Acknowledgements
10.2. Potential Sources of Resistance
Intra- and interspecific differences among and between coffee species respec- tively provide potential genetic variability, which is exploited for resistance against CWD. Intraspecific variability is the best and easiest to exploit since resistant individuals are easily released as new varieties without undergoing
hybridization, provided they posses other agronomic traits such as being high yielding; having resistance to other major diseases, mainly leaf rust and red blister disease and coffee berry disease (arabica only) and having good market qualities (big beans and good cup qualities). Where necessary, intraspecific hy- bridization is easier to carry out, and it is usually more successful.
C. canephora is particularly genetically variable (Plate 26), and the vari- ability is very diverse even among genotypes from the same locality (Musoli, 2007) or members of the same progenies, mainly because it is out breeding and its wide geographical distribution (Leakey, 1970). Musoli (2007) found that most of the genetic diversity among C. canephora populations at mo- lecular level was attributed to variations (heterozygosity) within individu- als. Similar studies carried out in DRC revealed diverse variability among C.
canephora populations in that country. C. arabica is relatively less genetically diverse, but nevertheless, the available diversity is high enough to be ex- ploited for resistance against CWD.
Open pollinated seedlings of different C. canephora populations, which included two cultivated distinct morphological types (‘erecta’ and ‘nganda’;
Plate 27) plus wild populations from Kibale and Itwara forests and a feral population from Kalangala, an isolated island in Lake Victoria, were assessed for CWD resistance through artificial inoculation in a screen house at COREC.
These studies showed significant (P = 0.001) genetic differences between the populations for CWD resistance. The disease (measured by plant mortality) progressed at different rates and to different final levels for the different popu- lations (Fig. 10.1). This illustrates the usefulness of a diverse germplasm pop- ulation when sourcing for resistance against CWD. Artificial inoculation of different genotypes of C. canephora in DRC and Tanzania also showed varying levels of resistance among the genotypes. Similar studies carried out on C. ara- bica in Ethiopia revealed differential reactions to CWD by genotypes from dif- ferent localities (Chapter 8). In Uganda, exploitation of intraspecific variability in C. canephora led to selection of eight high-performing, CWD-resistant clones,
0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0
wk4 wk5 wk6 wk7 wk8 wk9 wk10
% mortality
Time in weeks
Kibale Nganda Erecta Itwara Kalangala
Fig. 10.1. Progression of CWD among open pollinated progenies of different C. canephora populations.
which also have good bean and cup qualities and resistance to red blister dis- ease and leaf rust. These have been recommended for commercial cultivation.
In some parts of Uganda where C. canephora and C. arabica are cultivated side by side, CWD affected only the former species. Similar responses have been reported in Ethiopia, Tanzania and DRC. The difference observed be- tween the two species in Uganda was confirmed through artificial inoculation of seedlings of the two species in the screen house (Musoli et al., 2001). Inter- specific hybrids (F1, backcross 1 and backcross 2; arabica as a recurrent par- ent) of C. canephora and C. arabica (arabusta) grown close to C. canephora fields affected by CWD have remained unaffected since 2001, when the disease was first observed on C. canephora. The resistance of the arabusta hybrids was also confirmed through artificial inoculation of clones of their F1 progenies and F2 progenies at COREC. This implies that the resistance to CWD in C. arabica is dominant, and it can be introgressed into C. canephora through interspecific hybridization. However, deriving C. canephora-resistant varieties through arabusta interspecific hybrids needs several backcrosses to the C. canephora re- current parents This backcrossing is complex because the two species have dif- ferent ploidy levels and the crosses normally involves C. canephora artificially generated tetraploids.
Some level of host specificity to contemporary strains of G. xylarioides was noted through host–pathogen interaction studies. The strain currently affecting C. canephora in Uganda, DRC and Tanzania is specific to this spe- cies. Artificial cross inoculations carried out on young C. canephora and C. liberica half sib progenies in growth rooms showed that G. xylarioides iso- late (CAB003), which was obtained from C. canephora, causes severe mortal- ity only on C. canephora (Fig. 10.2a) (Musoli, 2007). Similar studies showed that the strain affecting C. arabica in Ethiopia is specific to this species (Chapter 8). In mixed coffee cultures of C. canephora and C. arabica within Ethiopia, the pathogen strains infect only C. arabica. Artificial inoculation of C. canephora under controlled growth room conditions using isolate CAB007 collected from C. arabica in Ethiopia showed that this strain induces early symptoms of CWD on C. canephora, but it is not fatal (Fig. 10.2b). The inter- action of a historical strain, DSMZ62457, isolated in the previous epidemic during the 1950s on C. canephora in CAR and evaluated through artificial inoculations in the growth room gave mixed reactions. Although Musoli (2007) showed that this strain was an aggressive pathogen of C. liberica and moderate on C. canephora (Fig. 10.2b), other studies showed that C. liberica isolate DSMZ62457 was pathogenic to C. canephora, C. liberica and C. arabica and that other isolates from C. arabica and C. canephora induced some symp- toms on C. liberica. Therefore, further studies are required to validate these findings. Field reports from DRC and Uganda suggest that certain C. liberica spp. are susceptible to the C. canephora isolate within these countries. These observations thus exclude C. liberica from being a source of resistance to CWD for introgression into arabica and robusta. Given such host–pathogen specificity, varieties with durable resistance, which can be used across the entire African region, irrespective of the prevailing pathogen strain, can be derived through Arabusta interspecific hybridization.