First report of powdery mildew caused by Erysiphe lonicerae var. Lonicerae on lonicera japonica in Korea

Japanese honeysuckle (Lonicera japonica Thunb., family Caprifoliaceae) is native to China, Japan, and Korea, and has become naturalized and is now invasive in Australia, New Zealand, Argentina, Brazil, Mexico, and much of the United States (Schierenbeck 2004). In Korea, it is widely planted as an effective groundcover and also as a fragrant ornamental. In October 2000, several vines exhibiting signs and symptoms of powdery mildew were found in Seoul, Korea. White superficial mycelia and conidia were present on both sides of the leaves as well as on young shoots, forming thin, irregular colonies. Infections often caused red-purplish discolorations and leaf senescence. Similar symptoms have also been found in Pyeongchang, Samcheok, Suwon, Gapyeong, and Busan from 2000 to 2015. Fourteen voucher specimens were deposited in the Korea University Herbarium (KUS). Conidiophores were unbranched, cylindrical, 50 to 130 µm, and composed of 3 to 4 cells. Foot-cells in conidiophores were slightly to moderately flexuous. Conidia produced singly were ellipsoid or doliiform, 27 to 40 × 15 to 21 µm, and devoid of distinct fibrosin bodies. Germ tubes were formed on the perihilar position of the conidium. The surface of the conidia had an angular/rectangular wrinkling pattern. Chasmothecia were dark brown, spherical, and scattered subgregariously, 100 to 140 µm in diameter. Each chasmothecium contained 4 to 6 asci. Appendages were 3 to 5 times dichotomously branched, aseptate or 1-septate, 11 to 17 per chasmothecium, 1.0 to 1.7 times as long as the chasmothecial diameter, and brown at the base, becoming paler toward the tip. Asci were short stalked, 4 to 6 spored, and 55 to 67 × 37 to 47 µm. Ascospores were ellipsoid-ovoid and 15 to 20 × 8 to 12 µm. The features were compatible with those of Erysiphe lonicerae var. lonicerae DC. (Braun and Cook 2012). The internal transcribed spacer regions of rDNA from KUS-F27344 and F28645 were amplified using primers ITS1/ITS4, and sequenced (Takamatsu et al. 2009). Phylogenetic analysis (MEGA 6) using the neighbor-joining method showed that two Korean isolates (GenBank Accession Nos. KT748729 and KT748730) grouped with E. lonicerae var. lonicerae on Lonicera spp. (GenBank Accession Nos. LC010020, LC009991, and LC009963). Pathogenicity was confirmed by gently pressing an infected leaf onto 10 leaves of a healthy, potted vine. Ten noninoculated leaves of similar features served as controls. All inoculated leaves developed symptoms after 8 to 10 days, whereas the control leaves remained symptomless. The powdery mildew infections associated with E. lonicerae var. lonicerae on Japanese honeysuckle have been recorded in Japan, China, and Switzerland (Farr and Rossman 2015). To our knowledge, this is the first report of powdery mildew caused by E. lonicerae var. lonicerae on Japanese honeysuckle in Korea. Our field observations suggest that the disease would be a serious threat to the widespread ornamental plantings of Japanese honeysuckle in Korea, especially in the case where the vines are growing in the shade.