First Report of Lotus Powdery Mildew Caused by Erysiphe takamatsui in China

Abstract

Lotus (Nelumbo nucifera Gaertn.) is an aquatic perennial plant of the family Nelumbonaceae, distributed in subtropical and temperate regions, also extensively planted as an ornamental flower. Furthermore, the rhizome is used as a vegetable and the roots and seeds in herbal medicine in China. In September 2018, some lotus leaves were found to be infected with a powdery mildew in a pond in Qingdao, Shandong Province, China. In the surveyed pond, from late September to mid-November, the disease incidence varied from 10 to 70%, and some leaves were almost completely covered with powdery mildew and turned brown and scorched. In several lotus ponds not far away from the surveyed one, a few powdery mildew–affected leaves were also found. Small colonies of powdery mildew were dispersed on the upper sides of the leaves, and the adjacent colonies coalesced over several days. Some chasmothecia were formed on the lesions in the middle of November. The voucher specimens are deposited in the Qingdao Agricultural University Herbarium (HMQAUPM18001). Superficial hyphae were hyaline, branched, and septate, 4.5 to 6.0 μm wide, with thin and smooth walls. Hyphal appressoria were 4.7 to 6.7 μm diameter, nipple shaped to lobed, mostly single, rarely in opposite pairs. Conidiophores were straight, 74.3 to 98.5 × 7.8 to 9.8 μm, and composed of three to four cells; foot cells were cylindrical, usually straight, occasionally somewhat curved to sinuous, 38.2 to 57.5 × 6.9 to 8.5 μm. Conidia were formed singly, ellipsoid-ovoid, subcylindrical to almost cylindrical, 35.2 to 41.2 × 17.2 to 19.2 μm, and showed a wrinkling pattern of the outer walls. Germ tubes were produced in perihilar position. Chasmothecia were scattered to gregarious, 102.2 to 129.8 μm in diameter; peridium cells were irregularly polygonal, 13.0 to 19.1 μm in diameter; appendages arising from the lower half were mycelioid, unbranched or occasionally with short lateral branchlets, 0.5 to 4 times as long as the chasmothecial diameter, brown at the base and becoming paler toward the apex. Asci were ellipsoid to obovoid, short-stalked, 62.0 to 70.9 × 39.8 to 46.4 μm, wall 1.9 to 2.9 μm thick, two- to eight-spored. Ascospores were ellipsoidal, 23.5 to 27.6 × 13.8 to 15.6 μm, colorless. The morphological characteristics were consistent with previous records of Erysiphe takamatsui (Braun and Cook 2012; Meeboon and Takamatsu 2015). To confirm the identification based on morphological traits, a molecular examination of the isolate HMQAUPM18001 was confirmed by sequence analyses of the rDNA spanning ITS1, 5.8S gene, ITS2 and the 28S rRNA gene using the primers ITS1/ITS4 (White et al. 1990) and NL1/NLP2 (Hirose et al. 2005; Mori et al. 2000), respectively. ITS and 28S sequences of HMQAUPM18001 have been deposited in GenBank under the accession numbers MK478870 and MK583929, respectively. BLAST analysis of the ITS (641 bp) and 28S (720 bp) segments both showed 100% homology with the ITS sequence of E. takamatsui strain TNS: F-52102 (GenBank accession no. AB916688) and 28S sequence of E. takamatsui strain MUMH 5399 (GenBank accession no. AB916690), respectively. Pathogenicity was confirmed through inoculation tests by pressing a diseased leaf onto young leaves of three healthy lotus plants. Three noninoculated plants were used as controls. Inoculated plants developed symptoms after 8 days, whereas the control plants remained symptomless. The fungus present on the inoculated plants was morphologically identical to that originally observed on diseased plants, fulfilling Koch’s postulates. Therefore, the pathogen on lotus was confirmed to be E. takamatsui. To our knowledge, this is the first discovery of E. takamatsui, hitherto only known from Japan, on lotus in the Chinese mainland (Farr and Rossman 2019). This is the second discovery of the sexual stage of E. takamatsui in the world, which is important to note because it has substantial implications on pathogen survival and diversity. Because this lotus powdery mildew may cause losses of ornamental value, as well as the quality as foodstuff and for medicine purposes, control measures such as improving air and light conditions, removing heavily infected leaves, and preinfection applying of sulfur agents should be considered.