First Report of Fusarium bubalinum Causing Stem Rot on Abelmoschus sagittifolius (Kurz) in Vietnam

In Vietnam, Abelmoschus sagittifolius (Sam Bo Chinh) is an increasingly important medicinal crop valued for its antioxidant, antidiabetic, and anticancer properties (Nguyen et al. 2024; Van Bay and Thong 2026). Its rapid industrialization into functional foods highlights the urgent need to protect this high-value species from emerging pathogens. In July 2025, during periods of high humidity in the rainy season, a severe stem rot outbreak was observed in A. sagittifolius plantations in Tra On, Vinh Long, Vietnam. Characteristic symptoms included irregular brown lesions on stems, accompanied by systemic wilting and chlorosis, while leaves remained attached to branches without abscission. To identify the causal fungal agent, branch tissue segments (3 × 3 mm) were excised from the margins of necrotic lesions, surface-sterilized in 70% ethanol for 1 min and 1% sodium hypochlorite for 2 min, rinsed three times with sterile water, and plated on potato dextrose agar (PDA) supplemented with streptomycin and ampicillin (100 µg/ml each). After incubation at 28°C for 7 days, a total of three isolates were obtained and purified using the single-spore isolation technique. A representative fungal isolate, SCT25, was selected for further analyses, and its morphological features were observed on PDA and carnation leaf agar (CLA). Colonies were white and cottony with pale-yellow centers and reached 62.5 mm in diameter after 7 days of incubation. Macroconidia were predominantly 3- to 5-septate, slightly curved, with foot-shaped basal cells and tapered apices, measuring 31.17 ± 4.79 × 4.84 ± 0.55 µm (length × width). Microconidia were oval to ellipsoidal, aseptate or one-septate. Chlamydospores were thick-walled, terminal or intercalary, and consisted of one to two cells. The morphological characteristics were consistent with those of species in the genus Fusarium (Xia et al. 2019). For molecular identification, the translation elongation factor 1-alpha ( tef1 ), the second largest subunit of RNA polymerase II ( RPB2 ), and calmodulin ( cmdA ) genes were amplified using primer pairs EF1/EF2 (Carbone and Kohn 1999), 7cf/11aR (O'Donnell et al. 1998; O'Donnell et al. 2009), and CAL-228-F/CAL2Rd (Yin et al. 2022), respectively. The resulting sequences were deposited in GenBank under accession numbers PZ436071 ( tef1 ), PZ436073 ( RPB2 ), and QB176273 ( cmdA ). Phylogenetic analyses of concatenated cmdA , RPB2 , and tef1 sequences and additional GenBank tef1 sequences supported the identification of SCT25 as Fusarium bubalinum . Pathogenicity tests were conducted on two-month-old A. sagittifolius plants using the cotton pad inoculation method. A 5-mL aliquot of conidial suspension (1 × 10 7 spores/mL) of isolate SCT25 was applied to surface-disinfested stems and incubated at 28°C for 7 days, while distilled water was used for the control group. Brown rot symptoms developed on inoculated stems within 14 days after inoculation and progressed to plant death by 21 days, whereas control plants remained symptomless. The fungus was consistently re-isolated from symptomatic tissues, and its identity was confirmed based on morphological characteristics and tef1 sequence analyses, thereby fulfilling Koch’s postulates. To our knowledge, this is the first report of F. bubalinum causing stem rot of A. sagittifolius in Vietnam. Given the economic importance of this crop, this report provides valuable information for the development of integrated disease management strategies.