First Report of Powdery Mildew Caused by Podosphaera xanthii on Cleome rutidosperma in China

Purple Cleome (Cleome rutidosperma DC.) is an annual herbaceous plant of the Cleomaceae family, which is native to tropical Africa. In the 1950s, it was introduced into tropical and subtropical regions of China as ornamental plants, and naturalized to Southern provinces. In China, this plant is commonly used for folk medicine for anti-inflammatory and anthelmintic activities (Ansari et al. 2016). In November 2024, powdery mildew symptoms were observed on 60% of purple Cleome plants on the Hainan Medical University campus (19° 58′ 53″ N; 110° 19′ 47″ E) in Haikou, Hainan Province, China. Powdery mildew colonies covered the leaf surfaces of affected plants, causing discoloration and defoliation. Mycelia were superficial and hyphal appressoria were nipple-shaped. Conidiophores (n =30) were unbranched, cylindrical, 95 to 274 × 11 to 14 µm, and produced three to five immature conidia in chains with a crenate outline. Foot cells (n =30) were cylindrical, straight or sometimes curved at the base, and 42 to 73 µm long. Conidia (n =100) were ellipsoid-ovoid to doliiform, 20 to 32 × 14 to 22 µm (length/width ratio = 1.1 to 2.0), with well-developed fibrosin bodies, and produced germ tubes from the lateral position. Based on these morphological characteristics, the pathogen was provisionally identified as Podosphaera xanthii (Braun and Cook 2012). The teleomorph was not observed. A specimen was deposited in the Hainan Medical University Plant Pathology Herbarium as HMCR-24. To confirm the identification, genomic DNA was extracted from mycelium, conidiophores, and conidia using a fungal DNA kit (Omega Bio-Tek, USA). The rDNA internal transcribed spacer (ITS) region was amplified with primers ITS1/ITS4 (White et al. 1990) and sequenced in both directions. The resulting 575-bp sequence was deposited in GenBank (accession no. PP178157). A BLASTn search in GenBank of this sequence showed 99% similarity with the ITS sequences of P. xanthii isolates from China (OR229712, MT260063, and MT739423), Thailand (LC270779), and Japan (AB525914). Additionally, the 28S rDNA gene was amplified using the primer pairs NL1 and NL4 (O´Donnell 1993). The 28S sequence (accession no. PP179514) shared 100% similarity with P. xanthii isolates (MT739424 and LC270779). Powdery mildew from Hainan sample belonged to the P. xanthii group with strong bootstrap values support 99% in maximum likelihood phylogenetic tree based on ITS and 28S gene sequences. To confirm pathogenicity, five healthy potted plants of C. rutidosperma were inoculated by gently pressing a powdery mildew-infected leaf onto 15 young leaves. Five non-inoculated plants served as controls. All plants were maintained in a greenhouse at 26°C, 70% relative humidity, with a 16-h photoperiod. After 7 days, the inoculated leaves showed powdery mildew symptoms whereas no symptoms were observed on the control plants. The fungal colonies observed on the inoculated plants were morphologically identical to those found on the originally infected leaves collected from Hainan. Based on the morphological characteristics and molecular identification, the fungus was identified as P. xanthii. This plant pathogen has been previously reported on C. rutidosperma in India (Nayak et al. 2018). To our knowledge, this is the first record of P. xanthii infecting C. rutidosperma in China. C. rutidosperma is often planted as an ornamental plant on the road, and we are concerned that it may serve as a new host, spreading this pathogen to other economic crops.