First report of fusarium oxysporum f. sp. cubense tropical race 4 causing fusarium wilt to bananas in Ecuador

Banana (Musa spp.) is one of Ecuador's most important export commodities, valued at USD 3.6 billion in 2024, equating to nearly 3% of the national gross domestic product (GDP). The industry notably contributes to the advancement of the socio-economy of the country by employing around 2 million people throughout the supply chain (Sistema de Información Pública Agropecuaria 2026). In August 2025, typical vascular wilt symptoms were observed in three Cavendish banana plants on a 7-ha farm located in the Santa Rosa canton in El Oro province, Ecuador (3°27′12.22″ S, 79°55′55.86″ W). Affected plants displayed wilting and progressive leaf yellowing. Upon internal examination of the pseudostem, a distinct dark red to brown vascular discoloration was observed (Figure 1). Pseudostem samples were collected from two diseased plants, from which fungal isolations were made. Small pseudostem pieces where healthy and diseased tissue met were surface sterilized with a 70% ethanol solution for 30 s, plated onto potato dextrose agar (PDA) amended with 0.04 g/L streptomycin, and incubated for 7 days at 25°C. The two fungal colonies that developed, one from each diseased plant, were then purified, single-spored and deposited in the culture collection at Stellenbosch University (CAV 6135 and CAV 6136). Cultural and morphological characteristics of the two isolates were studied by growing the cultures on PDA and carnation leaf agar (CLA). White colonies with purple centers developed on PDA, while falcate macroconidia with two to five septa, abundant microconidia borne in false heads on short monophialides, and globose chlamydospores were observed on CLA after 10 days. Observed characteristics were typical of those of the Fusarium oxysporum species complex (Leslie and Summerell 2006). Molecular identification was then performed using a F. oxysporum f. sp. cubense tropical race 4 (Foc TR4)-specific polymerase chain reaction (PCR; Li et al. 2013) and two qPCR assays (Aguayo et al. 2017; Matthews et al. 2020). DNA extracted from the two Ecuadorian isolates was positively identified with amplification similar to that of the positive control. Vegetative compatibility (VCG) testing was also conducted by pairing nit mutants generated from the two Ecuadorian isolates with universal VCG testers of VCG 01213/16, and heterokaryon formation confirmed complementation (Leslie and Summerell 2006) (Figure 2). To confirm pathogenicity, the two Ecuadorian isolates and a reference strain of Foc TR4 (CAV 789, from Australia) were infected on 2-month-old Cavendish banana plants. Three plants were inoculated per isolate, and the experiment was repeated. During inoculation, roots were immersed in a conidial suspension (1 × 106 conidia/mL) for 2 h, while the negative control plants were treated with sterile water. Plants were planted in a 1:1:1 soil:sand:peat substrate and kept under controlled environmental conditions (27°C, 60% relative humidity). Within 4 weeks, the Foc TR4-inoculated plants developed typical Fusarium wilt symptoms, including foliar yellowing and internal rhizome necrosis. Negative control plants remained symptomless (Figure 3). The pathogen was re-isolated from symptomatic tissues and identified as Foc TR4 with a loop-mediated isothermal amplification assay (Ordóñez et al. 2019), thus fulfilling Koch's postulates and confirming the presence of the pathogen in Ecuador. The detection of Foc TR4 represents a significant threat to banana production in Ecuador and highlights the importance of continued surveillance and containment efforts.