Erigeron annuus (L.) Pers., known as annual fleabane or eastern daisy fleabane, is native to North America and was unintentionally introduced to Korea in the 1910s (Park, 1995). It is now widely naturalized throughout Korea and was designated as one of the ten major introduced plants in Korea by the Korea National Arboretum. In September 2012, several dozen annual fleabanes were found to be heavily infected with powdery mildew. Symptoms first appeared as circular to irregular white patches, which subsequently showed abundant hyphal growth on both sides of the leaves. The same symptoms have continuously been found on annual fleabane throughout the country, where the disease incidence was often higher than 80%. Five voucher specimens were deposited in the Korea University Herbarium (KUS-F30208, 31414, 31774, 31784 and 32003). Hyphae were septate, branched, and 4.5 to 6.7 µm wide. Appressoria on the mycelium were lobed. Conidiophores (n = 30), measured 154 to 215 × 9 to 12.5 μm, were simple and produced 2 to 4 immature conidia in chains with a sinuate outline, followed by 2 to 3 cells. Foot-cells of conidiophores were straight, cylindrical, and 40 to 98 µm long. Conidia (n = 30) were hyaline, ellipsoid to barrel-shaped, measured 25.3 to 35.8 × 13 to 17 μm (length/width ratio = 1.62 to 2.31), lacked distinct fibrosin bodies, and showed reticulate wrinkling of the outer walls. Primary conidia were apically rounded and basally truncated and generally smaller than the secondary conidia. Germ tubes were produced on the subterminal position of conidia. No chasmothecia were observed. The structures described above were typical of the Euoidium anamorph of the genus Golovinomyces, and the fungus measurements were compatible with those of G. ambrosiae (Schwein.) U. Braun & R.T.A. Cook (Qiu et al., 2020). To confirm the identity of the causal fungus, the internal transcribed spacer (ITS) and large subunit (LSU) regions of rDNA from the five herbarium specimens were amplified with primers PM10/ITS4 for ITS and PM3/TW14 for LSU (Bradshaw and Tobin, 2020; Mori et al., 2000; White et al., 1990) and sequenced directly. The resulting sequence was deposited in GenBank (Accession No. OP788040-4 for ITS and OP788045-9 for LSU). Comparison with the sequences available in the GenBank database revealed that the isolates showed 100% sequence similarity with those of G. ambrosiae from the family Asteraceae (e.g., MT355557, MF612182, etc.). Pathogenicity was confirmed through inoculation by gently pressing diseased leaves onto the leaves of five healthy potted plants. Five non-inoculated plants served as controls. Plants were maintained in a greenhouse at 22 to 28°C. Inoculated plants developed signs and symptoms after seven days, whereas the control plants remained healthy. The fungus present on the inoculated plants was morphologically identical to that observed initially on diseased plants, fulfilling Koch's postulates. Powdery mildew infections of Erigeron spp. associated with Golovinomyces species have been known in the United States, France, and China (Farr and Rossman, 2022). To our knowledge, this is the first report of powdery mildew disease caused by G. ambrosiae on E. annuus outside of North America as well as in Korea. According to our field observation, powdery mildew infections were found only on annual fleabanes growing in shady areas, not in sunny places.
Keywords: annual fleabane; eastern daisy fleabane; introduced plant.