Status of Coffee Leaf Rust Resistance on Kenyan Commercial Resistant Cultivars

Background: Coffee Leaf Rust (CLR) is a fungal disease caused by Hemileia vastatrix Berk. and Br. The pathogen is constantly evolving leading to rapid break down in resistance of once resistant coffee varieties. To date, more than 49 races of the pathogen have been characterized all over the world and new races are continuously being characterized some of which are able to infect Robusta derivatives.
Aim: The objective of this study was therefore to re-examine the status of CLR resistance on Kenyan commercial resistant cultivars and investigate the pathogenic interaction between H. vastatrix isolates and their host genotypes.
Methodology: Hemileia vastatrix isolates were collected from naturally infected leaves of the host coffee genotypes and were inoculated on one Robusta coffee genotype and eight Arabica genotypes comprising of three Kenyan commercial cultivars (SL28, Ruiru 11 and Batian) and five museum genotypes (HDT, Mundo Novo, Pretoria, 110/2 and Bourbon) using leaf disks inoculation method. An infection scale of 1-6 was used to score the virulence of the pathogen isolates.
Results: There was significant variation among isolates on their virulence against the different genotypes. SL28, Pretoria and Mundo Novo were the most susceptible to most isolates while none of the isolates infected Ruiru 11, HDT and Robusta. All the isolates were able to infect Batian but none reached the stage of sporulation. Isolates from K7, SL34 and SL28 were found to be more virulent than those from Batian and Blue Mountain. Unlike the host genotype, the region from which the isolates were obtained was not found to play any role on the virulence of the isolates.
Conclusion: Although six additional races of H. vastatrix were recently detected in Kenya some of which are able to infect Robusta derivatives, the study confirmed that Kenyan genotypes in this group are still resistant against most races of H. vastatrix in Kenya. It was therefore deduced that either these new races are not yet wide spread in all coffee growing areas in Kenya or that there are other major and minor genes conditioning the coffee-rust interactions besides the SH genes.