First Report of Fusarium oxysporum f. sp. cubense tropical race 4 (VCG 01213/16) causing Fusarium wilt on banana in Bangladesh

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Fusarium oxysporum f. sp. cubense tropical race 4 (Foc TR4) is a major constraint to global banana production (Viljoen et al., 2020). The pathogen was first introduced into the Indian subcontinent in 2015, where it since caused significant damage to local banana production in India and Pakistan. In Bangladesh, banana is an important cash and food security crop. Between January and February 2025, typical Fusarium wilt symptoms which included progressive leaf yellowing and reddish brown discoloration of pseudostem and inner rhizomes were observed in five districts of Bangladesh including Tangail (24°14'59.4"N, 89°54'59.6"E), Rangpur (25°44'47.90"N, 89°15'5.98"E), Bogura (24°51'03.5"N, 89°22'15.9"E), Narsinghdi (23°55'14.5812''N, 43'7.7196''E) and Jamalpur (24°55'10.74"N, 89°56' 53.23"E). The disease was observed on Cavendish cultivars (AAA) such as Sagor, Nepali Sagor, Mehersagar and on local varieties Chinnichampa (AAB) and Sabri (AAB). Twenty-one pseudostem samples were collected from discolored vascular tissue to identify the causal agent. Vascular strands were surface disinfected with 70% ethanol and plated on potato dextrose agar (PDA) with streptomycin sulfate. After 5 days, developing fungal colonies were then purified and single-spored and maintained in the culture collection of the Bangladesh Agricultural University. The cultures were then further identified morphologically, with molecular detection methods and with vegetative compatibility group (VCG) analysis. Isolates were confirmed as F. oxysporum based on cottony white to pinkish colony growth on PDA and round to oval shaped microconidia, curved shaped macroconidia and chlamydospores on carnation leaf agar (Leslie and Summerell, 2006). For molecular identification, DNA was extracted and amplified with Foc TR4-specific primers (Li et al. 2013). Foc TR4 positive isolates were then sent to Stellenbosch University for VCG confirmation, where nit mutants were generated, and VCGs were confirmed based on the formation of heterokaryons with universal VCG 01213/16 tester strains (Leslie and Summerell, 2006). Pathogenicity testing of 19 selected Foc TR4 isolates was conducted on 2-month-old, tissue culture derived Cavendish banana plantlets. Ten plants per isolate were replanted into soil mixed with millet seeds colonized with Foc TR4 isolates (Ndayihansamaso et al. 2022). Control plants were treated with sterile distilled water. Plants were controlled at 27°C with 60% relative humidity. Within four weeks, inoculated plants developed typical Fusarium wilt symptoms, including progressive foliar yellowing and internal rhizome necrosis, while control plants remained asymptomatic. Pathogenicity testing was replicated. Koch’s postulates were completed by reisolating F. oxysporum from disease tissue and identification in Foc TR4-specific PCR (Dita et al. 2010) and qPCR (Matthews et al. 2020) assays. Foc TR4 poses a serious threat to banana production and the livelihoods of people of Bangladesh, considering the importance of the crop for income and food security. Local authorities have already begun to destroy infected plants in affected areas and initiated an extensive delimiting survey in other banana-producing regions to determine the distribution of Foc TR4. Public awareness campaigns, risk assessment and containment efforts will be initiated. The susceptibility of economically important local varieties will also be assessed.

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