First Report of <i>Fusarium solani</i> Causing Root Rot on <i>Amomum tsaoko</i> in China

Amomum tsaoko Crevost et Lemaire, a herbaceous species of the genus Amomum in the family Zingiberaceae, is primarily distributed in the Yunnan, Guangxi, and Guizhou provinces of China. The fruit of A. tsaoko has been widely used as both a spice and a traditional medicinal resource in China (Yang et al., 2022). In October 2024, an outbreak of root rot was observed affecting A. tsaoko plants in Nujiang Prefecture, Yunnan Province. The disease incidence and disease severity index were approximately 20% and 30, respectively. The affected plants displayed brown, water-soaked necrosis and decay of the roots, accompanied by wilting and yellowing of the aerial parts. To isolate and identify the causal agent, 5 × 5 mm sections of symptomatic root tissues were surface-sterilized with 75% ethanol for 30 s, followed by 1% NaOCl for 180 s, and rinsed three times with sterile water. The samples were then placed on potato dextrose agar (PDA) and incubated at 28°C in the dark for three days. Fungal isolates were purified by single-spore isolation, yielding four morphologically uniform isolates. The colonies exhibited dense, white mycelia and a cream-colored reverse. Microconidia were elliptical to long-oval, with 0 to 1 septa, and dimensions of 1.6 to 3.6 × 5.7 to 10.2 μm (average 2.5 × 7.3 μm, n = 30). Macroconidia were sickle-shaped, with 4 to 5 septa, and dimensions of 2.5 to 3.9 × 16.3 to 26.8 μm (average 3.0 × 22.1 μm, n = 30). Chlamydospores were nearly spherical, smooth-walled, with an average diameter of 4.3 μm (n = 30). For molecular identification, isolate AtF01 was randomly selected for further analysis. The nuclear ribosomal internal transcribed spacer (ITS) region, translation elongation factor 1-alpha (TEF1), and the second largest subunit of RNA polymerase II (RPB2) genes were amplified using primer pairs ITS1/ITS4 (White et al., 1990), EF1-728F/EF1-986R (Carbone and Kohn, 1999), and fRPB2-5F/ fRPB2-7Cr (Liu et al., 1999), respectively. BLASTn analysis of the ITS (accession no. PX242807), TEF1 (PX247234), and RPB2 (PX395932) sequences of AtF01 showed 99.29%, 99.37%, and 99.42% identity to the corresponding sequences of Fusarium solani (EU719658, DQ247541, EF469984), respectively. A phylogenetic tree was constructed based on concatenated nucleotide sequences of ITS, TEF1, and RPB2 using the maximum likelihood method under the Tamura-Nei model. The phylogeny revealed that AtF01 clustered closely with F. solani. Both morphological and molecular characteristics confirmed the identity of AtF01 as F. solani. To confirm pathogenicity, wounded roots were inoculated with a conidial suspension (1 × 106 conidia/ml) for 1 hour. Control plants were wounded and treated with sterile water. All plants were maintained in a growth chamber at 28℃ with a 12-h light/12-h dark photoperiod and 80% relative humidity. Each treatment included three replicates, with three plants per replicate. After 15 days, plants inoculated with AtF01 exhibited leaf yellowing, wilting, and root rot. These symptoms were similar to those originally observed on A. tsaoko in the field, while control plants remained symptomless. The fungus was re-isolated from infected tissues and was confirmed to be identical to the original inoculum through morphological and ITS sequence analysis, thereby fulfilling Koch’s postulates. F. solani has previously been reported to cause root rot on Coptis chinensis, Angelica acutiloba, and Paris polyphylla. To our knowledge, this is the first report of F. solani causing root rot on A. tsaoko in China. As Yunnan is a core production region for A. tsaoko, it is imperative to implement enhanced monitoring and management strategies for this disease.