First Report of Oak Anthracnose Caused by Apiognomonia errabunda on Oriental White Oak in Korea

Oriental white oak, Quercus aliena Blume, is native to East Asia including Korea. It is one of the major deciduous tree species in natural forests in Korea. In May 2012, several hundred trees were found to be heavily damaged by a previously unknown leaf disease in a forest near Songjiho Lake in Goseong County of central Korea. Leaf symptoms began as small, water-soaked, pale greenish to grayish lesions, which enlarged to follow the veins or midribs and to be bounded by them, often killing part of the leaf. Leaf distortion and blight resulted in the later stage of disease development. A number of grayish brown to nearly black acervuli were formed on the lesions, especially on the midribs and veins. Acervuli were mostly hypophyllous, intraepidermal, erumpent, circular to ellipsoid in outline, cushion-like, and 70 to 220 μm in diameter. Conidia (n = 30) were elliptical to fusiform-elliptical, occasionally obclavate, aguttulate or guttulate, hyaline, aseptate, and 7.5 to 20 × 5 to 7.5 μm (mean 14.6 × 6.1 μm). These morphological characteristics of the fungus were consistent with the description of conidial state of Apiognomonia errabunda (Roberge ex Desm.) Höhn. (3,4). Voucher specimens were deposited in the Korea University Herbarium (KUS). An isolate obtained from KUS-F26690 was deposited in the Korean Agricultural Culture Collection (Accession No. KACC46842). Fungal DNA was extracted with DNeasy Plant Mini DNA Extraction Kits (Qiagen Inc., Valencia, CA). The complete internal transcribed spacer (ITS) region of rDNA was amplified with the primers ITS1/ITS4 and sequenced. The resulting 549-bp sequence was deposited in GenBank (KC426947). This showed >99% similarity with sequences of A. errabunda (AJ888475 to 888477). For pathogenicity test, inoculum was prepared by harvesting conidia from 4-week-old cultures on potato dextrose agar. A conidial suspension (1 × 10⁶ conidia/ml) was sprayed onto young leaves of three potted seedlings. Three seedlings treated with sterile distilled water served as controls. Plants were covered with plastic bags to maintain 100% relative humidity for 24 h and then kept in a greenhouse (20 to 26°C and 60 to 80% RH). After 26 days, typical leaf spot symptoms, identical to the ones observed in the field, developed on the inoculated leaves. No symptoms were observed on controls. A. errabunda was reisolated from the lesions of inoculated plants, fulfilling Koch's postulates. Oak anthracnose associated with A. errabunda (including A. quercina) has been recorded in Europe and North America (1,4). Oak anthracnose of evergreen Quercus glauca Thunb. (syn. Cyclobalanopsis glauca (Thunb.) Oerst.) associated with A. supraseptata in Japan is not related to this disease (2). To our knowledge, this is the first report of oak anthracnose of Q. aliena globally and also the first finding of A. errabunda in Asia as well as in Korea. This pathogen is known as one of the major forest pathogens in oak stand in Europe and North America (3). Pending further studies, including a risk assessment, A. errabunda may be considered as a potentially new and serious threat in native and planted ranges of Q. aliena in Korea. References: (1) D. F. Farr and A. Y. Rossman. Fungal Databases. Syst. Mycol. Microbiol. Lab., Online publication, ARS, USDA, retrieved February 18, 2013. (2) S. Kaneko and T. Kobayashi. Trans. Mycol. Soc. Japan 25:11, 1984. (3) A. Ragazzi et al. Phytopathol. Mediterr. 46:295, 2007. (4) M. V. Sogonov et al. Mycol. Res. 111:693, 2007.