Uncontained spread of Fusarium wilt of banana threatens African food security

Why is banana among the most vulnerable crops?Banana is the most popular fruit worldwide [1] and a major staple food in tropical and subtropical regions where the majority of bananas is produced (Fig 1) [2].The importance of banana for food security is particularly relevant for East Africa (Burundi, Congo, Rwanda, Tanzania, and Uganda).Here, the East African Highland bananas (EAHBs) are the crucial cash crops and staple food for millions of people with the world's highest per capita banana consumption of 400 to 600 kg [3].Throughout this region, banana cultivation is embedded in complex mixed cropping systems by numerous small-scale farmers and households [4].Most edible bananas are seedless parthenocarpic diploids and triploid hybrids derived from the wild banana species Musa acuminata and Musa balbisiana [5].Although the wild, seeded bananas are genetically very diverse [5], the domestication of seedless and hence edible banana varieties resulted in a genetic bottleneck that limits genetic variation [5].On the local market, different clonal banana varieties are sold, in contrast to the global banana trade that is dominated by clonal Cavendish varieties [6].These large banana monocultures are extremely vulnerable to numerous diseases [7].A major concern for banana production is Fusarium wilt, a devastating vascular disease that withers banana plants (Fig 1).It is caused by soil-borne fungi belonging to the Fusarium oxysporum species complex.Strains that are able to infect banana were known as F. oxysporum f.sp.cubense (Foc), despite their well-known diversity [8].Recently, genotyping analyses confirmed several genetically distinct Foc lineages that were consequently recognized as individual Fusarium species [9].For instance, the Foc Race 1 strains, which drove a major epidemic of Fusarium wilt of banana (FWB) that almost eradicated the Gros Michel variety dominating the banana trade up to the 1960s in the last century [8], actually comprise a suite of different Fusarium species [9].Eventually, the resistant Cavendish clones saved the industry, and nowadays, 98% of the export market depends on them [6].However, already in 1967, FWB affected Cavendish in Taiwan [8] (Fig 1).The causal Fusarium strain, referred to as Tropical Race 4 (TR4), has recently been described as the new species, Fusarium odoratissimum [9].This modified nomenclature of FWB causing Fusarium spp. raised some controversy [10], and therefore awaits additional conclusive data.Most experts, however, agree that TR4 is a clonal lineage and genetically so dissimilar from other banana infecting Fusarium spp. that it is justifiably recognized as a new species.Next to Cavendish cultivars, TR4 affects a wide range of banana germplasm, including locally important varieties, such as the aforementioned EAHBs [11].The latter are essential for food security in the African Great Lakes Region where banana is a