Smoothlip cymbidium (Cymbidium kanran Makino) is one of the national endangered species in Korea and in situ and ex situ conservation measures have been implemented for its threatened habitat and population. Jeju Island in Korea is one of few places where wild populations of the plant remain. In 2012, leaf blotch and blight occurred on leaves of smoothlip cymbidium in Jeju Island throughout the growing season. Affected plants were collected in May and October to determine the causal agent. Leaves had multiple brownish lesions that often developed from the leaf tip or leaf margin, eventually coalescing together and proceeding toward the base. In the lesions, fungal fruiting bodies formed concentric rings and exuded salmon-colored spore masses when put in moist chambers for 1 to 2 days. Colletotrichum gloeosporioides was consistently isolated from the spore masses or the margins of fresh lesions. When cultured on potato dextrose agar (PDA), colonies were olive to gray on the upper side and dark gray to black on the reverse side. On affected leaves, mature acervuli were dark brown to black, waxy, subepidermal, circular to ellipsoid, and 142 to 255 μm in diameter. Setae on the acervuli were dark brown, acicular, and 67 to 103 μm long. Conidia were hyaline, cylindrical with both ends rounded, and 13.7 to 19.2 × 4.0 to 6.1 μm. Hyphopodia were lobed and 7.6 to 13.7 × 5.7 to 11.6 μm (43 to 157 μm2 in area). These morphological characteristics were consistent with descriptions of C. gloeosporioides (1,3). The identities of two representative isolates were confirmed by sequencing the internal transcribed spacer (ITS) regions and the large subunit (LSU) rDNA (GenBank Accession Nos. KC408373, KC408374, and KC408375). BLAST analysis of the sequences from each isolate against the GenBank database found 99% similarities to more than 30 accessions of C. gloeosporioides (e.g., AY266392, EU552111). Pathogenicity was tested with one isolate on four leaves obtained from four plants of asymptomatic smoothlip cymbidium. Two places on the epidermis of each leaf were gently pinpricked using a sterile dissecting needle. One disk (0.6 cm diameter) of either PDA containing the fungus or sterile water agar was placed on one of the two places on each leaf. Laboratory film and aluminum foil held the disks in place. Inoculated leaves were kept in a moist chamber for 24 hr. After 1 week, buff-colored lesions with young acervuli were found on all fungus inoculation sites but no lesions occurred with control inoculations. C. gloeosporioides was recovered from all inoculations, but not from the controls. To our knowledge, this is the first report of anthracnose caused by C. gloeosporioides on smoothlip cymbidium in Korea. The same fungus has been previously reported to cause anthracnose of other orchid plants in the genera Aspidistra, Cymbidium, and Dendrobium in Korea (2). This pathogen can pose a threat to smoothlip cymbidium.
References: (1) M. Du et al. Mycologia 97: 641, 2005. (2) The Korean Society of Plant Pathology. List of Plant Diseases in Korea, 5th ed. The Korean Society of Plant Pathology, Suwon, Korea, 2009. (3) B. C. Sutton. Pages 1-26 in: Colletotrichum: Biology, Pathology, and Control. J. A. Bailey and M. J. Jeger, eds. CAB International, Wallingford, U.K., 1992.
