Acknowledgements
8.7. Summary and Conclusions
The prospects of successful control of CWD rely principally upon deploy- ments of resistant coffee cultivars/lines. In this regard, full-fledged, indepen- dent, large-scale coffee collection and screening strategy should be planned hand in hand with efficient breeding programme. Because of the difficulty in screening for resistance under field conditions for soil-borne diseases, stan- dard seedling inoculation protocols should be employed. For a number of technical reasons, stem nicking (at 2 cm above the soil level) of late-cotyledon- stage (2 to 2.5 months old) coffee seedlings with inoculum concentration of 2 × 106 conidia/ml is recommended. The inoculated seedlings need to be placed in an air-conditioned room with higher humidity (>95%) and temper- ature of about 23°C for 1 week that ensure infection and then be transferred to the greenhouse (Plate 23). Disease parameters such as number of dead seedlings and number of days between inoculation and external symptom appearance (chlorosis, retarded plant growth and reduced leaf size, wilting and finally death) should be recorded for at least 6 months (preferably at fortnightly intervals), from which percentage death and incubation periods
can be computed and used for statistical analyses. Pieters and Van der Graaff (1980) reported the highest correlation between the field scores and death rates and incubation time in the seedling tests 6 months after inoculation.
The resistance of surviving seedlings of a particular cultivar/line needs to be further verified by re-inoculating grown seedlings (12 months old) in the screen house (following the same technique) and evaluated at least for one more year (Plate 24) because of the difficulties of proving host resistance un- der field conditions.
Following on from the detailed host–pathogen interactions supported by random amplified polymorphic DNA analysis, Girma et al. (2005) introduced the epithet formae speciales (special forms), G. xylarioides f. sp. abyssiniae(ana- morph:F. xylarioides f. sp. abyssiniae) for strains attacking C. arabica confined to Ethiopia and G. xylarioides f. sp. canephorae (anamorph:F. xylarioides f. sp.
canephorae) for strains specifically pathogenic to C. canephora and C. excelsa.
Rutherford (2006), based on the various molecular studies, reported that two clonal populations are responsible for the current CWD outbreaks in Africa, one composed of isolates obtained from affected C. arabica in Ethiopia (vari- ant ‘A’), and the other are isolates from affected C. canephora in DR Congo, Uganda, and Tanzania (variant ‘C’) (Chapter 7). Phylogenetic analysis of F.
xylarioides including the number of isolates obtained from CWD-affected coffee trees in Uganda has shown these to belong to the African clade of the Gibberella fujikuroi species complex, a clade replacing Fusarium section Liseola(Geiseret al., 2005). The appearance of new populations or strains of the pathogen also needs to be monitored in the field because G. xylarioides produces very abundant perithecia and ascospores in the stem bark of dead trees and stumps of coffee (Van der Graaff and Pieters, 1978; Flood and Bray- ford, 1997; Girma et al., 2001; Girma, 2004).
The preliminary histopathological observation of the interaction of C. canephora vs. G. xylarioides highlighted the hyphal colonization of xylem tissues and the intercellular spaces, blocking water movement that leads fi- nally to wilting of infected coffee trees. However, the underlying resistance mechanisms that may be morphological, physiological and/or biochemical involved in host defense systems of different cultivars/lines of Coffea species should also thoroughly be further studied. The resistance in C. canephora and C. liberica was supposed to be associated with rapid suberization in wounds and occurrence of caffeine and chlorogenic acid in high concentrations (Booth, 1971; Holliday, 1980). Understanding the gene(s) that govern resistance to CWD and mode of inheritance of the resistance mechanisms is of paramount importance to design and implement successful breeding programme.
Acknowledgements
The authors acknowledge the Department for International Development, UK, for funding part of the work done at CIRAD, Montpellier, and we are very thankful to all the staff at Jimma Agricultural Research Center (EIAR, Ethiopia) and CIRAD (France) for their unreserved technical assistance.
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9 Management of Coffee Wilt Disease
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