First Report of Powdery Mildew Caused by Golovinomyces asterum var. solidaginis on Invasive Weed Solidago gigantea in Korea
Authors: Cho S. E. , Park J. H. , Hong S. H. , Shin H. D. 
Published: March 2013

Solidago gigantea Aiton (syn. S. serotina Aiton), known as giant goldenrod, is native to North America and has been invasive in Europe and Asia. In Korea, this plant was accidentally introduced around the 1960s and has become widely naturalized by replacing indigenous plants and disrupting the native ecosystem (3). In October 2012, hundreds of giant goldenrod plants growing wild in riverine areas and roadsides were found affected by a powdery mildew in Busan, Korea. Voucher specimens were deposited in the Korea University Herbarium (KUS). Symptoms first appeared as circular to irregular white patches, which subsequently showed abundant hyphal growth on both sides of the leaves. Appressoria on the mycelium were nipple-shaped to moderately lobed. Conidiophores measured 120 to 240 × 10 to 12.5 μm, were arising laterally from hyphal mother cells, and produced 2 to 6 immature conidia in chains with a sinuate outline, followed by 2 to 3 cells. Foot-cells in conidiophores were 42 to 70 μm long and characterized by a distinctly curved base. Conidia were hyaline, ellipsoid to ovate, measured 28 to 42 × 17 to 24 μm (length/width ratio = 1.4 to 2.1), contained small oil drops, lacked distinct fibrosin bodies, and produced germ tubes on the subterminal position. No chasmothecia were observed. The morphological characteristics described above were typical of the Euoidium type anamorph of the genus Golovinomyces, and the fungus measurements and structures were consistent with those of G. asterum var. solidaginis U. Braun (1). To confirm the identity of the causal fungus, the complete ITS region of rDNA from isolate KUS-F27219 was amplified with primers ITS5 and P3 (4) and sequenced. The resulting 508-bp sequence was deposited in GenBank (Accession No. KC513763). A GenBank BLAST search of this sequence revealed >99% similarity with the ITS sequences of G. cichoracearum from Australia (GQ183940 ex Solidago sp.) and Japan (AB077625 ex S. altissima L. and AB077627 ex S. virgaurea subsp. asiatica Kitam. ex Hara). The G. cichoracearum isolates on Solidago spp. listed above are now placed in G. asterum var. solidaginis (1). Pathogenicity was confirmed through inoculation by gently pressing diseased leaves onto leaves of five healthy potted giant goldenrods. Five non-inoculated plants served as controls. Plants were maintained in a greenhouse at 24 to 30°C. Inoculated plants developed signs and symptoms after 7 days, whereas the control plants remained symptomless. The fungus present on inoculated plants was morphologically identical to that originally observed, fulfilling Koch's postulates. Association of S. gigantea with Golovinomyces powdery mildews has been known in North America, South America, Europe, New Zealand, Central Asia (Iran, Kazakhstan, and Kyrgyzstan), and Japan (2). To our knowledge, this is the first report of powdery mildew caused by G. asterum var. solidaginis on S. gigantea in Korea. Our field observations suggest that the powdery mildew could be a limiting factor to suppress the expansion of this invasive weed in Korea. 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) D. F. Farr and A. Y. Rossman. Fungal Databases. Syst. Mycol. Microbiol. Lab., Online publication, ARS, USDA, retrieved January 22, 2013. (3) S. M. Oh et al. Kor. J. Weed Sci. 22:280, 2002. (4) S. Takamatsu et al. Mycol. Res. 113:117, 2009.