Acknowledgements
7.4. Disease Cycle and Epidemiology
Although recent research has provided further information on the life cycle of G. xylarioides and on disease epidemiology, our understanding of the pathogen remains limited in comparison with many other fusaria and vascular wilt pathogens. Some studies of host–pathogen interactions have been undertaken (Chapter 8), but further in-depth research is required to clarify the primary infection points, investigate mechanisms of tissue colo- nization following infection, determine what physiological and biochemical responses are elicited by the plant and investigate the existence, role and interactions between genes conferring resistance in the host and virulence in the pathogen.
7.4.1. Host range
G. xylarioides would appear to have a narrow host range, coffee being its primary host and perhaps the only plant on which it is pathogenic. It is clear from observations of the occurrence of CWD in the field (including those of comprehensive and systematic surveys completed across East and Central Africa), the results of pathogenicity testing and isolations made from host substrates that the fungus is prevalent on the two most impor- tant commercially cultivated coffee species, C. arabica and C. canephora (Kilambo et al., 1997; Girma, 2004; Kilambo et al., 2004; Oduor et al., 2004;
Tshilenge-Djim et al., 2004; Kilambo et al., 2006) (see also Chapters 3, 4 and 5). The disease was also recently observed on excelsa coffee in Uganda, but only on plants held at a coffee germplasm conservation site (to the authors’ knowledge, commercial production of excelsa coffee in Africa is very limited). G. xylarioides was reported on wild C. canephora exhibiting
CWD symptoms in forest areas of Kibale and Itwara in west Uganda and confirmed as pathogenic to C. canephora in subsequent pot-based tests (Bieysse, 2005; Bieysse, 2006).
A range of other crops and weed species commonly found or cultivated in coffee farms affected by the disease in Uganda have been investigated as possible alternate hosts for G. xylarioides (Serani, 2000; Kangire et al., 2002;
Serani et al., 2007). However, the fungus was only recovered from the roots of the banana cultivar kayinja (syn. ‘pisang awak’) (Serani, 2000; Serani et al., 2007), often intercropped by farmers alongside coffee for production of banana juice and preparation of beer. This strain was recovered from within the root tissues and found to be pathogenic to C. canephora in subsequent host inoculation studies (Serani, 2000; Serani et al., 2007; G. Hakiza, personal com- munication, 2004). Pathogenicity to banana was not assessed. The extent of similarity, including genetic, between this strain and those recovered from cof- fee affected by CWD is otherwise unclear.
In DRC, a fungus identified as G. xylarioides on the basis of morphological and genetic characteristics (i.e. comparison of tef 1a sequences against those held for G. xylarioides on the FUSARIUM-SEQFUSARIUM-ID database;
Geiseret al., 2004), has been found on cinchona affected by Phytophthora canker (A. Buddie, P. Cannon and P. Kelly, personal communication, 2007). As yet, it is unknown whether this fungus is pathogenic to cinchona or indeed to cof- fee. G. xylarioides has also been found on cotton seed (Pizzinatto and Menten, 1991) and on rotting tomatoes obtained from fruit markets in Nigeria (One- sirosan and Fatunla, 1976). However, information supporting these reports is limited, and it is unknown whether these fungi differ morphologically, genetically or otherwise to those currently observed on coffee. Studies to in- vestigate and identify other potential hosts for G. xylarioides and to assess the pathogenic nature of the fungus on such hosts are otherwise limited. Insects have been suggested as possible vectors for CWD, but attempts to recover G. xylarioides from common pests of coffee (including coffee berry borer, Hypothenemus hampei), as well as from bees, termites and other insects found on affected coffee farms, have been unsuccessful (Rutherford and Flood, 2005;
G. Hakiza, personal communication, 2004).
7.4.2. Infection and colonization of coffee byG. xylarioides
G. xylarioides is considered to be an endemic, soil-inhabiting fungus. Al- though the life cycle and epidemiology of the fungus are not fully un- derstood, the mechanisms of host infection, colonization and symptom development are thought to be similar to those of other vascular wilt pathogens. Initial entry is considered primarily via the roots and lower stem, facilitated by the presence of wound sites as entry points. These may occur naturally as a consequence of farm management practices (see 6.4.3) or by livestock foraging at the base of the coffee tree. Once within the host tissues, the fungus moves within the vascular system and sur- rounding tissues, leading to restriction of water transfer through direct
physical occlusion with fungal material or by the host plant as a response to invasion. As with other vascular wilts, staining of the wood and roots is probably due to changes in phenol metabolism and may constitute a resistance response by the plant (Pegg, 1981; Beckman, 1987; Hillocks, 1992).
7.4.3. Fungal transmission and dynamics of disease spread
Vegetative and reproductive structures of the fungus, namely, microconidia, macroconidia, ascospores and mycelial fragments, may be readily dispersed by air, water (including rain splash and run-off ) and through human ac- tivity, perhaps attached to soil and plant debris (Van der Graaff and Piet- ers, 1978). A profuse production of conidia is noticed on faint mycelium patches developing mostly at the stem base under high-humidity conditions (Fraselle, 1950). Perithecia, often observed on coffee wood under natural conditions and often abundant during the rainy season, may constitute as a means of survival given the sparsity or absence of the thick-walled chla- mydospores produced by other fusaria (Flood and Brayford, 1997). Peri- thecial production has been reported to commence on CWD-affected trees within 2 weeks of the onset of the rainy season and to be more pronounced towards the stem base (Musoli and Hakiza, 2007). Day-to-day agricultural practice, exchange of planting material between growers, dissemination of germplasm by nurseries and distribution centres and transportation of cof- fee by traders are likely to disseminate fungal propagules and contaminated soil, providing an efficient means of disease spread on a local, national or regional scale (Chapters 3, 4 and 5). Wood cuttings, obtained as a means of vegetative propagation, are also exchanged and sold by growers and nurseries to establish new plantings. Coffee berries are obviously widely distributed and marketed and, if contaminated by G. xylarioides, may again provide an effective means of dispersal of the fungus. Isolation of G. xylari- oides from within coffee berries has been reported in Uganda (Serani et al., 2007).
The machete is an essential implement for coffee farmers in Africa and is used on a daily basis to perform a range of tasks including pruning and weed- ing. On-station trials confirm that wounding of coffee trees with a machete previously used on infested coffee wood is sufficient to transmit the pathogen to, and result in CWD development on, healthy and mature trees under field conditions. Coffee wood, an important source of fuel for farmers, also consti- tutes a source of infection for young coffee plants, while infested field soil may remain infective to young plants for several months following uprooting and removal of affected trees (Rutherford and Flood, 2005; Musoli and Hakiza, 2007; Chapter 3, this volume).
Studies to monitor the spatial and temporal development of CWD symp- toms at on-station and on-farm locations affected by CWD provide an insight on disease spread under natural conditions (Rutherford and Flood, 2005; Mu- soli, 2007; Musoli et al., 2008). In one instance, incidence of CWD in mature
plants of a range of susceptible clones increased from below 3% (9 plants) to 45% (166 plants) over a 2-year period with, on average, six plants developing external symptoms per month (Rutherford and Flood, 2005). Thirty months later, more than 90% of plants had either been killed or showed symptoms of the disease. The rate of increase in disease incidence also varied from clone to clone, suggesting different levels of susceptibility. The studies indicate that although initial foci of disease (in some cases individual trees) may be randomly distributed, these enlarge over time but in no particular direction (Plate 16). They also suggest that infection of adjacent trees, perhaps through root-to-root contact or short-distance dispersal of fungal material, is the pri- mary means of on-farm disease development (Rutherford and Flood, 2005;
Musoli and Hakiza, 2007). Although the point at which individual plants became infected in these studies is unknown and the precise mode of infec- tion is unclear, they provide the most accurate information to date on local- ized spread of CWD and do confirm the rapid and destructive effects of the disease once introduced. They also have important implications in terms of cultural management and the potential for eradicating CWD through early removal and destruction of plants as soon as symptoms become apparent (see Chapter 9). An illustration of the putative life cycle of G. xylarioides is provided in Fig. 7.1.
Fig. 7.1. Putative life cycle of the coffee wilt pathogen G. xylarioides (anamorph F. xylarioides), as composed by H. Maraite based on drawings from Van den Abeele and Vandenput (1956), Meiffren (1957), Booth (1971), von Blittersdorff and Kranz (1976) and Pochet (1988). (Courtesy of H. Maraite, Université Catholique de Louvain, Belgium.)