First Report of Powdery Mildew Caused by Podosphaera macularis on Humulus scandens in China

Abstract

Humulus scandens, (Lour.) Merr., is a climbing herb which are used as traditional Chinese medicine, a raw material for papermaking, making soap, and replacing hops H. lupulus. This herb is distributed in many provinces of China, including Sichuan province. During March and June 2022, powdery mildew was found on leaves of H. scandens in the modern agricultural high-tech demonstration garden of Sichuan Academy of Agricultural Science, Chengdu, Sichuan Province, China. Abundant white or grayish powdery colonies could be seen on the surface of leaves, and 30%-100% of leaf areas were affected. Some of the infected leaves were either chlorotic or senescent. About 90% of the observed plants showed powdery mildew symptoms. Conidiophores (n = 25) were 74.0 to 160.1 μm × 8.7 to 12.7 μm (on average 120.9 × 10.4 μm) and composed of cylindrical foot cells (length 31.9-72.9 μm, average 50.1 µm) and conidia (mostly 10 conidia) in chains. Barrel-shaped conidia with fibrosin bodies (n = 30) were 12.8 to 21.0 μm × 7.9 to 15 μm, on average 16.7 × 11.3 μm, with a length/width ratio of 1.5. Chasmothecia were not found. Based on these morphologic characteristics, the pathogen was initially identified as Podosphaera macularis (Braun and Cook 2012; Mahaffee et al. 2009). To confirm the identification, two isolates (PDLC0315 and PDLC0412) of P. macularis mycelia and conidia were collected, and mycelia and conidia were combined for a single DNA extraction from each isolate. With the total genomic DNA, the sequences of the internal transcribed spacer (ITS), 5.8S rRNA, the 18S and 28S large subunit ribosomal DNA (LSU) (Bradshaw and Tobin 2020; White et al. 1990), were bi-directionally sequenced and deposited in GenBank (ON862625.1 and ON862630.1). The ON862625.1 and ON862630.1 showed 100% similarity with sequences of P. macularis isolate CT1 (MH687414.1). Phylogenetic analyses based on the combined ITS and 28S rDNA sequences indicated that the two specimens, PDLC0315 and PDLC0412 formed a monophyletic clade together with sequences retrieved from Podosphaera macularis CT1 and Head quarter 31 (KX842348.1). The pathogenicity test with the fungus was confirmed by gently pressing the infected leaves onto three healthy wild plants from the same geographical location. Three uninoculated wild plants served as controls. Six inoculated and non-inoculated plants were placed in different growth chambers with a 16-h photoperiod at 22±2°C and 70% of relative humidity. After 10 to 14 days, powdery mildew colonies developed on inoculated plants. Non-inoculated control plants did not show any symptoms. The fungus on inoculated leaves was morphologically identical to that first observed in the garden. As far as we know, this study is the first report of powdery mildew disease in Humulus scandens caused by Podosphaera macularis in China. Rapid expansion and wild distribution of H. scandens could lead to increased powdery mildew risk in outdoor cultivation. Due to the invasive potential of the powdery mildew fungi, this record is important in the context of the range extension of Podosphaera macularis.