Atractylodes macrocephala is a perennial medicinal herb widely cultivated in China, valued for its diverse pharmacological properties, including anti-inflammatory, antitumor, gastroprotective, and metabolic regulatory activities (Gu et al. 2020). In August 2025, a previously undescribed rhizome rot disease was observed in a cultivation field in Luyi County, Zhoukou City, Henan Province, China (33.8076°N, 115.5368°E). The disease incidence was approximately 6.5%, affecting an estimated 800 plants. External symptoms consisted of irregular brown lesions covered with white filamentous mycelium. Cross-sections of symptomatic rhizomes revealed grayish discoloration of the internal tissues and emitted a distinct sour odor. To isolate the causal agent, symptomatic tissue pieces (~5 mm in diameter) were surface disinfected with 75% ethanol for 30 s, followed by 4% NaClO for 2 min, rinsed three times with sterile water, and placed on potato dextrose agar (PDA) plates amended with streptomycin sulfate (250 mg/L). After incubation at 28°C in darkness for 3 days, 15 morphologically similar fungal isolates (BZSF1 to BZSF15) were obtained by single-spore isolation. Colonies on PDA plates were circular, flat, and creamy white, with short velvety to nearly powdery mycelium arranged radially. Microscopic observation revealed septate, colorless hyphae with dichotomous branching. Mature hyphae fragmented into cylindrical, elliptical, or subglobose arthroconidia that were primarily unicellular, hyaline, and translucent, occasionally forming short chains. Conidia measured 2.86–12.31 ×2.41–5.80 µm (n=20). These morphological characteristics matched those of Geotrichum candidum (De Hoog et al. 1986). For molecular identification, the internal transcribed spacer (ITS) region and translation elongation factor 1-alpha (TEF‑1α) gene of a representative isolate BZSF1 were amplified using primer pairs ITS1/ITS4 and TEF‑728F/TEF‑1567R, respectively (Glass and Donaldson 1995; Carbone and Kohn 1999). BLASTn analysis of the obtained ITS (PX952898, 378 bp) and TEF‑1α (PX965723, 642 bp) sequences showed 99.73 and 100% identity to reference sequences of G. candidum (KJ817904 and MT346368, respectively). Phylogenetic analysis based on concatenated ITS and TEF‑1α sequences further confirmed that isolate BZSF1 clustered within the G. candidum clade. Pathogenicity tests were performed by irrigating healthy seedlings with a conidial suspension (1×106 conidia/mL), while control seedlings were treated with sterile water. All plants were incubated under controlled conditions (28 °C, 60 ±10% RH, 8 h light/16 h dark). The experiment was repeated three times. By 20 days post-inoculation (dpi), white mycelium developed on the epidermis of inoculated roots, accompanied by internal graying and a sour odor, matching field symptoms. Control roots remained asymptomatic. G. candidum was re-isolated from the infected roots, fulfilling Koch’s postulates. G. candidum is known to cause sour rot in various crops, including tomato, kiwifruit, and others (Wang et al. 2025; Cheng et al. 2021). Among medicinal plants, it has previously been reported only on Pseudostellaria heterophylla and Aloe vera (Chen et al. 2021; Fu et al. 2025). To our knowledge, this is the first report of G. candidum causing sour rot on A. macrocephala in China, an emerging threat that requires urgent research on host resistance, fungicide efficacy, and cultural control measures.
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