First Report of <i>Fusarium proliferatum</i> Causing <i>Bupleurum chinense</i> root rot in China

Bupleurum chinense, commonly known as “chai hu,” is an herbaceous plant known for its medicinal properties and is distributed throughout China (Ke et al. 2020). In August 2025, root rot was observed on B. chinense plants in Handan (36.55°N, 113.26°E) , Hebei Province, China (incidence rate 65%), plants in 250 ha were observed to be severely affected by a disease, causing a yield loss of 65%. The irregular brown lesions appear on the root which cracks longitudinally and may extend into the xylem tissue. Severe cases result in rotting of the root phloem, leaving only black vascular bundles. To isolate the causal agent, tissues (5×5 mm) from four symptomatic plants were removed from the border of lesions, surface sterilized in 75% ethanol for 30 s and 0.1% HgCl2 for 1 min, then rinsed three times with sterile distilled water, plated on potato dextrose agar (PDA) at 25℃, and incubated in the dark for 7 days. Two representative isolates, 202501 and 202502, were cultivated on PDA. Macroconidia of the isolates had three to five septa, and were slender, thin-walled, relatively straight, apical cell were curved. Microconidia were club shaped with a flattened base and 0-septate. Aerial mycelium were found in chains of varying, but usually moderate, length, false heads, or aggregates of a few microconidia. Conidiogenous cells were monophialides and polyphialides (Leslie and summerell 2006). The translation elongation factor (TEF-1α), and partial RNA polymerase second largest subunit (RPB2) were amplified (accession nos. TEF-1α: PX061601 and RPB2: PX549684) (Crous et al. 2009). When compared with other Fusarium species in GenBank, the isolate exhibited 100% (TEF-1α, MH535857) and 100% (RBP2, MT305154) similarity with Fusarium proliferatum (query coverage=100%, per-centage identity=100%). A phylogenetic tree was constructed in MEGA software (version 11.0.10) (Tamura et al. 2021) using the partial concatenated gene sequences. Maximum likelihood analysis revealed that the isolates were closely related to F. proliferatum (99% bootstrap). To test pathogenicity, a conidial suspension of 202501 and 202502 with a concentration of 1×106 conidia/mL was inoculated on 36 plants per isolate. For the control treatment, 36 plants were treated with an equivalent volume of sterile water. The experiment was repeated three times. The plants were placed in a greenhouse from 26 to 30℃ and 95% relative humidity. The typical symptom were observed 10 days after inoculation, except on the control samples where no symptoms were observed. The same fungus was successfully reisolated from the symptomatic plant tissue and was reidentified as F. proliferatum through morphological characteristics, TEF-1α and RBP2 sequencing analysis (accession nos. TEF-1α: PX780488 and RPB2: PX780487), thereby fulfilling Koch’s postulates. While F. proliferatum has been reported to cause corn sheath rot in China (Xu et al. 2008), this is the first study to report that F. proliferatum can infect B. chinense in China. The finding is greatly significant for preventing and controlling root rot in B. chinense and reducing economic losses for farmers.