Powdery Mildew of Pterocypsela indica Caused by Podosphaera xanthii in China

Abstract

Pterocypsela indica L. Shih (synonym Lactuca indica L.) is an annual or biennial herbaceous plant in the Asteraceae that has a worldwide distribution. In China, it is often used as a component for traditional Chinese medicine or as a leafy edible foraged from the wild. In September 2018 in Qianxi county (27.24 N, 106.01 E) of China, where a 2-ha field of P. indica was being planted, severe powdery mildew was observed on the leaves of P. indica. Approximately 35% of leaves on a plant were symptomatic, and about 40% of the plants were infected. Colonies were circular or irregular. White patches on the adaxial surface of the leaves appeared from June to September, and necrotic symptoms were observed in the autumn. To identify the pathogen, a voucher specimen (isolate CG) was examined for morphological characters using a microscope under 40× magnification. Each unbranched conidiophore was composed of a cylindrical foot cell followed by one to three short cells, ranging in size from 90 to 270 × 8 to 14 μm (n = 40). Conidia of the pathogen were ellipsoid-ovoid to barrel-shaped and measured 25 to 34 × 17 to 22 μm (n = 40). Fibrosin bodies were absent. Fungal hyphae were septate, branched, flexuous to straight, and up to 7 μm wide. These structures were identical with those of the anamorph of Podosphaera xanthii U. Braun & N. Shishkoff (Braun and Cook 2012), previously known as Podosphaera fusca (Cho et al. 2013). The pathogen was identified as P. xanthii on the basis of morphology and the sequence of ITS1-5.8s-ITS2 region of rDNA amplified by polymerase chain reaction with universal primers ITS-1 (5′-TCCGTAGGTGAACCTGCGGaaawf-3′) and ITS-4 (5′-TCCTCCGCTTATTGATATGC-3′) (White et al. 1990). The sequence submitted on December 6, 2017 (GenBank accession MG647826.1) of the ITS region exactly matched the sequences of P. xanthii accessions (e.g., GenBank accession nos. KX369541.1 and MG754404.1). Pathogenicity of the fungus was confirmed by performing Koch’s postulate as follows. Six healthy P. indica plants at the eight-leaf stage were selected for the test in the greenhouse. They were inoculated with conidia from an infected plant. Another six plants not inoculated were regarded as controls. After the inoculation, all plants were incubated in the greenhouse for symptom development. They were kept at room temperature, >70% relative humidity, 80 Lux, and 8 h light/day. The experiments were conducted twice. Fourteen days after incubation, powdery mildew symptoms appeared on the leaves of inoculated plants, and the pathogen morphology was similar to that of the original voucher, whereas control plants were still asymptomatic. Powdery mildew caused by P. xanthii has been reported on cucurbits, composites, and solanaceous crops in China (Fan et al. 2019) and on papaya in Korea (Joa et al. 2013). However, to the best of our knowledge, this is the first report of P. xanthii causing powdery mildew on P. indica in China. Owing to abundant dissemination of conidia of P. xanthii, the disease could be a threat to P. indica in Qianxi county of China, and this geography is right for P. xanthii to possibly spread to Lactuca sativa.