Virulence of colletotrichum kahawae strains and their effect on resistant arabica coffee varieties in Tanzania

The virulence of 26 Colletotrichum kahawae strains collected from different coffee growing areas in Tanzania was studied under laboratory conditions. Cultural and morphological characters on malt extract agar indicated variations amongst strains. Virulence tests both on detached coffee green berries and hypocotyls distinguished between highly and less virulence strains. Initial symptom appearance showed positive correlation to sporulation. Lesion size showed positive correlation to sporulation capacity and percent berry infection; and sporulation capacity and percent berry infection. At 15 0 C the virulence of C. kahawae strains was higher than at 20 0 C and 25 0 C. Biochemical test using esterases, acid phosphatases and alkaline phosphatases iso-enzymatic systems studied by iso-electric focusing revealed variation among Tanzanian C. kahawae strains from those obtained from other countries. Percent germination of conidia on wax surface extracts from green berries of resistant coffee genotypes indicated that the wax layer influences resistance to C. kahawae. Post-penetration growths of C. kahawae in hypocotyls measured at 24, 48 and 72 hrs after inoculation were higher in susceptible than resistant genotypes. Histological characterization of coffee genotypes revealed inhibition of fungal growth by callose formation around intracellular hyphae in resistant genotypes. Expression of coffee resistance to C. kahawae varied with the test isolates and the hybrids. However, coffee genotype 20498 (L2) showed the highest level of resistance to C. kahawae.

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