First Report of Causing Powdery Mildew on in China.

Astragalus mongholicus Bge. [A. membranaceus Bge. var. mongholicus (Bge.) Hsiao] is a highly valuable perennial medicinal plant mainly distributed in China, whose dry roots are known as Huangqi in traditional Chinese medicine for reinforcing vital energy, strengthening superficial resistance, and promoting tissue regeneration (Lin et al. 2000). A. mongholicus roots of high quality are produced in Northwest and North China. Since July 2021, powdery mildew outbreaks happened annually on the leaves of A. mongholicus in a plantation (123° 56' 40'' E, 47° 22' 20'' N) in Qiqihar city, Heilongjiang Province, China. Disease incidence reached 100% by October (Fig. 1A-C), causing severe impairment of growth. Powdery mildew spots of circular or irregular shapes emerged on upper surface of leaf, resulting in plentiful lesion specks. Dense white hyphae appeared chaotically intertwined. Hyphae were hyaline and highly flexuous, 5.3 - 10.7 μm in diameter (n = 20). Chasmothecia were globose or slightly ovoid-shaped and turned dark brown when matured. Chasmothecia (diameter: 135.2 - 222.9 μm, n = 20) existed abundantly on the diseased leaves in the fields. Conidiophores were 89.0 - 129.9 μm in length (n = 20) and composed of one cylindrical, straight foot cell, followed by two cells and one to three conidia. Conidia were slim ellipsoid-shaped, occasionally ovoid-shaped, measuring 14.6 - 24.7 μm by 6.4 to10.4 μm, length/width ratio was 1.8 - 3.0 (n = 30). Hyphal appressoria were nipple-shaped and appeared in singular, occasionally in pairs. Unbranched germ tube emerged reaching out of the germinating conidia while forming an acute angle with the long axis. Comprehensively, the pathogen exhibited micro-morphology of the genus Erysiphe. For molecular identification, pathogen was carefully scraped off diseased leaves for DNA extraction. We used the DNA samples of three biological replicates for the sequencing of the ITS rDNA fragment (primers by (White et al. 1990). All the samples resulted in an identical ITS sequence (deposited in GenBank as OQ390098.1). It displayed 99.83% identity with OP806835.1 of an E. astragali voucher collected in Iran (Fig. 1D-M, O). Hence, our pathogen was identified as an E. astragali stain. Additionally, we amplified the Mcm7 sequence (using primers by (Ellingham et al. 2019), deposited as OQ397582.1). We propagated 40-day-old A. mongholicus plants via germinating seeds in pot soil and performed pathogenicity tests. Firstly, we incubated detached healthy leaves of propagated plants with severely symptomatic leaves collected from the fields in petri dishes under saturated moisture content and room temperature. Powdery mildew symptoms emerged on each healthy leaf (n = 5) after two weeks. Further, we infected healthy plants (n = 5) by gently pressing and rubbing symptomatic leaves on each healthy leaf, and kept them in a greenhouse (24 ℃, 80% humidity, 16/8-hour light/dark cycle). After a month, symptoms emerged on a number of leaves of each infected plant. We performed micromorphology observation (Fig. 1N-P) and ITS sequencing to confirm that the results fulfilled Koch's postulates. Powdery mildew caused by E. astragali on A. strictus in Tibet (Wang and Jiang 2023) and on A. scaberrimus in Inner Mongolia (Sun et al. 2023) have been reported. Here we report powdery mildew caused by E. astragali on Astragalus mongholicus for the first time. These Astragalus spp. are all acknowledged to have medicinal values in China but their usages are quite different.