First Report of Soft Rot on Corydalis yanhusuo Caused by Pectobacterium versatile in China

Corydalis yanhusuo (Yanhusuo) is a traditional Chinese medicinal herb. From March to May 2023 and 2024, a previously unrecorded soft rot was observed in Panan County, Zhejiang Province, China (N29°12', E120°32'), with disease incidence of 10–50% (up to 80% in some fields). Initial symptoms were water-soaked spots on leaves that expanded into dark green, water-soaked lesions, causing leaf rot. Lesions extended to stems via petioles, leading to plant death. Small pieces (3 × 5 mm) from lesion margins were surface-sterilized, homogenized in sterile water, and streaked on LB agar. Four representative strains (YH01–YH04) were isolated and stored in 25% glycerol at -40°C. Colonies were circular, smooth, opaque, and yellowish to cream. Transmission electron microscopy revealed rod-shaped cells (1.5–2.0 × 0.8–1.1 µm) with peritrichous flagella. Fatty acid methyl ester (FAME) profiles (TSBA library v6.0) showed major peaks for 12:0, 14:0, 16:0, 16:1 ω7c/16:1 ω6c, and 18:1 ω7c, with absence of 13:0 2OH, consistent with Pectobacterium spp. (Cigna et al. 2021). The 16S rRNA gene sequences (~1,400 bp, amplified with primers 27F/1492R) (OQ568819–OQ568822) were ≥99% identical to Pectobacterium versatile (CP021894.1, CP065143.1). Multilocus sequence analysis (MLSA) of recA (Waleron et al. 2002), leuS (Portier et al. 2019), and dnaX (Slawiak et al. 2009) genes (GenBank accessions provided in Table S1) placed all four strains in a clade with P. versatile (82% bootstrap support). In pathogenicity tests, wounded leaves of cv. 'Zhehu No. 1' inoculated with 20 µL bacterial suspension (≈1×10⁸ CFU/mL) developed water-soaked lesions within 48 h (17–20°C), and lesions turned dark green with soft rot by 5 days; controls remained healthy. Unwounded spray inoculation with YH01 caused water-soaked spots at 10 days post-inoculation and severe rot by 20 days. Pectobacterium versatile has been reported on diverse hosts (Portier et al. 2019; Ma et al. 2021) but not on Papaveraceae. To our knowledge, this is the first report of P. versatile causing soft rot on C. yanhusuo, expanding its known host range.