NATURE OF RESISTANCE OF LYAMUNGU HYBRIDS TO Colletotrichum kahawae STRAINS

Abstract

Nature of resistance of Lyamungu coffee hybrids to coffee berry disease (CBD) caused by Colletotrichum kahawae were evaluated using conidia germination on cuticular wax extracted from the hybrids, stimulatory effects on the infection of the pathogen, and evidence of genetic resistance on studies using hypocotyls and green berries in the field. Significant differences (P £ 0.05) were found between the levels of conidia germination on the extracted wax of the coffee genotypes. Nine coffee genotypes showed no conidia germination and five showed conidia germination percentage of below 20%. Conidial germination percentage levels of up to 90% were found on CBD susceptible variety N39. Light micrographs showed that hyphal growth of C. kahawae strains were restricted in epidermal cells and cortex of CBD resistant genotypes 20498 and 20509. Callose and thick walls were observed in cortex cells of resistant genotypes 20498 and 20509. Field evaluation also revealed same genotypes which exhibit highest levels of CBD resistance. In order to understand resistance of Lyamungu coffee hybrids, pathogenicity of 26 C. kahawae strains from different eco-agricultural zone in Tanzania was studied under laboratory conditions. Pathogenicity tests both on detached green coffee berries and hypocotyls drew a distinction between highly and less pathogenic strains. The C. kahawae strain 2006/14 was found to be the most pathogenic because it showed high sporulation capacity and also induces CBD symptoms 3 days after infection both on green coffee berries and hypocotyls.