First Report of Cirsium arvense var. integrifolium as a new host of Podosphaera xanthii in Sichuan, China.

Cirsium arvense (L.) Scop. var. integrifolium Wimm. & Grab. (syn. Cirsium setosum (Willd.) MB.), a perennial herbaceous plant in the Asteraceae family, is widely distributed across Europe, Australia, Asia, North Africa, North and South America (Nischwitz and Newcombe 2004; Yang et al. 2024). In China, its tender stems and leaves are utilized as a vegetable and also a traditional medicine because of its heat-clearing efficacy (Jiang et al. 2013). In July 2023, symptoms of powdery mildew were observed in C. arvense var. integrifolium populations in Deyang city (N30°35', E104°37'), Sichuan Province, China. Approximately 40.38% (63/156) of the surveyed plants showed symptoms. The disease manifested as irregular small white, powdery patches, typically on the upper surface of leaves, which progressed to extensive hyphal growth covered over 90% of the leaf surface. With the disease advanced, the infected leaves wilted and senesced. Voucher specimens of symptomatic leaves (CaPM-ZL) as representative samples have been deposited at China West Normal University. The conidiophores were unbranched, straight, and measured 78.47 to 246.69 µm (avg. 154.60±49.59 μm) × 4.98 to 29.50 µm (avg. 13.58±8.06 μm) (n= 30). The conidia, produced singly, were hyaline, ellipsoid-ovoid to nearly cylindrical with fibrosin bodies, measuring 53.12 to 72.47 μm (avg. 59.82±6.41 μm) in length and 25.86 to 41.72 μm (avg. 33.85±4.87 μm) in width (n=30). Scanning electron microscopy revealed the conidia were turgid and smooth, with gentle contractions or bulges at both poles. The pathogen was provisionally identified as Podosphaera xanthii based on morphological characteristics (Braun and Cook 2012). To confirm the identification, the internal transcribed spacer region (ITS) and nuclear ribosomal large subunit (LSU) for CaPM-ZL was amplified using primers ITS4/PM5 and LSU1/LSU2, and analyzed using the BLASTn search of GenBank, respectively. The ITS (PP951890) and LSU (PQ644087) sequences were deposited into GenBank and showed 100% similarity with P. xanthii sequences (ITS: MT260063, KY388504; LSU: OP765401, PP897382). Based on the concatenated sequences of ITS and LSU, the phylogenetic analysis was performed in MEGAX with maximum likelihood method (Kumar et al. 2016) clustered CaPM-ZL into the Podosphaera xanthii clade with a 99% bootstrap support value. Combining morphological and phylogenetic analyses, CaPM-ZL was conclusively identified as Podosphaera xanthii. To evaluate pathogenicity, leaves of 3 healthy potted C. arvense var. integrifolium plants (3 leaves per plant) were inoculated by gently pressing them with diseased leaves, while 3 no-contact plants served as controls. The inoculated plants were isolated from the controls in a greenhouse maintained at 27±1°C, with a 14-hour light period and 80% relative humidity. After 7 days, the inoculated plants exhibited powdery mildew symptoms, while the control remained asymptomatic. Morphological characteristics and sequencing of inoculated pathogen were matched those of naturally infected plants. P. xanthii has been previously reported on Acalypha indica (Wu et al. 2024), Siraitia grosvenorii (Pan et al. 2024), and Vigna unguiculata (Zhang et al. 2024) in China. To the best of our knowledge, this is the first report of powdery mildew caused by P. xanthii on C. arvense var. integrifolium in China. This species as a new host of P. xanthii suggests that controlling the spread of this disease may be more challenging, and will contribute to an increased understanding of the host range. Our finding will provide the fundamental knowledge for future powdery mildew diagnosis and control.