Colletotrichum acutatum J. H. Simmonds was isolated from diseased leaves of ornamental Kalmia latifolia L. (mountain laurel) cvs. Carousel and Peppermint on plants imported from the United States to Edinburgh, Scotland, in December 1999. Symptoms included sunken, desiccated, darkened necrotic areas, primarily at the leaf tip. Necrotic areas advanced toward the leaf base and were bordered by purple/red pigmentation. Isolations were made from salmon colored conidiomata that developed on abaxial leaf surfaces following incubation in a humidity box at 25°C for 7 to 10 days. White aerial mycelia, becoming gray to grayish beige, and producing salmon to orange colored conidial masses, formed on potato dextrose agar after 10 to 14 days. Conidia were hyaline, aseptate, fusiform to slightly irregular, and measured 13.4 to 13.8 × 4.3 to 4.9 μm. Both morphological and conidial characteristics were consistent with the description of C. acutatum (2). The identity of isolates was further verified by positive plate-trapped antigen ELISA of conidial preparations using a species-specific monoclonal antibody (1). Pathogenicity was assessed by inoculating the adaxial surface of healthy leaves of both cultivars of the imported plants with colonized agar disks and a range of spore suspensions (30, 300, and 3,000 spores delivered in 30 μl volumes) from test fungal isolates and a confirmed laboratory strain (three replicates per treatment). To ensure inoculum uptake, two 5 mm2 areas of cuticle on either side of the mid-rib of each leaf were lightly scratched with a sterile hyperdermic needle prior to inoculation. Inoculated leaves were incubated in a humidity box at 25°C for up to 3 weeks. Symptom development was progressive but relatively slow on both cultivars. The relatively slow development on artificially infected leaf material may be partly attributable to residual fungicide treatment as prescribed by the Scottish Plant Health Service at the time of planting out. Symptoms produced on fruits (apple, banana, and strawberry), inoculated with both test and laboratory strains of the fungus, were identical. Symptoms did not occur on control leaves or fruits inoculated with sterile distilled water or uninoculated agar disks. Koch's postulates were confirmed by consistently reisolating isolates with morphological and immunological characteristics identical to the fungal isolate used to initially inoculate test material. Over the same period, additional symptoms, identical to those originally described at the time of interception, continued to develop on leaf tips of both Kalmia cultivars. Additional isolations from this material were characterized as C. acutatum. Identification of representative isolates was confirmed by CABI Bioscience, Egham, UK, where a reference culture (accession number IMI 384569) has been deposited. As advanced symptoms were observed immediately on arrival of this consignment in the UK, original infection is thought to have occurred prior to importation. This is the first report of C. acutatum infecting K. latifolia.
References: (1) I. Barker et al. 1994. Pages 179-182 in: Assays for Plant Pathogenic Fungi: Identification, Detection and Quantification. A. Schots, F. M. Dewey, and R. Oliver, eds. CABI International, Wallingford, UK. (2) B. J. Dyko and J. E. M. Mordue. CMI Descriptions of Pathogenic Fungi and Bacteria. No. 630, 1979.