Tolerance in colletotrichum coffeanum noack to benomyl and other related systemic fungicides
Two benomyl tolerant strains of Colletotrichum Coffeanum Noack were isolated from infected green berries from Migaa and Kibubuti estates in Kiambu District. The estates were known to have a history of benomyl coffee berry disease (CBD) control programme. The sensitive strain was isolated from a plot that had never been sprayed with fungicides of Coffee Research Station, Ruiru, in Kiambu District. An account is given of the mycelial growth, pigmentation and sporulation of benomyl tolerant and sensitive strains grown under similar conditions on synthetic media. Slight differences were observed in pigmentation, sporulation, zonation and radial growth of mycelia. It was noted that conidia from the benomyl torelant strains showed a slight delay in germination compared to those of the sensistive strain . In all cases spore suspensions containing lower numbers of conidia gave lower levels of germination and took longer time to germinate compared with the spore suspensions containing higher numbers of conidia. Conidia washed several times after harvesting from malt extract agar (MEA) also gave lower level of germination compared to the unwashed conidia and washed conidia in glucose solutions. Higher doses of benomyl lowered percent germination and also delayed germination of conidia. In vitro mycelial growth of benomyl tolerant strains was less inhibited on MEA containing 1000 ug/ml benomyl than that of a sensitive strain on MEA with 1.0 ug/ml benomyl. Differences in sporulation were found among all strains in vitro. No reduction of conidial size was observed. Conidia of tolerant strains, harvested from MEA amended with different concentrations of benomyl, did not lose their viability and were able to germinate in distilled sterile water. All strains in culture produced colour variants by sectoring.