Identification, Characterization, and In Vitro Fungicide Sensitivity of Paraphoma radicina, the First Reported Cause of Root Rot in Polygonatum sibiricum

<i>Polygonatum sibiricum</i>, a perennial medicinal and edible herb, has recently suffered severe root rot outbreaks in China, threatening its yield and quality. From 2023 to 2024, plants showing rhizome darkening and necrosis were observed in Tai'an, Shandong Province. A fungal isolate (HJ2B3) was consistently recovered from symptomatic tissues and confirmed as pathogenic through in vitro detached rhizome and potted plant inoculations. The isolate produced globose to subglobose, setose pycnidia and hyaline, aseptate conidia. Morphological characteristics, combined with <i>ITS</i>, <i>EF1-α</i>, and <i>β-tubulin</i> sequence analyses and phylogenetic evidence, identified the pathogen as <i>Paraphoma radicina</i>. In vitro fungicide assays using the mycelial growth rate method showed that difenoconazole, prochloraz, and pyraclostrobin were the most effective, with EC₅₀ values of 0.06, 0.32, and 0.36 mg/L, respectively, whereas tetramycin and azoxystrobin exhibited lower inhibitory activity. This study reports <i>P. radicina</i> as the causal agent of <i>P. sibiricum</i> root rot for the first time, expands the known host range of this pathogen, and provides the first baseline data on its in vitro sensitivity to five fungicides. These results offer practical guidance for chemical control and further evaluation under greenhouse or field conditions in <i>P. sibiricum</i> cultivation.