First Report of Powdery Mildew Caused by Podosphaera xanthii on Sigesbeckia orientalis in China

Abstract

Sigesbeckia orientalis L., (St Paul’s wort) is an annually grown natural herb of Asteraceae with a long therapeutic history for a wide range of inflammation-related diseases in China (Zhong et al. 2019). In June 2020, typical symptoms of powdery mildew were observed on 30% of wild S. orientalis plants grown along the roadsides and gardens in Minjiang University, Fuzhou, China. Circular to irregular white powdery fungal colonies were observed on both surfaces of the leaves and young stems, causing necrosis and premature senescence. Fungal hyphae were epigenous, flexuous to straight, branched, and septate. Appressoria on the hyphae were nipple-shaped or nearly absent. Conidiophores were straight, 30 to 210× 8 to 12 μm, and produced 3 to 7 immature conidia in chains with a crenate outline. Foot-cells were cylindrical, 45 to 75 ×10 to 12 μm, followed by 1 to 2 shorter cells. Conidia were hyaline, ellipsoid-ovoid to barrel-shaped, 25 to 38 × 18 to 23 μm with distinct fibrosin bodies. Germ tubes were produced from a lateral position on the conidia. Chasmothecia were not observed on the infected leaves. Based on anamorph characteristics, fungus was identified as Podosphaera xanthii (Castagne) U. Braun & N. Shishkoff (Braun and Cook 2012). For molecular identification, total genomic DNA was extracted (Mukhtar et al. 2018) from fungal colonies on infected leaves of five collections separately. For each DNA sample, the part of LSU and ITS regions were amplified using primers LSU1/LSU2 and ITS1/ITS4 (Scholin et al. 1994; White et al. 1990), respectively. A BLAST search revealed 100 % sequences similarity with P. xanthii sequences reported on Ageratum conyzoides (KY274485), Eclipta prostrata (MT260063), Euphorbia hirta (KY388505), Sonchus asper (MN134013), and Verbena bonariensis (AB462804). Representative sequences (ITS: MZ613309; LSU: MZ614707) of an isolate were deposited in GenBank. The phylogenetic analysis also grouped the obtain sequences into P. xanthii clade. Pathogenicity was confirmed by gently pressing the infected leaves onto young leaves of five healthy one-month-old S. orientalis plants, while three non-inoculated plants were used as controls. All plants were maintained in a greenhouse at 25 ± 2°C. After, seven days, white powdery colonies were observed on inoculated plants, whereas controls remained mildew-free. On inoculated leaves, the fungus was morphologically and molecularly identical to the fungus on the original specimens. P. xanthii has been reported as a significant damaging pathogen on a wide range of plants in China (Farr and Rossman 2021). To our knowledge, this is the first report of powdery mildew caused by P. xanthii on S. orientalis in China as well as worldwide. S. orientalis is one of the most important commercial Chinese medicinal herbs and the occurrence of powdery mildew is a threat to its production, quality, and marketability. References: Braun, U., and Cook, R. T. A. 2012. The Taxonomic Manual of the Erysiphales (Powdery Mildews). CBS Biodiversity Series 11: CBS. Utrecht, The Netherlands. Farr, D. F., and Rossman, A. Y. 2021. Fungal Databases. Syst. Mycol. Microbiol. Lab., USDA ARS, 9 October 2021. Mukhtar, I., et al. 2018. Sydowia.70:155. Scholin, C. A., et al. 1994. J. Phycol. 30:999. White, T. J., et al. 1990. Page 315 in: PCR Protocols: A Guide to Methods and Applications. Academic Press, San Diego, CA. Zhong, Z., et al., 2019. Chin. Med. (U. K.) 14, 1–12. 10.1186/s13020-019-0260-y