First Report of Powdery Mildew Caused by Podosphaera xanthii on Heliotropium indicum in Hainan Province, China

Heliotropium indicum L. is an annual herb of the Boraginaceae family, which is widely distributed throughout tropical and subtropical regions in the world (Chaipon et al. 2019). In China, this plant is commonly used as folk medicine for wound healing, fever, insect bites, abdominal pain and malaria treatments (Chaipon et al. 2018). In January and November 2024, powdery mildew symptoms were observed on 50% of H. indicum plants on Haidian Island (20° 3′ 52″ N; 110° 18′ 35″ 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, 77 to 148 × 12 to 18 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 32 to 59 µm long. Conidia (n =100) were ellipsoid-ovoid to doliiform, 23 to 33 ×15 to 22 µm (length/width ratio = 1.3 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 HMHI-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 directly. The resulting 577-bp sequence was deposited in GenBank (accession no. PP179515). A BLASTn search in GenBank of this sequence showed 100% similarity with the ITS sequences of P. xanthii isolates from China (OP765400 and MT739423), Taiwan (MT131253 and PV056449), and Thailand (LC270780). Additionally, the 28S rDNA gene was amplified using the primer pairs NL1 and NL4 (O´Donnell 1993). The 613-bp 28S sequence (accession no. PP179516) shared 100% similarity with P. xanthii isolates (ON365842, OP765401, and LC371331). 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 H. indicum 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 24 to 30°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 found on the originally infected leaves collected from Hainan Province. Based on the morphological characteristics and molecular identification, the fungus was identified as P. xanthii. P. xanthii has been previously reported on H. indicum from Taiwan (Amano 1986). To our knowledge, this is the first record of P. xanthii infecting H. indicum in Hainan province, China. We are concerned that the pathogen could become a threat to the widespread planting of H. indicum in the future.