Strains of Colletotrichum coffeanum, the causal agent of coffee berry disease, tolerant to benzimidazole compounds in Kenya

The detection of Colletotrichum coffeanum tolerant to methyl ester of benzimidazole 2-carbamic acid (carbendazim) and a related benzimidazole compound, cypendazole, followed increases in levels of coffee berry disease observed on Coffea arabica in experimental plots sprayed for 2 yr with these compounds. Sporulation by the pathogen on naturally infected berries removed from carbendazim-, cypendazole- or benomyl-sprayed plots was not checked by a further application of 0–05 % (a.i.) of any of the compounds. Nearly all the isolates from these berries were capable of some growth on agar media containing 1000 ppm (a.i.) of either carbendazim or cypendazole. However, only a few could tolerate 1000 ppm of benomyl and the inability of this compound to reduce sporulation on berries infected with tolerant strains was presumably due to its rapid conversion to carbendazim within the host tissue. Less than 1 ppm of carbendazim, cypendazole or benomyl was needed to give 50% inhibition of conidia of the normal strain. Against the most tolerant strains, however, the LD 50 was > 100 ppm of carbendazim and about 30 ppm of benomyl. Whether isolated from unsprayed or benzimidazole-sprayed plots, all isolates of Colletotrichum acutatum, a saprophytic cohabitant of lesions initiated on berries by C. coffeanum, showed the highest degree of tolerance to benzimidazole compounds. No tolerance of either fungus to the ‘conventional’ fungicide captafol was detected.