Powdery Mildew of <i>Leonurus japonicus</i> Caused by <i>Neoerysiphe galeopsidis</i> in China

Leonurus japonicus is an annual or perennial herbaceous plant belonging to the Lamiaceae. It is renowned for its medicinal properties, including promoting blood circulation, regulating menstruation, acting as a diuretic to reduce swelling, and clearing heat and detoxifying the body. In December 2024, powdery mildew-like signs and symptoms were observed on leaves of L. japonicus plants (n=50) cultivated in a park (150 m2) in Xinxiang city, Henan Province, China (35.21° N, 113.56° E). A specimen (NG-ZM20241202) was stored in the Xinxiang Key Laboratory of Plant Stress Biology. The infected leaves were covered with white and thin masses. More than 80% of plants (n=50) exhibited these signs and symptoms. White colonies were on both leaf surfaces of the leaves, covering about 80% of the leaf area. Foot cells of the fungus (n = 40) were straight and 35 to 60 μm long. Cylindrical conidiophores were 65 - 120 × 6 - 18 μm. The ellipsoid-ovoid-shaped conidia were 20 - 40 × 16 - 22 μm with a length/width ratio of 1.2 to 2.2. These morphological characteristics were similar to Neoerysiphe galeopsidis (Braun and Cook 2012). Following a previously reported method (Zhu et al. 2022a), the internal transcribed spacer (ITS) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) regions of a selected specimen of the fungus were amplified and sequenced with primers ITS1/ITS4 (White et al. 1990) and GAPDH1/GAPDH3R (Bradshaw et al. 2022). The resulting sequences were deposited in GenBank (Accession No. PV460745 and PV423823). BLASTn analysis indicated that the ITS and GAPDH sequences were respectively 100% (592/592 bp) and 98.84% (255/258 bp) identical to N. galeopsidis (MK452604 and ON075656). In the phylogenetic tree, the isolated N. galeopsidis PV460745 clustered in the same subclade with the known N. galeopsidis isolates (ON073888, MK452604). Therefore, the morphology and molecular analysis indicated that the pathogen was N. galeopsidis. To confirm the fungal pathogenicity, inoculation assays were conducted. According to the method previously described (Zhu et al. 2022b), the leaves of three healthy L. japonicus plants (10 cm high) were inoculated with fungal conidia by gently pressing the surfaces of infected leaves onto the leaves of healthy plants. Three untreated plants served as controls. Both the control and inoculated plants were separately placed in greenhouses (humidity, 60%; light/dark, 16 h/8 h; temperature, 18°C). At 8-10 days post-inoculation, distinct powdery mildew signs developed on leaves of inoculated plants, whereas control plants showed no significant changes compared to pre-inoculation status. The pathogenic fungus was re-isolated and showed same morphological characteristics with the original specimen (NG-ZM20241202). The experiments were repeated three times and the same results were obtained. While this powdery mildew fungus was reported on Lagopsis supina (Zhang et al. 2024) and on Lamium album subsp. barbatum (Choi et al. 2023). To the best of our knowledge, this is the first report of N. galeopsidis on L. japonicus in China. The outbreak of this powdery mildew severely compromises plant health, thereby diminishing both the ornamental and medicinal value of L. japonicus. Therefore, the identification and confirmation of N. galeopsidis infecting L. japonicus not only expands the known host range of this pathogen but also establishes a critical foundation for future disease control strategies.