Acknowledgements
10.5. A Perspective for Developing CWD-Resistant Varieties
Basing on the information above, a practical perspective for successfully de- veloping CWD-resistant varieties is given below. The perspective outlined is for C. canephora, and it might not be suitable for C. arabica because the two species have different reproductive systems and the interaction of C. arabica with the G. xylarioides strain is different. We have not attempted to give a possible breeding strategy for developing CWD-resistant C. arabica commer- cial varieties because we lack basic information on type of resistance and heritability of the resistance in this species. A schematic representation of the breeding outline is given in Fig. 10.5.
10.5.1. En masse germplasm screening
As already indicated, there is variability among C. canephora genotypes and populations for CWD resistance. Thus, en masse screening of seedlings and clones derived from different populations through artificial inocula- tion leads to identification of CWD-resistant genotypes quickly. In Uganda, 1519 CWD-resistant genotypes have so far been identified through en masse screening of coffee seedlings and cuttings derived from germplasm available in conservation plots at the research institutes and from coffee trees surviv- ing in wilt-devastated gardens. The conservation plots at Kawanda Agri- cultural Research Institute were the immediate and most available source and therefore provided the biggest amount of seedlings and cuttings so far screened. Open pollinated seedlings and rooted cuttings were raised from various genotypes in these collections at different dates following routine nursery procedures of raising coffee seedlings and rooted cuttings (MAAIF, 1995). The seedlings and cuttings, when at 6–8 months old, were inoculated with field isolates of the pathogen prepared from a specimen obtained from a commercial C. canephora clone 257/53. The plants were inoculated using the root dip method and a standardized inoculum (1.3 × 106 conidia per millilitre). All inoculated plants were incubated at room conditions in the screen house as they were monitored for CWD symptoms (Plate 25). At the end of each trial, any healthy-looking plants were re-inoculated 6 months after the first inoculation. These re-inoculated plants were again incubated in the screen house and monitored for the disease symptoms. Again after another 6 months, healthy-looking plants were selected for another inoc- ulation. Survivors of these re-inoculations were considered resistant and potential clones for future varieties. All the resistant genotypes were then planted in mother gardens and each of them cloned through cuttings to raise clones for establishing field evaluation trials (preliminary variety evaluation trials).
Vegetative propagation (2)
Mass vegetative propagation Screening local
germplasm (2)
On-station field assessments and selection (6)
Vegetative propagation (3)
On-farm adaptation & adoption evaluation & selection (6)
Vegetative propagation of mother stock for farmers (2)
Screening exotic germplasm (4)
Hybridization (1) 1. Local R x CC 2. Local R x R 3. Local R x exotic R 4. CC x exotic
On-station field assessment and selection (6)
Vegetative propagation (3)
On-station polycross seed gardens (3)
On-farm progeny evaluation &
selection (6)
Mass seed propagation
Seedling varieties Clonal varieties
Tissue culture + nodal cuttings
Fig. 10.5. Proposed scheme for developing C. canephora varieties resistant to CWD. Figures in parentheses are the minimum time in years a particular breeding stage could take. R = resistant, CC = current commercial varieties.
Open pollinated seeds and semihard wood cuttings were obtained from coffee trees surviving in wilt-devastated gardens. Seedlings and rooted cut- tings were, respectively, raised from these materials for inoculation and se- lection of resistant genotypes following the procedure described above.
It was observed that a very small proportion (3%–15%) of the plants raised from the on-station germplasm plots survived the first inoculation. The rate of survival varied between progenies (Fig. 10.2), perhaps due to relative resistance of their parents. However, the proportion of survivors after the first inoculation from among seedlings and cuttings raised from genotypes surviv- ing in wilt-devastated gardens was relatively higher (15%–20%). This implies that mother plants surviving in wilt-devastated gardens have some degree of resistance, although this resistance might not be strong enough to withstand the inoculum pressure given in the screen house. It has also been observed that in the re-inoculation, about 50%–70% of the re-inoculated plants die.
In Tanzania, more than 270 resistant genotypes have been identified through mass screening of germplasm. The resistant genotypes have been planted in mother gardens for vegetative multiplication.
10.5.2. Field germplasm screening
Germplasm screening can also be carried out in field observation plots.
However, field evaluations tend to handle relatively fewer plants, and the assessment is prolonged over many years, yet one cannot ascertain that re- sistant genotypes selected through field observations can withstand high disease pressures under different agroecological conditions unless the trials are validated under a range of agroecological conditions. At COREC, clones in a field trial were assessed for CWD resistance starting 1999 to 2006. It was noted from this field trial that clones respond to the disease differently and one clone (J/1/1) was still resistant by November 2007 (Table 10.2). How- ever, rooted cuttings raised from the resistant clone, when tested for resis- tance in the screen house, succumbed to the disease although the level of disease among its cuttings was lower than that among cuttings of the other clones within this trial.
10.5.3. Multiplication of the CWD-resistant varieties
Because C. canephora is predominantly an out-crossing crop, it is anticipated that all the individuals are heterozygous, and therefore, the CWD-resistant genotypes are expected to segregate for most traits including the resistance to CWD. It is therefore recommended that these genotypes are propagated entirely using vegetative methods, particularly rooted nodal cuttings or tis- sue culture. At the COREC, the former is being used for propagating materi- als earmarked for both further evaluation and farmers’ use whereas the latter exclusively dedicated to raising planting materials for farmers’ use from so- matic embryogenesis.
10.5.4. Field evaluation of the resistant varieties
It is obvious that not all the CWD-resistant genotypes have all desired qualities of commercial varieties. Therefore, the CWD-resistant genotypes must undergo preliminary field evaluation, where they are evaluated for yield, resistance to other diseases (coffee leaf rust [CLR] and red blister disease in Uganda) and liquor and physical bean qualities. Because this initial evaluation involve many genotypes, it is ideal to first have a simple trial where all the available genotypes are assessed and genotypes with higher potential are selected for further evaluation in different agroecologi- cal localities. As of November 2007, 1519 CWD-resistant clones had been identified in Uganda, and these have been planted in single rows of six trees in an on-station trial at Kituza. To date, 25 superior clones have been selected from among 167 clones that have undergone the initial on-station evaluation. The 25 clones are being multiplied through nodal cuttings for further evaluation in different agroecological areas so as to validate their performance and adaptation to different localities. Good yielding (at least 2 t of green beans) clones that are resistant to CLR and red blister disease and have good liquor and physical bean qualities will be recommended for farmers’ use, either across the entire country or for particular agroeco- logical localities, depending on the performance of individual clones in the multilocation trials.
10.5.5. Hybridization for CWD resistance
Although CWD takes precedence at the screening stage, yield, quality and resistance to CLR, red blister disease and other major coffee diseases are im- portant traits for commercial C. canephora varieties, particularly in Uganda.
CWD-resistant clones that might not be high yielding and have low potential for cup and bean qualities and resistance to other diseases can be crossed with resistant genotypes and other lines that posses these complementary traits. The hybrid progenies can then be evaluated for all the traits starting either at the screen house through field trial testing or in field trials only. The former approach ensures that genotypes that proceed to the field are all re- sistant to CWD. The latter approach is less efficient for CWD resistance since it relies on field responses only, but it retains more genes especially those in moderately resistant genotypes to be tested that could have been eliminated in the more exacting screen house tests.
The current commercial clones (1s/2, 1s/3, 1s/6, 223/32, 257/53 and 258/24) used in Uganda have the complementary traits, but they are suscep- tible to CWD. The commercial clones yield 2.5–3 t of green coffee beans per hectare per year, have good bean qualities (18–22 g hundred beans weight, over 90% retained by screen 18/64) and have good cup qualities. Hybrids have been generated through artificial pollinations to combine the CWD re- sistance and the complementary traits found in these commercial clones. The progenies generated have been planted in field trials at Kituza for evaluation
as individual trees. The individual genotypes that will be within acceptable limits of the traits will be selected, multiplied as clones and planted in multi- location trials in different agroecological localities for re-assessment. Clones that shall perform well in these trials will be selected for release to farmers as clonal varieties either at agroecological zone level or for the entire country depending on the performance of the variety.
Hybridization can also be between the CWD-resistant clones possessing complementary traits. The hybrid progenies generated in such crosses are also evaluated as individual trees for resistance against CWD and for field performance in the other traits. Good-performing individuals can be selected, cloned and planted in multilocation trials for adaptation and adoption tests.
Good-performing clones will then be selected and released to farmers.
If there is an entire progeny of a cross between resistant and susceptible parents that perform well (resistant to CWD, CLR and red blister disease;
have good qualities and are high yielding), then parents of such progenies can be planted in polycross seed gardens for production of seeds to be given out to farmers. It is anticipated that progenies involving parents from differ- ent populations, particularly from distant geographical locations, shall bene- fit from hybrid vigour derived from double heterozygosity of the parents. As already indicated above, given the specificity of the pathogen populations affecting the different commercial coffee species, resistant varieties can be derived through interspecific hybridizations, bearing in mind the complica- tions associated with such hybridizations and the difficulty to derive a vari- ety of desired quality.
As known from many breeding programmes, incorporation of CWD re- sistance genes into commercial clones that should also have good quality traits is likely to take a very long time, and at times, it is a gamble. Therefore, molecular techniques can be adopted to facilitate the breeding and selection process. Studies had been initiated at CORI to characterize CWD resistance using molecular markers, and it is our hope that a follow-up of these studies shall continue, which should lead to mapping of the resistance genes. Map- ping studies could be initiated using the double haploid CWD-susceptible/
resistant parents and their progenies to identify molecular markers and or quantitative trait loci (QTL) associated with CWD resistance to assist in breeding resistant varieties and isolation of resistance genes for creating bac- terial artificial chromosome libraries. Use of double haploids shall minimize the effect of heterozygosity.
10.5.6. Grafting
Because currently the mechanism of resistance against the CWD pathogen is not known, we are not certain whether a resistant rootstock can prevent the pathogen from reaching the scion. However, if it is established that this phenomenon is possible, then grafting scions of varieties with good agronomic characteristics but which are susceptible to CWD on to a CWD- resistant rootstock should be a good and probably quicker means of deriving
appropriate planting materials for farmers of both C. canephora and C. arabica.
In Uganda, grafting of current commercial clones on to the CWD-resistant clones is being evaluated for (i) compatibility between the scion and root- stock and (ii) the ability of the rootstock to prevent the CWD from getting through the vascular system of the rootstock to the scion. If these results are satisfactory, then grafting should be adopted for continuing with large-scale multiplication of the current commercial varieties.
Rootstock of other Coffea species can also be explored for this purpose. Suc- cessful interspecific grafting involving C. canephora and C. liberica spp. has been reported (Couturon, 1993). However, Bertrand et al. (2001) reported depress- ing effects of C. liberica ssp. rootstock on yield and quality of C. arabica scion varieties. Therefore, other agronomic properties of the grafted varieties such as yield and quality should be studied and well understood before recommend- ing grafting for producing planting materials for farmers on a large scale.