First Report of Powdery Mildew caused by Erysiphe astragali on Sphaerophysa salsula

Sphaerophysa salsula (Pall.) DC., also known as Yang Liao Pao, belongs to the Leguminosae family and is the only existing species in the Sphaerophysa genus. S. salsula is tolerance to cold, high salt, and alkaline soil, it is widely cultivated in China as a forage crop, and used as a Chinese folk medicine to treat hypertension (Ma et al., 2002). In 2023, signs and symptoms similar to powdery mildew were found on S. salsula planted in Tumd left (40.515°N, 110.424°E), Baotou City, Inner Mongolia Autonomous Region, China. The white powdery substance covered 90% of the leaf area, and the infected plants showed weak growth and senescence. More than 80% of plants (n=200) had these powdery mildew-like symptoms. Hyphal appressoria are solitary, conidiophores have few branches and septa. Conidia are cylindrical to clavate, 25-32 μm long and 8-15 μm wide (n=30), conidia form single subapical germ tubes, straight to curved-sinuous, with swollen apex or distinctly lobed conidial appressorium. Based on these morphological characteristics, the fungus was tentatively identified as an Erysiphe sp. (Schmidt and Braun 2020). Fungal structures were isolated from diseased leaves, and genomic DNA of the pathogen was extracted using the method described by Zhu et al. (2022). The internal transcribed spacer (ITS) region was amplified by PCR using the primers PMITS1/PMITS2 (Cunnington et al. 2003) and the amplicon sequenced by Invitrogen (Shanghai, China). The powdery mildew strain, named as KMD (GenBank accession no.: PP267067), showed an identity of 100% (645/645bp) with Erysiphe astragali, a powdery mildew reported on Astragalus glycyphyllos in Golestan, Iran (GenBank: OP806834) and identity of 99.6% (643/645bp) with Erysiphe astragali (GenBank: MW142495), a powdery mildew reported on A. scaberrimus in Inner Mongolia, China (Sun et al. 2023). Pathogenicity tests were conducted by brushing the conidia from infected S. salsula leaves onto leaves of four healthy plants, while four control plants were brushed in the same manner. All the treated plants were placed in separate growth chambers maintained at 19°C and 65% humidity, with a 16 h light/8 h dark photoperiod. Nine days after inoculation, the treated plants showed powdery mildew symptoms, while the 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. No report on the occurrence of powdery mildew on S. salsula plants has been found previously. The occurrence of this destructive powdery mildew may adversely affect the cultivation of S. salsula. Identifying the pathogen of powdery mildew will support future efforts to control and manage powdery mildew on S. salsula.

First Report of Erysiphe astragali Causing Powdery Mildew on Astragalus mongholicus 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.

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.