First Report of <i>Fusarium suttonianum</i> Causing Root Rot on <i>Curcuma kwangsiensis</i> in Guangxi, China

Curcuma kwangsiensis, a perennial herb of the Zingiberaceae family, is widely cultivated in southern China for its medicinal roots (rhizomes and tuberous roots). However, disease problems have become increasingly severe with the expansion of cultivation areas. In September 2025, severe root rot was observed in approximately 6.67 hectares of C. kwangsiensis plantations in Guangxi, China, with disease incidence reaching up to 80% in continuous cropping fields. Initial symptoms included leaf yellowing and wilting, followed by plant death. Roots initially developed local necrotic lesions, which gradually expanded into dark brown rot, ultimately leading to complete disintegration of the root system. Typical diseased plants were collected for pathogen isolation and identification. Small root segments (approximately 5 × 5 mm) were excised from lesion margins, surface-sterilized with 75% ethanol for 30 s and 2% NaOCl for 2 min, rinsed five times in sterile water, dried, and placed on potato dextrose agar (PDA) at 28°C in the dark. Six morphologically consistent isolates were obtained. Colonies produced abundant white, fluffy aerial mycelia with white or pale-yellow reverse. Macroconidia were sparse, hyaline, falcate or slightly curved, 1–3 septa, measuring 20.6–42.3 × 3.6–6.1 μm (n=50). Microconidia were abundant, hyaline, falcate, ovoid to reniform, 0–1 septa, 7.2–16.1 × 2.8–6.7 μm (n=50), and formed in false heads on monophialides. Chlamydospores were globose, smooth-walled, single or in pairs, 6.7–8.1 μm in diameter. Based on these morphological characteristics, the fungus was preliminarily identified as Fusarium sp. For molecular identification, genomic DNA of representative isolate Fsu1 was extracted. The internal transcribed spacer (ITS), translation elongation factor 1-α (TEF1-α), and RNA polymerase II second largest subunit (RPB2) genes were amplified using primers ITS1/ITS4, EF-1H/EF-2T, and RPB2-5F2/RPB2-7cR, respectively (O'Donnell et al. 2022). BLAST analysis showed that all obtained sequences shared &gt; 99% homology with Fusarium suttonianum. Specifically, the ITS, TEF1-α, and RPB2 sequences of isolate Fsu1 (GenBank accession nos. PZ176576, PZ211876, PZ211877) shared 100%, 99.89%, and 100% identity to F. suttonianum, respectively. Maximum likelihood phylogenetic analysis based on the concatenated sequences showed that isolate Fsu1 clustered with the type strain of F. suttonianum in the same clade. For pathogenicity test, five healthy C. kwangsiensis seedlings were slightly root-wounded, and 50 mL of spore suspension (1×10⁶ conidia/mL) was inoculated into the soil around roots. Controls were treated with sterile water. All plants were grown in a greenhouse. After 30 days, all inoculated plants developed symptoms identical to field observations, including leaf yellowing, wilting, and dark brown root rot, while controls remained asymptomatic. The fungus was re-isolated from diseased roots and identified as F. suttonianum by morphological and molecular analyses, fulfilling Koch's postulates. F. suttonianum has been reported causing root rot on various plants, including Plukenetia volubilis, Phaseolus vulgaris, and Vigna unguiculata (Delgado-Mera et al. 2025, Sousa et al. 2025). To our knowledge, this is the first report of F. suttonianum causing root rot on C. kwangsiensis in China. This finding provides a foundation for developing effective disease management strategies for C. kwangsiensis production.