First Report of Brown Rot Caused by Porogramme epimiltina on Gastrodia elata in China.

Abstract

Gastrodia elata Blume is a valuable medicinal plant in China with great significance in medicine (Li et al. 2023). From 2022 to 2023, G. elata tuber rot occurred in about 50 households in the main cultivation areas of G. elata (27°39' N, 104°16' E) in Yiliang County, Zhaotong, Yunnan Province, southwest China. The planting area of G. elata was 776 ha, and the incidence rate was 10%. Symptoms present as light brown lesions on the surface of the tuber, sunken, soft and foul-smelling. Infected G. elata tubers were randomly collected from each household, packed into transparent plastic bags, and strains were isolated in the laboratory as follows. The tubers of 15 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. Two fungal strains (Charliezhao 425 and 433) with the same morphological characteristics were obtained from the samples. Colonies were whitish and grew rapidly, irregularly turning pale orange at the edge or center of the mycelium pad on a two-week-old petri dish, and finally dark red，spore oval to spherical, 2.7 to 5.3 × 2.3 to 3.5 μm (n=50). The morphological characteristics of the isolates resembled Porogramme epimiltina (Mao et al. 2023; Kubayashi et al. 2001). Genomic DNA of two representative isolates (Charliezhao 425 and 433) was extracted using the DN14 cetyltrimethylammonium bromide rapid plant genome extraction kit (Aidlab Biotechnologies Co., Ltd, Beijing). The ITS and TEF1 genes were amplified by polymerase chain reaction using the primers ITS1/ITS4 (White et al. 1990) and EF1-983F/EF1-2218R (Rehner et al, 2005), respectively. All sequences were deposited in GenBank (accession no. OR905803, OR905804 for ITS, OR939812, OR939813 for TEF1). A BLASTN homology search with the ITS nucleotide sequences showed that they had 98.99 to 99.15% identity with P. epimiltina isolate OP997539 (588/594 bp) and isolate OP997539 (584/589 bp), respectively; and the TEF1 sequences had 95.41 to 95.59% % identity to isolates OP556566 (540/565 bp) and isolate OP556566 (542/567 bp), respectively. To complete Koch's hypothesis, the surfaces of 5 mature and healthy G. elata tubers were disinfected with 1% NaClO solution for 1 minute, rinsed with sterile water 5 times, and dried at 25 ℃ for 30 minutes. Conidial suspensions (106 spores/ml) were collected from two isolates (Charliezhao 425 and 433) and sprayed on G. elata tuber, and the control treated with distilled water. All G. elata tubers were incubated at 25℃ with 80% relative humidity. The experiment had three replicates. After 7 days of culture, there were obvious rotten and smelly on the inoculated tubers. No symptoms were observed in the control groups. The pathogen was re-isolated from all inoculated birch tubers and confirmed as P. epimiltina by morphological and molecular analysis, which fulfilled Koch's hypothesis. Gastrodia elata is a valuable and extensively used herbal Traditional Chinese Medicine with a wide range of clinical applications. As far as we know, this is the first report of P. epimiltina causing brown rot of G. elata in China.