Limb dieback of flowering dogwood caused by Colletotrichum acutatum

In 1991 and 1992, an array of symptoms consisting of leaf spots, defoliation, limb and twig dieback, canker-like deformities, and mortality of flowering dogwood trees (Cornus florida L.) was observed in several nurseries in north and central Florida. Older affected trees produced flower buds, but the buds did not open. Many trees were destroyed because of unsightly dead limbs and deformed trunks. In 1998-99 the disease re-appeared in north Florida nurseries and once again caused extensive damage. The disease is currently present in nurseries and landscapes and represents a serious threat to nursery production of flowering dogwood in Florida. Colletotrichum acutatum J. H. Simmonds was consitently isolated from diseased leaf, pith, and wood samples, from acervuli produced in abundance on diseased lives, twigs, branches of small and large trees, and from lesions on diseased, rooted cuttings. A modified bud grafting technique was used to insert small pieces of agar containing mycelia of the pathogen beneath the bark of small trees. Typical symptoms developed within 5 months on inoculated trees from which the suspected pathogen was consistently re-isolated. In greenhouse experiments, conidia of the isolated pathogen were used to inoculate small dogwood trees. Within 2 to 3 weeks, a slowly-developing leaf spot was produced on inoculated plants. After 3 months, numerous acervuli were produced on inoculated leaves and on adjacent small twigs from which the same pathogen was consistently re-isolated. Inoculated trees died within 9 months after inoculation. Based upon the size and morphology of conidia and growth characteristics of the fungus on agar medium, the pathogen was identified as a Colletotrichum acutatum. Analysis of fungal DNA by the polymerase chain reaction and comparison of PCR producs with those from other Colletotrichum species confirmed the fungus as a pathotype of C. acutatum.