First Report of Powdery Mildew Caused by Podosphaera elsholtziae on Dracocephalum tanguticum in China

Abstract

Dracocephalum tanguticum Maxim. is endemic to the Qinghai Tibetan Plateau. It has been widely used in Tibetan traditional medicine to treat pneumonia and respiratory inflammation, but powdery mildew has seriously affected the quality of the plants and reduced the economic value. In August 2017, severe powdery mildew was observed on D. tanguticum in Huangnan (35.31°N, 101.93°E, altitude 3,300 m), China. Voucher specimens were deposited in the Herbarium of Plant Pathology at Qinghai University under accession number QHU2017043. This powdery mildew fungus is morphologically characterized as follows. Mycelium was observed on the entire surface of the leaves and stem, and more than 70% of leaf areas were affected; hyphae hyaline, thin-walled, smooth, 3 to 6 µm wide; hyphal appressoria nipple shaped; conidia ellipsoid-ovoid to doliiform, 22 to 30 × 12 to 17µm; chasmothecia amphigenous, scattered to gregarious, dark brown, globose or subglobose, 67 to 83 µm in diameter; peridium cells irregular shaped, 8 to 37 µm in diameter; appendages usually few, mostly fewer than six, arising from the lower half, myceloid, simple, usually unbranched, 0.3 to 2 times as long as the chasmothecial diameter, 3 to 7 µm wide, brown below and paler toward the tip, septate, wall thin, smooth; ascus broadly ellipsoid-ovoid, sessile or with a very short stalk, 60 to 71 × 50 to 58 µm, six- to eight-spored; ascospores broad ellipsoid-ovoid or subglobose, 11 to 18 × 8 to 15 µm, colorless. This fungus was identified as Podosphaera elsholtziae based on the morphology of its asexual and sexual morphs (Braun and Cook 2012). To confirm the identification, the internal transcribed spacer (ITS) region were amplified. Genomic DNA was extracted from chasmothecia using Chelex-100 (Hirata and Takamatsu 1996; Walsh et al. 1991). The ITS5/P3 and PM5/ITS4 primers were used to amplify the ITS region by nested polymerase chain reaction, and the cloned fragments were sequenced. The ITS region sequences were deposited in GenBank (accession no. MK402076). A BLAST search analysis of the two sequences revealed 99% identity with P. elsholtziae infecting Ajuga reptans Linn. in Japan. Based on the ITS rDNA phylogenetic tree, the sequences retrieved from the specimen clustered within the same clade (bootstrap value 97%) with P. elsholtziae (AB026142). Cladistic trees were constructed using the maximum parsimony method with the Kimura two-parameter substitution model in MEGA 5.0. Branch robustness was assessed via bootstrap analysis with 1,000 replicates. Phylogenetic analysis data agreed with morphological characteristics (Takamatsu et al. 1999, 2009). Pathogenicity tests were performed by rubbing infected leaves onto healthy leaves and stems of four D. tanguticum seedlings. Four healthy seedlings were kept separately as a noninoculated control. Inoculated and noninoculated plants were maintained in a greenhouse at 22 to 25°C. Inoculated plants developed signs and symptoms after 17 days, whereas control plants remained symptomless. The morphology of the fungus on inoculated leaves was identical to that originally observed on diseased plants. To our knowledge, this is the first report of powdery mildew caused by P. elsholtziae on D. tanguticum worldwide.