First Report of Powdery Mildew Caused by Erysiphe macleayae on Corydalis edulis in China

Abstract

Corydalis edulis Maxim. is a common ornamental plant in China. It is also an important Chinese medicinal herb for its antipyretic action and the ability to flush out toxic materials. In October 2016, severe powdery mildew infections were observed on C. edulis in Mengdatianchi (35.82°N, 102.69°E), China. Voucher specimens were deposited in the Herbarium of Plant Pathology at Qinghai University under accession no. QH2016001. This powdery mildew is morphologically characterized as follows. Mycelium amphigenous, thin, effuse or in irregular patches, often around leaf veins, sometimes covering the entire surface, white, evanescent to persistent; hyphae sparingly branched, thin-walled. Hyphal appressoria well-developed, lobed, 3 to 8 µm diam. Conidiophores erect, arising from the top and mostly toward one end of the mother cell, 40 to 150 µm long, foot-cells straight to often curved-sinuous, 25 to 120 μm long and 6 to 10 µm wide, followed by 1 to 3 cells shorter than or about as long as the foot cell. Conidia ellipsoid-ovoid, 25 to 60 × 10 to 29 μm, length/width ratio 1.8 to 3.4. Chasmothecia 70 to 100 μm diam., solitary or in groups, with dark brown mycelioid, unbranched appendages, 0.5 to 7 times as long as the chasmothecial diameter, 5 to 9 µm wide, septate, thin-walled, brown at the base and paler toward the apex. Asci 2 to 5 per chasmothecium, saccate-clavate, subsessile to short-stalked, 40 to 75 × 25 to 60 μm, and 2- to 5-spored. Ascospores ellipsoid-ovoid, rarely slightly curved, 18 to 38 × 12 to 18 μm, often with oil drops, colorless to yellowish. This fungus was identified as Erysiphe macleayae based on the morphology of its asexual and sexual morphs (Braun and Cook 2012). To confirm identification, the 28S rDNA and ITS region were amplified. The ITS5/p3 and PM5/ITS4 primers were used to amplify the ITS region by nested PCR. Primers LSU1/LSU2 were used to amplify the 28S rDNA, and the cloned fragments were sequenced. The 28S rDNA and ITS region sequences were deposited in GenBank (accession nos. KX817183 and KX817184). A BLAST search analysis of the two sequences revealed 100% identity with E. macleayae infecting Macleaya cordata (Willd.) R.Br. in Japan. Based on the ITS rDNA phylogenetic tree, the sequences retrieved from the Chinese specimen clustered within a strongly supported clade (bootstrap value 100%) with E. macleayae (AB016048). Cladistic trees were constructed using the neighbor-joining method with the Kimura 2-parameter substitution model in MEGA 5.0. Branch robustness was assessed via bootstrap analysis with 1,000 replicates. Phylogenetic analysis data agree with morphological characters (Takamatsu et al. 1999, 2009). For Koch’s postulate, inoculated and noninoculated plants were maintained separately in different rooms of a greenhouse at 20 to 24°C. Inoculated plants developed signs and symptoms after 12 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 E. macleayae on C. edulis worldwide. Infection of Corydalis by E. macleayae is very unusual since this powdery mildew species naturally occurs on Chelidonium majus, Meconopsis cambrica, and Macleaya spp. In inoculation experiments, it could be demonstrated that E. macleayae from Chelidonium majus was able to infect Argemone, Glaucium, and Papaver spp. (Pastircakova et al. 2016). The present record of this species on C. edulis supports a wider host range of E. macleayae among papaveraceous genera.