First report of powdery mildew caused by Golovinomyces artemisiae on Chrysanthemum zawadskii var. Latilobum in Korea

Chrysanthemum zawadskii var. latilobum (Maxim.) Kitag. [syn. Dendranthema zawadskii var. latilobum (Maxim.) Kitam.] is a perennial herb of the Asteraceae, and has been used in traditional medicine in Korea (Shim et al. 2012). Recently, horticultural varieties have been developed and widely planted in gardens as ornamentals. In August 2014, dozens of plants (cv. Hongyoung) exhibiting powdery mildew symptoms were found in a private garden in Hongcheon (37°41′00″N; 127°52′52″E), Korea. Powdery mildew colonies first appeared as thin white patches, which progressed to abundant hyphal growth on both sides of the leaves and stems. Severe infection caused leaf withering and premature senescence, reducing the aesthetic value of the plants. Three voucher specimens were deposited at the Korea University Herbarium (KUS-F28391, F28872, and F29010). Appressoria on the mycelium were indistinct to nipple-shaped. Conidiophores were cylindrical, measured 120 to 200 × 9 to 11 µm, and produced 2 to 5 chains of immature conidia with a sinuate outline. Foot-cells of conidiophores were straight, cylindrical, and 60 to 90 μm long. Conidia were hyaline, ellipsoid to barrel-shaped, measured 28 to 40 × 16 to 22 μm with a length/width ratio of 1.5 to 2.0, lacked distinct fibrosin bodies, and showed reticulate wrinkling of the outer walls. Germ tubes were produced on the perihilar position of the conidia. Primary conidia were apically rounded, basally subtruncate, and smaller than the secondary conidia. No chasmothecia were observed until the shoots and leaves senesced and died in November and December. The structures described above were typical of the powdery mildew of the genus Golovinomyces, and the fungus measurements were similar to those of G. artemisiae (Grev.) V.P. Heluta (Braun and Cook 2012). The complete internal transcribed spacer (ITS) regions from KUS-F28391 were amplified with primers ITS5/P3 and sequenced (Takamatsu et al. 2009). The resulting sequence of 506 bp was deposited in GenBank (accession no. KY072929). The obtained ITS sequence shared >99% similarity with G. artemisiae on Chrysanthemum spp. from Japan (AB077654 and AB769433). Pathogenicity was confirmed through inoculation by gently dusting conidia onto leaves of five healthy potted plants. Five noninoculated plants served as controls. Inoculated plants developed symptoms after 5 days, whereas the control plants remained symptomless. The fungus present on the inoculated plants was identical morphologically to that originally observed on diseased plants. Associations of Chrysanthemum and Golovinomyces have been reported globally (Takamatsu et al. 2013). Chrysanthemum zawadskii was recorded as host plant of G. cichoracearum sensu lato in Switzerland, but C. zawadskii var. latilobum has never been known to be affected by a powdery mildew (Farr and Rossman 2016). Our field observations suggested that powdery mildew infections pose a serious threat to health of this plant, especially in shady areas of the gardens.