First Report of Brown Leaf Rot Disease on Dendrobium nobile Caused by Xylaria flabelliformis in Guizhou Province, China.

Abstract

Dendrobium nobile is the largest species of the Orchidaceae family and produces dendrobine, a compound with medicinal properties (Sarsaiya et al., 2020a; Sarsaiya et al., 2024; Qian et al., 2024). The accumulation of dendrobine in D. nobile is regulated by various pathogenic fungi, which directly and indirectly influence dendrobine biosynthesis (Sarsaiya et al., 2019a; Sarsaiya et al., 2019b). In a field planted with D. nobile in Guizhou Province, China, small lesions were initially observed on the upper part of the leaves from May to June 2019, which later developed into larger brown necrotic leaf lesions. Over time, these lesions greatly impacted the medicinal value (dendrobine) and productivity of the plant. A pure culture of Xylaria flabelliformis from infected wild D. nobile leaves was recovered and subsequently cultured on potato dextrose agar (PDA) at 25 °C for 5 days. Xylaria flabelliformis grew slowly and was composed of white mycelia. Colonies were initially white, with a regular margin, and formed stromata that consisted of mycelia sterilia without ascospores. We identified the strain as Xylaria flabelliformis based on its morphological characteristics (Liu et al., 2007) and by sequencing elongation factor-1α (EF-1α). The length of the DNA sequence of EF-1α that was used for the analysis of Xylaria flabelliformis was 1188 bp. BLASTx (nucleotide 6-frame translation-protein) analysis using the National Center for Biotechnology Information database showed that the obtained protein sequence (BLASTx protein accession no.: UTS95822.1, BLASTn nucleotide sequence accession no.: MW508334.1) had the highest similarity (98.21%) with the X. flabelliformis hypothetical protein (TRX95197.1) based on a thorough phylogenetic comparison with other Xylaria species. Healthy D. nobile seedlings were planted in pots and sterilized. The terminal leaves were excised from all pre-sterilised D. nobile seedlings and inoculated with Xylaria flabelliformis mycelial plugs, whereas sterile PDA plugs and moist cotton plugs were used as controls. All seedlings were maintained under optimum temperature and humidity conditions (25 °C and 80%, respectively) for seven days for observation and analysis. All experiments were performed in triplicate. After the incubation period, brown leaf rot lesions were observed for the first time on the inoculated D. nobile leaves, but no symptoms were observed on the leaves of the two control groups (sterile PDA plugs and moist cotton plugs). To complete Koch's postulates, Xylaria flabelliformis was re-isolated and identified from all diseased tissues by DNA sequencing of the EF-1α. It was determined for the first time that Xylaria flabelliformis can cause brown leaf lesions in D. nobile. Moreover, the pathogenicity of Xylaria flabelliformis in D. nobile has not been previously reported (Mead et al., 2019; Meng et al., 2019; Sarsaiya et al., 2019a; Sarsaiya et al., 2020b; Chen et al., 2023; Rinchen, 2023; Cao et al., 2024). To the best of our knowledge, this is the first report of BLRS lesions in D. nobile leaves caused by Xylaria flabelliformis in Guizhou Province, China. Identification of Xylaria flabelliformis as a pathogen of D. nobile is crucial for advancing effective management and control practices against brown leaf rot disease. This discovery provides valuable insights into the development of targeted strategies to mitigate the impact of Xylaria flabelliformis on D. nobile, safeguard medicinal properties such as dendrobine, and enhance overall productivity.