First Report of Mucor Rot Caused by Mucor circinelloides on Gastrodia elata in China

Gastrodia elata Blume is used as a traditional Chinese herbal medicine and is widely planted throughout China (Zhu et al. 2019). From September to October 2022, G. elata tuberous rot occurred in 47 households in Yiliang County, Zhaotong City (27°39' N, 104°16' E), Yunnan Province, China, within a major G. elata production area covering 776 ha, with an incidence rate of 10 to 15%. Symptoms manifested as black and brown lesions on the tuber surface, which were concave, soft, foul-smelling, and surrounded by grayish-brown tissue . Three infected G. elata were randomly collected from each household, placed in transparent plastic bags, and pathogen isolation was conducted in a laboratory. Infected G. elata tubers were surface-sterilized with 0.5% NaOCl for 2 min, rinsed five times with sterile water, and dried. Symptomatic tissues from the margin between necrotic and healthy tissues were cut into 5 × 5 mm pieces, placed onto potato dextrose agar (PDA), and incubated at 28 ºC in the dark for 3 days. Hyphal tips of fungi growing from the samples were transferred onto new PDA plates and incubated until they produced conidia. Five fungal isolates (Charliezhao 417, 418, 419, 420, and 421) with the same morphological characteristics were obtained from the samples. Colonies tended to be yellow or light grey, and produced sporangiospores that were sub-globose, ellipsoid, or irregular, measuring 4.0 to 9.7 × 2.5 to 4.6 μm (n = 50). The morphological characteristics of the isolates resembled Mucor circinelloides (Wagner et al. 2020). Genomic DNA of two representative isolates (Charliezhao 417 and 418) was extracted using the DN14 cetyltrimethylammonium bromide rapid plant genome extraction kit (Aidlab Biotechnologies Co., Ltd, Beijing). The ITS and RPB1 genes were amplified by polymerase chain reaction using the primers ITS1/ITS4 (White et al. 1990) and Af/Cf (Matheny et al. 2002), respectively. All sequences were deposited in GenBank (accession no. OQ612709, OR028949 for ITS, OQ621439, OR033135 for RPB1). A BLASTN homology search with the ITS nucleotide sequences showed that they had 99.5 to 99.2% identity with M. circinelloides isolate KR056083 (603/606 bp) and isolate KJ588204 (617/622 bp), respectively; and the RPB1 sequences had 99.89% to 99.75% identity to isolates KJ588206 (874/875 bp) and isolate KJ588206 (803/805 bp), respectively. To complete Koch's postulates, five mature, healthy G. elata tubers were surface disinfected with 1% NaClO solution for 1 min, rinsed with sterile water, and dried at 25℃ for 30 min. A conidial suspension (106 spores/ml) collected from two isolates (Charliezhao 417 and 418) was sprayed onto G. elata tubers, and the control treated with distilled water. All G. elata tubers were incubated at 25 ºC with 80% relative humidity. The experiment had five replicates. After 7 days of incubation, there were obvious brown spots on inoculated tubers; no symptoms were observed on the controls. The pathogen was re-isolated from all inoculated G. elata tubers and confirmed as M. circinelloides by morphological and molecular analyses, completing Koch's postulates. This is the first report of M. circinelloides causing G. elata mucor rot in China. The tubers of G. elate are often employed in the treatment of headaches, convulsions and neurodegenerative disorders (Manavalan et al. 2012). Thus, the declining yield of G. elate due to persistent obstacles related to continuous cropping and diseases poses a potential threat to the development of the G. elate industry.