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First Report of Powdery Mildew Caused by Golovinomyces ambrosiae on Helianthus salicifolius in Korea

Zhao, T. T., Cho, S. E., Kim, J. Y., Choi, I. Y., Shin, H. D.
Plant disease 2018 v.102 no.7 pp. 1453
Ambrosia trifida, Golovinomyces, Helianthus annuus, Helianthus salicifolius, Helianthus tuberosus, Rudbeckia, ambient temperature, appendages, appressoria, asci, ascospores, autumn, calyx, conidia, conidiophores, discoloration, dusting, fungi, germ tube, greenhouses, herbaria, host plants, internal transcribed spacers, leaves, ornamental plants, powdery mildew, risk, stems, summer, Korean Peninsula
Helianthus salicifolius A. Dietr. (tribe Heliantheae, family Asteraceae), called willowleaf sunflower, is native to the central U.S.A. and was recently introduced into Korea for ornamental purposes. During summer and autumn of 2017, powdery mildew symptoms were observed on willowleaf sunflowers grown in Hwaseong, Korea (37°13′21″N; 127°02′34″E). Powdery mildew colonies were circular to irregular, forming white patches on the upper surface of the leaves, stems, and sepals. Infection usually resulted in premature senescence of leaves and discoloration of flowers. A representative specimen was deposited in the Korea University Herbarium with accession no. KUS-F30175. Hyphal appressoria were nipple-shaped or indistinct. Conidiophores were 130 to 254 × 10 to 12 μm, and produced conidia in chains with sinuate outline. Foot-cells of conidiophores were erect, cylindrical, and 90 to 136 μm long. Conidia were ellipsoid to barrel-shaped, 30 to 38 × 20 to 26 μm with a length/width ratio of 1.4 to 1.9, and devoid of distinct fibrosin bodies. Primary conidia were apically rounded and basally subtruncate. Germ tubes were produced at perihilar position of the conidia. Chasmothecia were subglobose to globose, dark brown, and 95 to 130 μm in diameter. Appendages were mycelioid, simple, brown to olivaceous brown, and 0.5 to 1.5 times the chasmothecial diameter. Asci were 2-spored, clavate-saccate, and 60 to 83 × 36 to 50 μm. Ascospores were ellipsoid to ovoid, 20 to 25 × 18 to 23 μm, and colorless. The morphological characteristics and host genus were in agreement with those of Golovinomyces ambrosiae (Schwein.) U. Braun & R.T.A. Cook (Braun and Cook 2012). To confirm the identification, the internal transcribed spacer (ITS) regions of KUS-F30175 were amplified and sequenced with the primers of ITS5/P3 (Takamatsu et al. 2009). The sequence was deposited in NCBI (accession no. MG678042). It showed over 99% similarity with G. ambrosiae on host plants of the tribe Heliantheae of Asteraceae, e.g., H. tuberosus (AB769420, AB767419), H. annuus (KM657962), Ambrosia trifida (JF907589), Rudbeckia paniculata (AB769422). Inoculation tests were conducted by gently dusting fresh conidia from infected leaves onto leaves of three healthy willowleaf sunflower plants. Three noninoculated plants were selected as controls. These plants were kept in a greenhouse at room temperature. White colonies developed on the inoculated plants after 7 days, while noninoculated plants were free of symptoms. The fungus on inoculated plants was morphologically in agreement with that observed originally on the diseased plants. Although Helianthus-Golovinomyces associations have been known globally, H. salicifolius infected with G. ambrosiae (formerly G. cichoracearum var. latisporus) has been reported in Germany and Switzerland (Farr and Rossman 2017). To the best of our knowledge, this is the first report of G. ambrosiae on H. salicifolius in Korea. Considering the increasing demand of this ornamental plant and the rapid expansion of this disease, the powdery mildew poses a potential risk to health of willowleaf sunflowers in Korea.