First Report of Powdery Mildew Caused by Cystotheca wrightii on Quercus glauca in Korea.

Abstract

Quercus glauca Thunb. (syn. Cyclobalanopsis glauca (Thunb.) Oerst.), known as ring-cupped oak or Japanese blue oak, is a dominant tree species commonly found in evergreen forests in East Asia (2). In May 2012, hundreds of Q. glauca were found heavily affected by a powdery mildew in several locations of Jeju Islands, Korea. Symptoms on overwintered leaves appeared as circular to irregular blackish violet to dark brown felt-like growths with numerous chasmothecia on abaxial leaf surfaces. New infections on current-year leaves started in early summer and were characterized by typical white patches with abundant sporulation on abaxial leaf surfaces. In early autumn, with formation of special aerial hyphae and without further sporulation, the patches turned light brown to brown. Formation of chasmothecia was noticed from late autumn. The adaxial leaf surface was free of powdery mildew growths and exhibited yellowing and discoloration. Voucher specimens were deposited in the Korea University Herbarium (KUS). Special aerial hyphae were falcate to curved, aseptate, at first hyaline, later deep brown to purplish brown, thick-walled, and 80 to 140 × 6 to 15 μm. Conidiophores were 175 to 245 × 10 to 12 μm, and produced 2 to 4 immature conidia in chains with a sinuate outline. Foot-cells of conidiophores were cylindrical and 80 to 120 μm long. Conidia were lemon- to barrel-shaped, 26 to 35 × 17 to 24 μm with a length/width ratio of 1.2 to 1.6, and devoid of fibrosin bodies. Primary conidia were apically conical and basally subtruncate. Germ tubes produced in the lateral to perihilar position of conidia were long and slender (3 μm wide). Chasmothecia were scattered or gregarious, partly embedded with special aerial hyphae, dark brown, spherical, 55 to 70 μm in diameter, and contained a single ascus. Chasmothecial peridia consisted of two layers. Exoperidia (outer layer) were composed of dark brown, polygonal cells 10 to 20 μm wide. Endoperidia (inner layer) consisted of hyaline, polygonal cells 10 to 15 μm wide. Appendages were basally attached, mycelioid, rare or few, and pale brown to rusty brown. Asci were short stalked, 55 to 72 × 35 to 46 μm, had a terminal oculus 10 to 20 μm wide, and contained 8 ascospores. Ascospores were oblong-elliptical, 22 to 27 × 10 to 12.5 μm, subhyaline, and contained 1 or 2 oil drops. The specific measurements and characteristics (especially falcate aerial hyphae) were consistent with those of Cystotheca wrightii Berk. & M.A. Curtis (1). Fungal DNA was extracted by the Chelex method. The internal transcribed spacer (ITS) region of rDNA of KUS-F27309 was amplified with primers ITS5/P3 and sequenced directly (4). The resulting 589-bp sequence was deposited in GenBank (Accession No. KF735066). A BLAST search in GenBank showed that the Korean isolate had 100% homology with C. wrightii on Q. glauca from Japan (AB000932). Powdery mildews of Q. glauca associated with C. wrightii have been known in Japan, Taiwan, and China (1,3), but not in Korea. Finding of C. wrightii on Q. glauca could pose a potential threat to other evergreen oak species in southern part of Korea.