Powdery Mildew of Xanthium strumarium Caused by Podosphaera xanthii in Central China

Xanthium strumarium, known as cocklebur, is an annual herb and has been used in traditional Chinese medicine. In October 2020, powdery mildew-like disease signs and symptoms were observed on X. strumarium grown in a crop field, Xinxiang city, Henan Province, China (35.36076° N, 113.93467° E). The specimen (PX-XS2023) was stored in Xinxiang Key Laboratory of Plant Stress Biology. White colonies in irregular or coalesced circular shaped-lesions were abundant on both ad- and abaxial surfaces of leaves and covered up to 99 % of the leaf area. Some of the infected leaves were senesced. More than 70 % of plants (n = 130) exhibited these signs and symptoms. Conidiophores were straight or slightly curved, 55 to 160 × 11 to 13 μm composed of foot-cells, shorter cells and conidia. Conidia were ellipsoid to oval, 29 to 40 × 14 to 20 μm (n = 50), with a length/width ration of 2.0 to 2.5, containing fibrosin bodies. Dark brown to black chasmothecia were found on infected leaves. The appendages were mycelium-shaped and at the base of scattered or gregarious chasmothecia (n = 50, 70 to 120 μm in diameter). Asci were 55 to 80 × 50 to 65 μm (n=30). These morphological characteristics were consistent with those of Podosphaera xanthii (Braun and Cook 2012). The internal transcribed spacer (ITS) region and Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) region of the fungus (PX-XS2023) were amplified and sequenced with primers ITS1/ITS4 (White et al. 1990) and GAPDH1/GAPDH3R (Bradshaw et al. 2022) according to a previously reported method (Zhu et al. 2022). The resulting sequences were respectively deposited into GenBank (Accession No. MW300956 and PP236083). BLASTn analysis indicated that the sequences were respectively 99.82 % (564/565) and 100% (272/272) identical to P. xanthii (MT260063 and ON075658). The phylogenetic analysis indicated that the strain PX-XS2023 and P. xanthii were clustered into a same branch. Therefore, the causal agent of powdery mildew on X. strumarium was P. xanthii. To conduct pathogenicity assays, mature leaves of five healthy X. strumarium (height in 50 centimeters) were inoculated with fungal conidia by gently pressing surfaces of infested leaves onto leaves of healthy plants (Zhu et al. 2020). Five untreated plants served as controls. The controls and inoculated plants were separately maintained in greenhouses (humidity, 60%; light/dark, 16 h/8 h; temperature, 18°C). Eight days post-inoculation, signs of powdery mildew were detectable on inoculated plants, however, the controls were asymptomatic. Thus, the fungal pathogen was morphologically and molecularly identified and confirmed as P. xanthii. This powdery mildew caused by P. xanthii was previously reported on X. strumarium in Korea, Russia and India (Farr and Rossman, 2021). In addition, P. xanthii was recorded on X. strumarium in Xinjiang Province, China (Tai 1979). However, this is the first report of P. xanthii on X. strumarium in central China, where is around 3000 km away from Xinjiang Province with geographically differences. The sudden presence of powdery mildew caused by P. xanthii may adversely affect plant health and thus reduce medical value of X. strumarium. Therefore, the identification and confirmation of P. xanthii infecting X. strumarium enhance the knowledge on the hosts of this pathogen in China and will provide fundamental information for disease control in the future.