Occurrence of Powdery Mildew Caused by Erysiphe astragali on Astragalus scaberrimus in Inner Mongolia, China

Astragalus scaberrimus Bunge, a perennial herb, is widely distributed in North and central China, Russia, and Mongolia (POWO, 2023). Due to its tolerance to drought, cold, high salt, low nutrients and alkaline soil, this plant is widely cultivated in China as a forage crop, for water and soil conservation, and for its medicinal properties (Meng, 2015). In 2022, powdery mildew-like signs and symptoms were seen on leaves of A. scaberrimuns cultivated on the campus of Inner Mongolia Agricultural University, Hohhot, Inner Mongolia China. White powder-like masses covered up to 99% of the leaf area with infected plants showing weak growth and senescence. More than 70% of plants (n = 180) exhibited these powdery mildew-infected symptoms. Conidiophores were 70-120 μm long (n = 20) and composed of a basal foot cell, followed by two cells and a conidium. Cylindrical- or ovoid-shaped conidia were 30-45µm long by 9-15 µm wide (n = 20). Brown or light-brown chasmothecia were 100-140 μm in diameter, with flexuous appendages. Based on these morphological characteristics, the fungus was tentatively identified as an Erysiphe sp. (Braun and Cook, 2012; Schmidt and Braun, 2020). Fungal structures were isolated from diseased leaves and genomic DNA of the pathogen was extracted utilizing the method described by Zhu et al. (2022). The internal transcribed spacer (ITS) region was amplified by PCR employing the primers PMITS1/PMITS2 (Cunnington et al., 2003) and the amplicon sequenced by Invitrogen (Shanghai, China). The sequence for the powdery mildew fungus (GenBank accession no.: MW142495) showed 100% identity (645/645 bp) with Erysiphe astragali, which was reported on A. glycyphyllos in Golestan province, Iran (accession no. OP806834). Pathogenicity tests were conducted by brushing the conidia from infected A. scaberrimus leaves onto leaves of four healthy plants, while, the four control plants were brushed in the same manner. All the treated plants were placed in separate growth chambers maintained at 19℃, 65% humidity, with 16 h light/8 h dark photoperiod. Nine days post inoculation, powdery mildew disease signs appeared on inoculated plants, whereas control plants remained asymptomatic. The same results were obtained for two repeated pathogenicity experiments. The powdery mildew fungus was reisolated and identified as E. astragali based on morphological and molecular analysis, thereby fulfilling Koch's postulates. E. astragali causing powdery mildew on A. glycyphyllus were previously reported in Germany with Genbank accesion number of MZ265150 and MZ265151 (Bradshaw et al., 2022). This, to our knowledge, is the first report of powdery mildew caused by E. astragali on A. scaberrimus. The severe occurrence of this destructive powdery mildew disease on A. scaberrimus may adversely affect the utility of the plant for soil conservation or cultivation for medicinal purposes. Identifying the causal agent of powdery mildew will support efforts for the future control and management of diseases on A. scaberrimus.