First Report of Powdery Mildew Caused by Podosphaera xanthii on Solena amplexicaulis in China

Solena amplexicaulis (Lam.) Gandhi is an important medicinal plant of the family Cucurbitaceae. The plant has been used in traditional herbal medicine because it contains numerous anti-inflammatory, antinociceptive, and antidiabetic compounds (State Administration of Traditional Chinese Medicine 1999). It is identified as a common raw material in traditional Chinese medicine and grows wild in crop fields in tropical and subtropical regions in China. In January 2018, powdery mildew on the leaves of S. amplexicaulis plants was observed around a greenhouse in Hainan Experimental Station of Northeast Agricultural University (18°19′34″ N, 109°48′28″ E), Sanya, China. There were about 120 S. amplexicaulis plants in an area of 150 m². Powdery mildew colonies first appeared as white spots on the upper surface of the leaves and stems of the plants. Later, white hyphae covered the leaves and the petiole. Later in the season, entire leaves yellowed and senescence occurred. Isolated conidia from infected samples were ellipsoid-ovoid to barrel-shaped, with distinct fibrosin bodies visible in their cytoplasm, and measured 27 to 36 × 18 to 24 μm (n = 30) with a length/width ratio from 1.5 to 2.0. Conidiophores were unbranched, straight, 80 to 260 × 7 to 15 µm in size, and produced three to five immature conidia in chains with a crenate outline. Foot cells of conidiophores were cylindrical and 35 to 72 μm long, with slight constrictions at basal septa, and followed by one to three short cells. Fungal hyphae were septate, branched, and flexuous to straight and up to 7 µm wide with indistinct to slightly nipple-shaped appressoria. These structures are typical of the genus Podosphaera. Partial sequences of the 18S small subunit ribosomal (ITS1), 5.8S ribosomal (ITS2), and large subunit ribosomal RNA gene were amplified from conidia collected from infected leaves using universal primers ITS1/ITS4 (White et al. 1990), and purified PCR products were sequenced. Sequences were deposited in GenBank under accession numbers MK809309 and MK809310. BLASTn analysis of the ITS sequence (569 bp) revealed ≥99% sequence identity with respective sequences of Podosphaera xanthii isolates from Lagenaria siceraria (MG719984.1), Citrullus lanatus (KX369541.1), and Abelmoschus esculentus (MG754404). Consequently, on the basis of morphological and molecular characteristics the fungal species causing disease on S. amplexicaulis was tentatively identified as P. xanthii (Braun and Cook 2012). A pathogenicity test was performed by dusting conidia of an infected leaf onto young leaves of five healthy S. amplexicaulis plants, with noninoculated plants as controls. Powdery mildew symptoms were observed on inoculated leaves after 6 days at 19 to 27°C (night/day) and 75% relative humidity, whereas the control plants remained asymptomatic. The morphological features of the powdery mildew on the inoculated leaf discs were similar to the original fungal isolate. Powdery mildew, caused by P. xanthii, is one of the major diseases of the Cucurbitaceae affecting a wide range of host species worldwide, including in China (Farr and Rossman 2017). To the best of our knowledge, this is the first report of powdery mildew caused by P. xanthii on S. amplexicaulis in China. The leaves, roots, and fruits of S. amplexicaulis are all important in traditional herbal medicine. Powdery mildew would be a serious threat to production of S. amplexicaulis in this region.