Dissemination of Fusarium Wilt of Banana in Mozambique Caused by Fusarium odoratissimum Tropical Race 4

Abstract

Fusarium wilt of banana (FWB) is a serious soil-borne fungal disease. In the previous century, FWB already destroyed Gros Michel-based banana cultures in Central America, and currently, the disease threatens all major banana-producing regions of the world. The causal agents of these epidemics, however, are diverse. Gros Michel was infected by a wide range of Fusarium species, the so-called Race 1 strains, whereas the contemporary Cavendish-based cultures are affected by Fusarium odoratissimum, colloquially called Tropical Race 4 (TR4). TR4 was reported in Mozambique on two commercial banana farms in 2013, but no incursions were found outside the farm boundaries in 2015, suggesting that the disease was under control. Here we report the presence of TR4 outside of these farm boundaries. We obtained fungal samples from 13 banana plants in smallholder and roadside plantings at various locations throughout northern Mozambique. These samples tested positive for TR4 by molecular diagnostics and in greenhouse pathogenicity assays. The results were confirmed with re-isolations, thereby completing Koch’s postulate. To study the diversity of TR4 isolates in Mozambique, we selected five samples for whole-genome sequencing. Comparison with a global collection of TR4 samples revealed very little genetic variation, indicating that the fungus is clonally spreading in Mozambique. Furthermore, isolates from Mozambique are clearly genetically separated from other geographic incursions, and thus we cannot trace the origin of TR4 in Mozambique. Nevertheless, our data demonstrate the dissemination of TR4 in Mozambique, underscoring the failure of disease management strategies. This threatens African banana production.

Uncontained spread of Fusarium wilt of banana threatens African food security

“…During surveys in 2015, no suspicious wilting symptoms were detected outside the farms; hence, TR4 was declared to be under control [ 28 ]. However, recently, wilting symptoms were observed outside the farm boundaries and subsequent analyses confirmed the dissemination of TR4 to other distant locations ( Fig 1 ) [ 30 ]. A comparison of 5 fungal strains, isolated from FWB symptomatic banana plants at various locations, to the TR4 II5 reference isolate clearly confirmed TR4 as the causal organism [ 30 ].…”

“…However, recently, wilting symptoms were observed outside the farm boundaries and subsequent analyses confirmed the dissemination of TR4 to other distant locations ( Fig 1 ) [ 30 ]. A comparison of 5 fungal strains, isolated from FWB symptomatic banana plants at various locations, to the TR4 II5 reference isolate clearly confirmed TR4 as the causal organism [ 30 ]. The analyzed isolates show little genetic diversity [ 30 ], suggesting that local transmission occurs through a single clonal lineage.…”

“…A comparison of 5 fungal strains, isolated from FWB symptomatic banana plants at various locations, to the TR4 II5 reference isolate clearly confirmed TR4 as the causal organism [ 30 ]. The analyzed isolates show little genetic diversity [ 30 ], suggesting that local transmission occurs through a single clonal lineage. However, the isolates could not be linked to other worldwide reported TR4 incursions since sequencing data typically only include isolates from the first official disease reports that lack sampling depth to address local diversity and dissemination ([ 30 ]; S1 Dataset).…”

“…The analyzed isolates show little genetic diversity [ 30 ], suggesting that local transmission occurs through a single clonal lineage. However, the isolates could not be linked to other worldwide reported TR4 incursions since sequencing data typically only include isolates from the first official disease reports that lack sampling depth to address local diversity and dissemination ([ 30 ]; S1 Dataset). Consequently, comparative analyses of the genomes from new versus previous incursions enables only provisional associations.…”

“…Consequently, comparative analyses of the genomes from new versus previous incursions enables only provisional associations. The accumulation of unique genetic variation across TR4 strains in Mozambique suggests an extended time of local spread [ 30 ]. We, therefore, can neither robustly link the origin of TR4 in Mozambique with incursions in other countries, nor declare the newly discovered TR4 strains as independent novel incursions.…”