Actinidia arguta (Siebold & Zucc.) Planch. ex Miq., known as hardy kiwi, is a perennial vine native to Japan, Korea, northern China, and Russian Siberia. It produces a small fruit resembling the kiwifruit. The fruits, referred to as hardy kiwifruit, are edible and often sweeter than kiwifruit. Picking of wild hardy kiwifruits in autumn is a popular ecotourism activity in Korea. In 2000, a powdery mildew disease on the abaxial surface of hardy kiwi leaves was found in Korea. Additional findings of the powdery mildew between 2000 and 2013 showed that the disease of hardy kiwi commonly occurs in Korea. The infected leaves were frequently distorted when young, then becoming prematurely chlorotic and defoliated. Chasmothecia were abundantly formed by September. Voucher specimens (n = 21) were deposited in the Korea University Herbarium (KUS). Mycelia were hypophyllous, thinly effuse, initially forming patches, finally covering the whole abaxial leaf surface. Hyphal appressoria were hook-shaped or often branched, and single or opposite in pairs. Conidiophores were erect, cylindrical, 160 to 300 × 5 to 7.5 μm with straight foot-cells (65 to 115 μm long), basal septum elevated up to 15 μm, and produced conidia singly. Conidia were obpyriform to clavate, papillate at the apex, 55 to 75 × 20 to 32 μm, and devoid of fibrosin bodies. Germ tubes were produced at the terminal and lateral positions of conidia. Chasmothecia were blackish brown, depressed globose, and 190 to 250 μm in diameter. Appendages arising around the equatorial zone of chasmothecia were 9 to 15 in number, acicular with a bulbose base, 1 to 1.3 times as long as the chasmothecial diameter, hyaline throughout, and aseptate. Penicillate cells crowded on the upper part of the chasmothecia were ampulliform, numerous, 50 to 87.8 μm long, and 12.5 to 17.5 μm wide. Asci were 13 to 20 in a chasmothecium, olivaceous brown, 60 to 90 × 22.5 to 40 μm, and short stalked. Ascospores were 2 in an ascus, ellipsoid-ovoid, pale olivaceous, and 32.5 to 40 × 16.5 to 20 μm. On the basis of the morphological characteristics, this fungus was identified as Phyllactinia actinidiae (Jacz.) Bunkina (1). To confirm the identification, the complete internal transcribed spacer (ITS) regions of rDNA of three specimens (KUS-F23673, F26240, and F26308) were amplified using primers ITS5/P3 and sequenced (4). The resulting sequences were deposited in GenBank (Accession Nos. KJ703014, KJ703015, and KJ703016). GenBank BLAST search with the three isolates showed >99% similarity with the results for P. actinidiae on hardy kiwi from Japan (AB080489, AB080500, and AB080508). Actinidia-Phyllactinia associations were recorded in China, Japan, Taiwan, Korea, Russia, and Turkey (2,3). However, P. actinidiae on A. arguta was known only from Japan (3). To our knowledge, this is the first record of P. actinidiae on hardy kiwi in Korea. There has been no finding of powdery mildews on commercial varieties of golden kiwi (A. chinensis) and fuzzy kiwi (A. deliciosa) in Korea. Common occurrence of Phyllactinia powdery mildew on hardy kiwi in Korea can be a potential threat to the commercial kiwi industry.
References: (1) U. Braun and R. T. A. Cook. Taxonomic Manual of the Erysiphales (Powdery Mildews), CBS Biodiversity Series No.11. CBS, Utrecht, 2012. (2) I. Erper et al. Scand. J. Forest Res. 27:432, 2012. (3) D. F. Farr and A. Y. Rossman. Fungal Databases. Syst. Mycol. Microbiol. Lab., Online publication. ARS, USDA, March 10, 2014. (4) S. Takamatsu et al. Mycol. Res. 113:117, 2009.
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