First report of Botrytis cinerea causing grey mold on Corydalis yanhusuo in Panan, China.

Abstract

Yanhusuo (Corydalis yanhusuo (Y. H. Chou & Chun C. Hsu) W. T. Wang ex Z.Y. Su & C.Y. Wu), a perennial herbaceous plant of the Papaveraceae family and genus Corydalis, is also known as Yuanhu and used as medicine by its tuberous roots. It is mainly planted in Zhejiang, Jiangsu and Anhui provinces of China, with the best quality produced in Panan County of Zhejiang province. Yanhusuo has the effects of promoting blood circulation, invigorating the flow of qi and relieving pain, and is widely used in Chinese traditional medicines. In surveys carried out in summer of 2020-2023, grey mold disease was found occurred on C. yanhusuo in Panan County. This disease begins at April, and lasts to July, with incidence of 20% to 70%. The diseased plants showed a large number of gray mold layers adhere to the leaves. When the disease infects from the leaf tips, it form V-shaped lesions; when the leaves are severely infected, the entire leaves die, shrink, curl, and have a large number of gray mold layers on the surface. To identify the causal agent of this leaf disease, diseased leaves were collected from Yanhusuo field at Panan County of Zhejiang province in China since 2020, and tissues at the junction of the healthy and diseased areas were cut off, disinfected with 75% ethanol for 30 seconds, rinsed with sterile water for 3 times for 1 minute each time, air-dried under sterile conditions, and then were inoculated in PDA medium and cultured in a 25℃ incubator. After 2-3 days, picked the edge hyphae of the fungi that grew on the PDA plate and cultured them on a new PDA plate. After 5 days, picked the single spore and inoculated on a new PDA plate for continuous cultivation until pure culture strains were obtained. Thirty strains were isolated from 30 samples that collected from 3 Yanhusuo fields in Panan County. One of the thirty purified strains was named "YH8" for further identification. When cultured on PDA medium, mycelia were initially whitish and turned gray with age. The hyphae accumulate into clusters, and no sclerotia are produced during the cultivation. The conidiophores are slender, septate. The base of the conidiophore is enlarged or slightly enlarged. The conidiophore often has branches and produces a large number of conidia, which are similar to grape clusters. The conidia are monosporous, ovoid, and colorless, 6.08 μm-12.76 μm×8.42 μm-19.34 μm, with an average size of 9.55 μm×14.50 μm. To further identify the species, YH8 genomic DNA was extracted, and the internal transcribed spacer (ITS), heat shock protein (HSP60), and glyceraldehyde-3-phosphate dehydrogenase (G3PDH) genes were amplified with the primers ITS1/4 (White et al. 1990), HSP60-F/HSP60-R, and G3PDH-F/G3PDH-R (Staats et al. 2005), respectively. A multilocus phylogenetic tree was constructed with the ITS, HSP60, and G3PDH reference sequences, and the sequences of PCR amplicons (Genbank number: PP388281, PP376066 and PP376067) were 100% (518 bp out of 518 bp), 99% (994 bp out of 995 bp) and 100% (880 bp out of 880 bp) identical to the Botrytis cinerea strain 5-3, respectively, and the grouping of strain YH8 was supported by 99% bootstrap value. To fulfill the Koch's postulates, spore suspension (approximately 103 CFU/mL) of YH8 was sprayed onto leaves of 3-week Yanhusuo seedlings, and sterile water was sprayed as negative control, 15 seedlings for each treatment, and the experiments was repeated for times. The seedlings were incubated in a growth chamber under 28℃ and 80% humidity. Seven days after inoculation, leaves of noninoculated controls were green and healthy, while the seedlings inoculated with spore suspension of YH8 showed lesions and molds, which were same with field symptoms. The causal pathogen was then reisolated from the lesions, and the gained pathogen showed same colony and spore morphology with YH8, which suggested the confirmation of Koch's postulates. Based on the morphological characteristics and molecular identification, the strain YH8 was identified as B. cinerea. To our knowledge, this is the first report of B. cinerea causing gray mold on the Corydalis yanhusuo in China. This report will provide guide to growers and local technicians for diagnostic and controlling grey mold disease of Yanhusuo.