Auricularia fibrillifera is a valuable edible mushroom due to its nutritional and medicinal properties. In May 2025, a bacterial rot disease emerged on fruiting bodies of A. fibrillifera in a mushroom cultivation facility (approximately 0. 05 ha) located in Guangchang County, Jiangxi Province, China (26°51′41″N, 116°19′32″E), where the disease incidence attained 30%. Initially, the fruiting bodies of A. fibrillifera exhibited water-soaked lesions. These lesions progressively enlarged, generating purulent tissue formation. Finally, severe soft rot causing fruiting body deformation and foul odor. Four symptomatic tissues from diseased fruiting bodies were rinsed and macerated in sterile water. Serial dilution of the resulting suspension was spread onto Luria-Bertani (LB) medium and incubated at 26 °C for 48 h. Subsequently, dominant colonies (yellowish, convex, circular, and smooth) were purified by streaking on LB plates at least three times. Six isolates were obtained from four samples collected randomly. Two isolates (JFRL 02-06 and JFRL 02-10) were randomly selected for further analysis. Biochemical assays of these isolates indicated positive reactions for catalase, oxidase, β-galactosidase, urease, gelatinase, and nitrate reduction, while H2S production, indole production, and Voges-Proskauer (VP) tests were negative, consistent with those of Sphingobacterium siyangense SY1 (type strain) described by Liu et al. 2008. The 16S rRNA and two housekeeping genes (rpoB and cpn60) from both isolates were amplified and sequenced. These sequences were deposited in GenBank under the following accession numbers: 16S rRNA (PX908888; PX462059), rpoB (PX910631; LC897700), and cpn60 (PX910630; LC897701). BLASTN analysis showed respective sequence identities of 99. 9% (1387/1388 bp), 99. 8 % (1806/1810 bp), and 98. 7% (458/464 bp) with the corresponding loci of S. siyangense MMO-142 (GCA₀37148175). Maximum-likelihood phylogeny confirmed that isolate JFRL 02-06 and JFRL 02-10 formed a well-supported clade with S. siyangense MMO-142, supported by a 99% bootstrap value. Collectively, the pathogen was identified as S. siyangense based on its morphological, biochemical, physiological, and molecular characteristics. For the pathogenicity test, the bacterial suspensions (108 CFU/ml) were spread onto six healthy fruiting bodies of A. fibrillifera, using sterile distilled water as a negative control, and then incubated at 25°C with 90% relative humidity. Experiments were performed in duplicate. Four days later, the inoculated fruiting bodies exhibited yellowing and a water-soaked appearance. Six days later, the yellowing intensified, and the fruiting body became severely rotten with a foul smell. Treated fruiting bodies presented symptoms resembling natural infections, whereas controls remained healthy. S. siyangense was consistently reisolated from the infected fruiting bodies and verified through morphological and molecular analyses, thus satisfying Koch’s postulates. Previous studies documented several Sphingobacterium species causing brown spot or rot in cultivated mushrooms, including Pleurotus ostreatus and Sparassis latifolia (Chen et al. 2024; Lv et al. 2021). To our knowledge, this is the first report of S. siyangense causing bacterial rot on A. fibrillifera in China.
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