First report of powdery mildew caused by Podosphaera spiraeae on Japanese spiraea in Korea

Japanese spiraea (Spiraea japonica L.f., Rosaceae) is native to Japan, China, and Korea. It is naturalized throughout North America and Europe and has the potential to become an invasive weed (Wilson and Hoch 2009). Nevertheless, many varieties are widely planted for their ornamental value worldwide. In May 2014, several hundred plantings (cv. Hwangkeum) were found damaged by a powdery mildew with 100% disease incidence in a public garden (37°46′17.0″ N; 128°52′12.4″ E) of Gangneung City, Korea. White colonies were present on young stems, both sides of the leaves, and inflorescence, detracting from their beauty in landscape plantings. Infected leaves later showed partial distortion and diffuse red-purple discoloration. Voucher specimens (n = 3) were deposited in the Korea University Herbarium (KUS). Hyphae were flexuous to straight, branched, septate, 5 to 7 μm wide, and had nipple-shaped appressoria. Conidiophores were cylindrical, 95 to 188 × 8 to 10 μm, and produced 2 to 4 immature conidia in chains with a crenate outline. Foot-cells of conidiophores were straight, 48 to 88 μm long, sometimes constricted at the base, and with basal septum elevated up to 8 μm. Conidia were ellipsoid-ovoid to doliiform, 25 to 35 × 12 to 16 μm with a length/width ratio of 1.5 to 2.3, and had distinct fibrosin bodies. Primary conidia were apically rounded and basally subtruncate. Germ tubes were produced on the perihilar position of conidia. Chasmothecia were not found. These structures were typical of the powdery mildew Euoidium anamorph of the genus Podosphaera. The structure and measurements were consistent with the anamorphic state of P. spiraeae (Sawada) U. Braun & S. Takam. (Braun and Cook 2012). To confirm the identity of the fungus, the complete internal transcribed spacer (ITS) region of ribosomal DNA from KUS-F28608 was amplified and sequenced using universal primer pair ITS1 and ITS4. The resulting sequence of 670 bp was deposited in GenBank (Accession No. KT290556). A GenBank BLAST search of ITS sequence showed >99% similarity with that of P. spiraeae on S. japonica from China (KF500426) and on S. cantoniensis from Japan (AB525940). A pathogenicity test was confirmed through inoculation by gently pressing a diseased leaf onto five healthy leaves of potted 2-year-old seedlings. Five noninoculated leaves served as controls. Inoculated leaves developed signs and symptoms after 5 days, but the noninoculated leaves remained symptomless. The fungus presented on the inoculated plants was morphologically identical to that originally observed on diseased plants. Powdery mildew infections of Japanese spiraea associated with P. spiraeae have been recorded in Japan, China, Poland, and Switzerland (Xing et al. 2014; Farr and Rossman 2015). In Korea, P. spiraeae was recorded on S. prunifolia var. simpliciflora (Farr and Rossman 2015). To our knowledge, this is the first report of powdery mildew caused by P. spiraeae on Japanese spiraea in Korea. Our field observations suggested that powdery mildew infections pose a serious threat to health of this plant, especially in shady areas of the gardens.