology (somatic hybridization and genetic transformation). While the taxonomy of Citrus is in flux, the fruits of cultivated varieties are easily identified and also popular food commodities around the world (Gmitter et al., 2009; Liu et al., 2012).
Saunt, (2000) indicates that the initiation of budding practices from the 1840‟s, were as a direct result of the devastation caused by root rot pathogens, later identified as Phytophthora species. Therefore citrus fruits are commercially produced from the budding union of two trees, the rootstock and the scion. Citrus fruit production boomed from the 1960s increasing from 30 million tons to over 105 million tons at the end of 2006, with half this production being orange fruits (Saunt, 2000; Liu et al., 2012). Commercial citrus production expanded to over 140 countries in that time, rendering the fruit globally available and popular for uses beneficial to human nutrition, as animal feeds and other industries (Saunt, 2000; Gmitter et al., 2009; Liu et al., 2012).
Commercial citrus fruit production is rated at the third most important after apples and banana fruits on a global scale (Singh and Rajam, 2009). Brazil and the United States of America lead the world‟s orange fruit production figures (Gmitter et al., 2009; Singh and Rajam, 2009; Liu et al., 2012), with averages of 17, 813 and 8, 217 million metric tons recorded respectively between 2007 and 2012. During the same period the European Union and Russia were the world leaders of orange fruit importers, averaging 866.6 and 520.8 metric tons respectively, with the Republic of South Africa satisfying most of their requirements, reportedly exporting 968.4 metric tons of fruit annually (Fig. 2.1).
Citrus scions and rootstock
Citrus plants belong to the genus Citrus L., subtribe Citriae, family Rutaceae and subfamily Aurantioideae (Saunt, 2000; Gmitter et al., 2009). The economically important species of scions or fruiting citrus, cultivated around the world, include Citrus sinensis (L.) Osbeck (sweet orange), C. reticulata Blanko (mandarin), C. x paradisi Mac. f. (grapefruit), C. limon (L.) Burm. f. (lemon), C. aurantiifolia (Christm) Swing. (lime), C. aurantium (L.) (sour orange) and C. grandis (L.) Osbeck (pummelo) (Saunt, 2000; Mabberley, 2004; Singh and Rajam, 2009; Castle, 2010). In South Africa Citrus sinensis (Eureka lemons) and Citrus reticulata (Nules clementines) are the commonly cultivated fruiting citrus (Meitz-Hopkins et al., 2014).
Scions are budded to rootstock in nurseries, and Castle, (2010) highlights rootstocks that have formed the global portfolio for citrus industries over the last 100 years to date (Table 2.1).
Phytophthora nicotianae root rot in citrus
Citrus trees within the different global regions of citrus production are attacked by different species of the pathogen alone, in a complex with another Phytophthora or other pathogens (Matheron et al., 1998; Graham and Feichtenberger, 2015). Phytophthora nicotianae Breda de Haan (syn. P. parasitica Dastur), P. palmivora (E. J. Butler) and P. citrophthora (Smith & Smith) are primary pathogens responsible for damping off in citrus nurseries and trunk, foot and root rots of trees in orchards around the world (Widmer et al. 1998; Graham and Timmer, 2004; Savita et al., 2012; Adaskaveg et al., 2014; Meitz-Hopkins et al., 2014; Graham and Feichtenberger, 2015). P. nicotianae is reported to be more common in the tropical and subtropical citrus regions such as the citrus growing regions of Brazil (Mourao et. al. 2008; Boava et al., 2011; Graham and Feichtenberger, 2015), Florida (Graham, 1990; Timmer et al., 1991; Graham and Feichtenberger, 2015), Egypt (Ahmed et al., 2012) and South Africa (Burger, 2001; Thompson et al., 1995; Nagel et al., 2013; Meitz-Hopkins et al., 2014) causing root rot and tree die-back.
P. nicotianae has adapted to such warmer conditions where its optimal temperatures for growth prevail and allow it to grow, out competing other Phytophthora species (Siviero et al., 2006; Panabières et al., 2016). The early infection stages of root rot in feeder or fibrous roots of citrus rootstocks, is characterized by soft water-soaked lesion formations followed by disintegration of the outer cortex or sheath of the roots which impairs uptake of water and nutrients (Graham and Timmer, 2004; Adaskaveg et al., 2014; Graham and Feichtenberger, 2015). Rot results in white thread-like stele protruding from decaying tissues characterizing the latter stages of the disease (Adaskaveg et al., 2014). In mature trees, root rot results in tree decline characterized by foliage yellowing, leaf drop and die-back of twigs and branches (Graham and Timmer, 2004; Adaskaveg et al., 2014; Graham and Feichtenberger, 2015). The pathogen reduces tree vigour and health resulting in lowered fruit quality and yields that impact negatively on profits (Graham and Feichtenberger, 2015). Phytophthora often go unidentified as primary causal agents of tree decline disease allowing propagule build up and dissemination (Tsao, 1990).
Pathogenic Phytophthora species are the most important primary causal agents of rot in citrus causing several diseases, of which trunk rot (gummosis) and root rots are considered to be the most relevant from the perspective of both research and industry (Laviola et al., 1990; Mourao et. al. 2008; Castle, 2010; Adaskaveg et al., 2014; Graham and Feichtenberger, 2015). Through scientific observations and grower feedback, researchers respond and focus their efforts to select the most desirable citrus rootstocks from the range of planting material being developed for their prevailing environment (Castle, 2010; Roose, 2014).
CHAPTER 1
General Introduction
1.1 Background and motivation for the study
1.2 Research Objectives
1.3 Project Outline
1.4 Conclusion
1.5 References
CHAPTER 2
Literature Review
2.1 Background on Citrus
2.2 The genus Phytophthora
2.3 Metabolomics for Plant Protection
2.4 Conclusion
2.5 References
CHAPTER 3
Screening of citrus rootstocks for tolerance against the root-rot pathogen Phytophthora nicotianae under greenhouse conditions
3.1 Introduction
3.2 Materials and Methods
3.3 Results and Discussion
3.5 Conclusion
3.6 References
CHAPTER 4
Metabolite fingerprinting of citrus rootstocks for disease tolerance related biomarkers against P. nicotianae: using MassLynx 4.1
4.1 Introduction
4.2 Materials and Methods
4.3 Results and Discussion
4.4 Conclusion
4.5 References
CHAPTER 5
Metabolite-marker based predictive model for P. nicotianae tolerance in citrus rootstocks
Abstract
5.1 Introduction
5.2 Materials and methods
5.3 Results and Discussion
5.4 Conclusion
5.5 References
CHAPTER 6
Summary
6.1 Introduction
6.2 References
