First Report of Powdery Mildew Caused by Golovinomyces circumfusus on Scutellaria baicalensis in China.

Abstract

Scutellaria baicalensis Georgi is a member of the plant family Lamiaceae and is widely cultivated in China as a medicinal plant (Wang et al. 2021). In August 2022, an investigation of a 10-hectare field of S. baicalensis in Longxi county, Gansu province, China (35°18' N, 104°57' E) found that approximately 90% of the plants were infected with 70% leaves symptomatic. Initially, the thin, radial, irregular white colonies appeared on the adaxial surface of the plant leaves. Subsequently, the white colonies expanded and thickened to cover the entire adaxial surface of the leaves, and gradually spread to the stems and abaxial surface of the leaves. Finally, infected leaves dried out and defoliated prematurely. Tissue from infected plants was collected in order to identify the pathogen. Microscopic observations showed that hyphae were septate, branched, flexuous to straight, 3 to 9 µm wide. Appressoria were solitary and slightly to distinctly nipple-shaped. Conidiophores were erect, straight or somewhat flexuous, 105 to 145 × 8 to 13 µm in size (n=30). The conidia were ellipsoid to ovoid in shape, 25 to 39 × 12 to 21 µm (n=50), without obvious fibrosin bodies. Chasmothecia were globose or oblate, scattered to gregarious, dark brown, and 85 to 140 μm in diameter. Appendages were born equatorially on the ascomata, unbranched, 0.5 to 2.5 times as long as the chasmothecial diameter, interlaced with each other, and colorless. Asci were 5 to 12 per chasmothecium (n=30), obovate or suborbicular, 40 to 68 × 20 to 35 µm (n=30), mostly containing 2-3 ascospores. Ascospores were ellipsoid-ovoid, 14 to 24 × 10 to 15 μm (n=30). The fungus was identified as Golovinomyces circumfusus based on these morphological characteristics (Braun and Cook 2012). To validate the identity, the pathogen identification was confirmed through multi-locus phylogeny using internal transcribed spacer (ITS), 28S large ribosomal subunit, intergenic spacer (IGS) and beta-tubulin (TUB2) genes amplified. (Qiu et al. 2020). The resulting sequences were registered in GenBank (OR186707 for ITS, OQ861003 for 28S rDNA, OR193383 for IGS, OR205893 for TUB2). The phylogenetic tree was obtained using maximum parsimony (MP) in PAUP 4.0b. The phylogenetic analysis of the multi-locus sequences indicated that the strain HQ1 and G. circumfusus (type) clustered together on the same branch, confirming its identification. Pathogenicity tests were performed on potted 1-year-old plants of S. baicalensis according to Koch's postulates. Three plants were inoculated by gently pressing the naturally infected leaves onto 30 healthy leaves. Three non-inoculated healthy plants served as controls. Inoculated and control plants were placed in separate growth chamber (light/dark, 14 h/10 h; humidity, 70%; temperature, 25℃). After 12 days, the inoculated plants developed typical powdery mildew symptoms, but the control leaves remained without symptoms. The fungus on inoculated leaves was re-isolated, sequenced and confirmed as G. circumfusus based on morphological characteristics and molecular identification. Powdery mildew caused by G. circumfusus has been reported affecting Eupatorium cannabinum (Asteraceae) in Germany (Qiu et al. 2020), and Bidens pilosa var. radiata and Passiflora. edulis f. flavicarpa in Taiwan (Lin et al. 2020). There is no previous report about G. circumfusus causing powdery mildew on S. baicalensis. Moreover, although powdery mildew has been reported on S. baicalensis in China, the pathogen was not further identified (Zheng et al. 2010). To our knowledge, this is the first report of powdery mildew caused by G. circumfusus on S. baicalensis in China and worldwide.