First Report of Powdery Mildew Caused by Podosphaera xanthii on Spathodea campanulata in Hainan Province, China

The flame tree, Spathodea campanulata P. Beauv., is a large deciduous tree in the family Bignoniaceae. This ornamental garden plant originated in equatorial Africa, and it is widespread in tropical regions of China (Wang et al. 2019). In China, S. campanulata is commonly used as folk medicine to treat fever, malaria, and diabetes (Świątek et al. 2022). In December 2025, powdery mildew symptoms were observed on 60% of S. campanulata seedlings on Haidian Island (20° 3′ 50″ N; 110° 21′ 16″ E) in Haikou, Hainan Province, China. Powdery mildew colonies covered the leaf surfaces and stems of affected plants, causing discoloration and defoliation. Mycelia were superficial and hyphal appressoria were nipple-shaped. Conidiophores (n =30) were unbranched, cylindrical, 101 to 261 × 9 to 17 µm, and produced five to nine immature conidia in chains with a crenate outline. Foot cells (n =30) were cylindrical, straight or sometimes curved at the base, and 26 to 65 µm long. Conidia (n =100) were ellipsoid-ovoid to doliiform, 22 to 36 × 15 to 24 µm (length/width ratio = 1.2 to 1.8), with well-developed fibrosin bodies, and produced germ tubes from lateral positions. 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 HMSC-25. To confirm the genus identification and ascertain a putative species, genomic DNA was extracted from mycelium, conidiophores, and conidia using a fungal DNA extraction kit (Omega Bio-Tek, USA). The rDNA internal transcribed spacer (ITS) region was amplified with primers ITS1/ITS4 (White et al. 1990) and sequenced directly. The resulting 575-bp sequence was deposited in GenBank (accession no. PX901355). A BLASTn search of GenBank showed 100% sequence similarity with the ITS sequences of P. xanthii isolates from China (MH143483), Korea (MG754404), and Mexico (JQ728480). Additionally, the 28S region was amplified using the primer pairs NL1 and NL4 (O´Donnell 1993). The 613-bp 28S sequence (accession no. PX901358) shared 100% similarity with P. xanthii isolates (AB936276, LC270782, and MT739424). Phylogenetic analysis placed the isolate within the P. xanthii clade with strong bootstrap support (99%) in maximum likelihood phylogenetic tree based on ITS and 28S gene sequences. To confirm pathogenicity, five healthy potted plants of S. campanulata were inoculated by gently pressing a powdery mildew-infected leaf onto 15 young leaves. Five noninoculated plants served as controls. All plants were maintained in a greenhouse at 25°C, 70% relative humidity, with a 16-h photoperiod. After 7 days, inoculated leaves showed powdery mildew symptoms whereas no symptoms were observed on control plants. The fungal colonies observed on inoculated plants were morphologically identical to those observed on the original infected leaves. Based on the morphological characteristics and molecular identification, the fungus was identified as P. xanthii. In different countries, Oidium sp. has been previously reported on S. campanulata from Australia, Colombia, and Venezuela (Amano 1986). To our knowledge, this is the first record of P. xanthii infecting S. campanulata in Hainan Province, China. S. campanulata is often planted as ornamental street trees, and S. campanulata may serve as a potential host for this pathogen, spreading this pathogen to other economic crops.