First Report of Fusarium Wilt Tropical Race 4 in Cavendish Bananas Caused by Fusarium odoratissimum in Colombia

Abstract

Fusarium wilt of bananas, commonly called Panama disease, is caused by a suite of Fusarium species. Fusarium odoratissimum (previously known as Fusarium oxysporum f.sp. cubense) comprises Tropical Race 4 (TR4) (Maryani et al., 2018), which is highly aggressive on Cavendish bananas as well as many other banana varieties (Ploetz 2015). Since the 1990's TR4 has spread across Asia until it surfaced outside this region in Jordan in 2013 (Garcia-Bastidas et al., 2014). Subsequently, a succession of TR4 incursions in banana-growing regions in Asia, the Middle East, the Indian subcontinent, Africa and even Europe was reported (Zheng et al., 2018). Thus far TR4 was not reported in Latin America. Here, we report the occurrence of TR4 in samples originating from the department of Guajira in the northeast of Colombia, which is one of the leading global banana producing countries (~550.000 ha. banana and plantain production, with 66,000 ha. for export). Typical symptoms, such as wilting and chlorosis of leaves and vascular discoloration, were observed in at least hundred plants in June 2019 in four farms totaling more than 175 ha. This area was placed under quarantine by the Colombian Agricultural Institute (ICA) and a massive eradication effort was started. In total, 22 arbitrarily pseudostem samples were collected from symptomatic Cavendish plants from two farms located 3.8 km apart. These were processed for fungal isolation and characterization as described by Zheng et al. (2018). White fungal colonies developed from surface sterilized (1% NaClO) tissue on potato dextrose agar (PDA). Three single-spore isolates (Col 2, Col 4 and Col 17) that phenotypically resembled F. odoratissimum ( Maryani et al., 2018). Subsequently, these isolates were analyzed using three molecular diagnostics targeting two independent regions of the TR4 genome. The first region was assayed according to Dita et al., (2010) and with the commercial Clear®Detections TR4 kit. The second region was tested with a Loop‐Mediated Isothermal Amplification (LAMP) assay (Ordonez et al., 2019). These tests identified the three isolates as F. odoratissimum TR4 (Figure 1). Further analyses were undertaken by whole-genome sequencing using the Illumina platform (MiSeq Kit V3) and subsequent phylogenetic analyses using the TR4 reference (Foc II5, see PRJNA73539 and PRJNA56513, GenBank) as well as available additional Fusarium spp. (Asai et al., 2019; Ordonez et al., 2015; Maryani et al., 2018; Yun et al., 2019; Zheng et al., 2018). This confirmed that the three Colombian isolates firmly group together with other F. odoratissimum TR4 isolates (Figure 1). Finally, we tested whether the three isolates were able to cause Fusarium wilt in Cavendish. The pathogenicity assays and inoculum production were according to Garcia-Bastidas et al. (2019) and included water controls, a Race 1 isolate, which is not pathogenic on Cavendish, the three Colombian isolates Col 2, Col 4, and Col 17, and the II5 TR4 reference strain. Five plants per isolate were inoculated, all plants inoculated with the Colombian TR4 isolates and the II5 reference showed typical symptoms of Fusarium wilt after four weeks. After six weeks, internal symptoms were recorded (Figure 2), and affected tissue was collected from all plants for re-isolation of the causal fungus. All affected tissues and the re-isolated strains were positive with the aforementioned TR4 diagnostics. No isolates were recovered from water controls or plants inoculated with Race 1, which remained asymptomatic (Figure 2). Taken together, our results confirm the presence of TR4 in Colombia. Despite major efforts currently undertaken to cordon off the affected farms, this is an imminent threat for Colombia and the entire region.