Occurrence of Ck-1 gene conferring resistance to Coffee Berry Disease in Coffea arabica cv. Ruiru 11 and its parental genotypes

Resistance to Coffee Berry Disease (CBD) in Coffea arabica cv. Ruiru 11 is known to be controlled by among others, the T (Ck-1) gene from Robusta coffee (Coffea canephora Pierre). The cultivar reportedly presents significant variability in resistance to CBD. Previous work identified a microsattelite marker Sat 235 which was linked to CBD resistance and mapped it onto the introgressed C. canephora fragment which carries the Ck-1 gene. This study was aimed at utilizing the Sat 235 marker to assess the occurrence of the Ck-1 gene in Ruiru 11 sibs and their parental genotypes. The test genotypes used were CBD resistant Robusta coffee, non introgressed C. arabica cv. caturra, 14 Ruiru 11 parental genotypes with varying reaction to CBD and 34 Ruiru 11 sibs. Evaluation of CBD resistance was conducted in the laboratory using hypocotyl inoculation method. Seeds of the test genotypes were sown in plastic boxes filled to half-depth with sterilized river sand and arranged in a completely randomized design with three replications. Six weeks after sowing, the seedling hypocotyls were inoculated with a conidial suspension of C. kahawae standardized to 2.0 × 106 spores/ml. Disease scoring was conducted 4 weeks after inoculation on a scale of 1 to 12. To confirm occurrence of the Ck-1 gene, genomic DNA was then extracted from the test genotypes and amplified with the microsatellite primer Sat 235 and electrophoresed on a 6% denaturing polyacrylamide gel. All the genotypes containing the Ck-1 gene were expected to show phenotypic resistance to CBD and to show similar banding pattern as Robusta and HDT while the ones lacking the gene were expected to show phenotypic susceptibility to CBD and to similar banding pattern as Caturra and SL28. The study observed that all Ruiru 11 sibs that were evaluated contained the Ck-1 gene. The study also provided further evidence that the fragment amplified by SSR primer Sat 235 is linked to CBD resistance.