Colletotrichum trifolii is the principal causal agent of anthracnose of alfalfa in the mid-Atlantic United States.  Local isolates of C. destructivum and C. dematium f. truncata were weakly pathogenic; however, one isolate of C. destructivum from Canada was moderately pathogenic to alfalfa.  An isolate of Colletotrichum found in a greenhouse was classified in the composite group, C. gloeosporioides.  It produced anthracnose-type stem lesions on experimental strains and cultivars of alfalfa resistant or susceptible to C. trifolii.  Seedlings simultaneously inoculated with a mixture of C. trifolii and local isolates of C. destructivum or with C. trifolii and C. dematium f. truncata were significantly less damaged than those inoculated with C. trifolii alone.  The two weakly pathogenic species produced many acervuli on stem lesions caused by C. trifolii.  Similar results were obtained when plants were inoculated with C. trifolii 5 days after inoculation with the two weakly pathogenic Colletotrichum spp.  When plants were inoculated with C. trifolii alone and with C. trifolii before inoculation with C. destructivum and C. dematium f. truncata, disease incidence (anthracnose) was similar on all plants.