Most modern cultivated varieties (cultivars) of edible bananas originated from two wild, seeded banana ancestors; Musa acuminata and Musa balbisiana. Banana is plagued by several major diseases of which, arguably, Fusarium wilt caused by infection with Fusarium oxysporum f. sp. cubense (Foc) is one of the most destructive. The disease can cause up to 100% yield loss and has the capacity to decimate commercial banana cultivation. Fusarium wilt caused by Foc tropical race 4 (TR4) is estimated to threaten >80% of the world’s banana crop, including cooking banana cultivars that provide food for approximately 400 million people. TR4 is a single clonal line of Foc, which has been disseminated by humans from its origin in South East Asia to distances as far as Jordan and Mozambique. Since Foc is a vascular pathogen, once infection is detected, there is no conventional way of combating the disease.
Traditional breeding of banana is expensive, time- and space-consuming. The key goals of this project was to create a resistance against Fusarium wilt in banana. To do so, I first established a greenhouse pathogenicity assay using small banana plantlets that allows quick resistance screening even in small greenhouses in temperate countries. We will publish this method open access so that it can replace current, cumbersome pathogenicity testing, which requires bigger plants and therefore lots of greenhouse space. Genetic engineering of existing cultivars is one solution to combat current threats of banana production. Initially, we planned to introduce a resistance gene and inactivate a susceptibility gene for Foc TR4 to create resistance. Neither of these strategies turned out to be perfect. In contrast to the interaction of tomato with F. oxysporum f. sp. lycopersici, the receptor kinase encoding susceptibility gene does not seem to be a susceptibility gene for banana, since the Foc effector peptide RALF that is targeting this receptor kinase is not a strong virulence factor for Foc. We are currently preparing the manuscript for submission to Molecular Plant Pathology. Likewise, the tomato resistance protein I-3 seems to requires additional tomato factors and is therefore not suitable for transfer in banana. Since, genetically engineered bananas might have faced difficulties with consumer acceptance and would have been difficult to de-regulate, we decided to change our strategy and to explore biologicals as a means to control Fusarium wilt disease.
Biologicals are antagonistic microbial strains or molecules. They are a promising solution to control Fusarium wilt, especially for smallholder farmers, because they are inexpensive and offers potentially season-long disease control. In collaboration with a US biotech company, I successfully identified a bacterial strain that is able to inhibit Foc growth. If this strain is active in field trials, it could become a game changer in the control of banana Fusarium wilt.
