Ageratum conyzoides L. is a member of the Asteraceae and a competitive weed for crops in the subtropics and tropics. In September 2016, we observed that a plant community of A. conyzoides was 85% infected with powdery mildew in a forest park at Fujian Agriculture and Forestry University (FAFU), Fuzhou, China. Initially, circular to irregular white powdery colonies or thinly effuse white mycelial patches formed on both sides of the leaves and on the stems. In later stages, entire leaves were covered with white mycelium, followed by leaf yellowing and senescence. Fungal hyphae were septate, branched, flexuous to straight, and 5 to 7 μm wide. Appressoria were indistinct to slightly nipple-shaped and solitary. Conidiophores (80 to 215 × 10 to 12 μm) were erect and unbranched, slightly constricted at the basal septa, with cylindrical foot-cells (35 to 70 × 10 to 12 µm) and 1 to 3 cells shorter than the foot-cell. Conidia (25 to 40 × 14 to 23 μm) were hyaline, catenescent, ellipsoid-ovoid to barrel-shaped, with a length/width ratio of 1.3 to 2.0, and containing distinct fibrosin bodies. Simple to forked germ tubes were observed on subterminal or lateral position of germinating conidia. No chasmothecia were observed in the collected infected samples. Based on the morphology of the imperfect state, the isolate was identified as Podosphaera xanthii (Castagne) U. Braun & N. Shishkoff (Braun and Cook 2012). A 565-bp amplicon (KY274485) of the internal transcribed spacer (ITS) region generated with primers ITS1/ITS4 (White et al. 1990) was 100% identical with other P. xanthii sequences (AB040351, AB462804) and showed phylogenetic grouping with P. xanthii isolates from Asteraceae and Verbenaceae. Pathogenicity tests were completed by dusting conidia from an infected leaf onto young leaves of three asymptomatic, potted plants and covered with plastic bags. Noninoculated plants were used as controls. Inoculated leaves showed powdery mildew symptoms after 6 days at 20°C and 80% relative humidity in green house. Control plants exhibited no disease symptoms. The fungal morphology on inoculated leaves was identical to that observed on initially diseased leaves. A voucher specimen (HMAS 247182) was deposited in Institute of Microbiology Chinese Academy of Sciences (CAS) Herbarium Mycologium (HMAS), Beijing, China. Previously, Euoidium agerati has been mentioned as powdery mildew pathogen on A. conyzoides in Taiwan (Braun and Cook 2012). However, this pathogen is quite different from the present collection by not having fibrosin bodies in conidia. Powdery mildew of A. conyzoides caused by P. xanthii has been recorded in Pakistan and India (Gautam 2015; Mukhtar et al. 2013). In China, A. conyzoides is used as medicinal plant to treat a variety of conditions and diseases. Incidence of P. xanthii on this plant will add information on pathogen’s range and will be helpful for future investigation. To our knowledge, this is the first report of P. xanthii on A. conyzoides in China.