Fungal disease casts a long shadow over banana production worldwide, with Fusarium wilt perhaps the most well-known threat. Its spread is prolific, with projections that it could hit 17% of the global banana growing area by 2040, potentially resulting in $10 billion of losses.
Although banana research has historically trailed well behind that for other staple crops, this is, perhaps unsurprisingly, changing fast. The race to stop Fusarium in its tracks is well and truly on .
The pathogen Fusarium oxysporum forma specialis Cubense is behind what is fast becoming this global problem. It affects the roots of banana plants, and has a long history. Present-day problems are caused by a genetic subtype known as tropical race 4, or TR4. It was first seen outside Southeast Asia in 2014, and has since spread to 17 more countries, with incursion into all banana-producing areas now more or less inevitable.
The scale of the problem is driving a search for novel control options, as is resistance to a range of commercial fungicides on the market – albeit the issue of resistance is nuanced. Work by University of Exeter researchers showed that resistance is only to classes of fungicide that target a single site on the fungus, and that some of those that target multiple sites can still prove effective.
The global crop protection giant Syngenta has also recently created a product based on a new active ingredient, tymirium, which has efficacy against the disease. But it is no ‘magic bullet’, and as is the case with many pathogens and pests which threaten valuable crops, it is more likely that integrated approaches, employing adapted versions of old techniques and contemporary innovations, could bring the most telling results.
Better breeding
Beyond fungicides, genetics is very likely to play a major role — but here, there are different schools of thought.
The one put forward by Professor Gert Kema and his team at Wageningen University in the Netherlands is that after years of the globally dominant Cavendish banana proving resistant to previous strains of Fusarium, the genetic uniqueness of TR4, and Cavendish’s susceptibility to it, means it’s time for a change.
“Banana has, I think, one of the last and worst examples of a global monoculture. But you can crash the monoculture by simply delivering new, diverse, genetically diverse germplasm to the market. We as customers are not used to that, because everything we buy is Cavendish,” Kema said.
“Our ultimate goal is to replace Cavendish [with] a suite of new, innovative, genetically diverse varieties. There, you address the underlying issue, and that’s genetic uniformity. We don’t have any principal objection to genetic modification or genome editing, on the contrary, but for us, it’s more a tool for gene validation.”
The team has sequenced a broad range of wild banana varieties to find resistance genes, as well as hundreds of strains of both Fusarium and Mycosphaerella fijiensis, the fungus that causes black sigatoka, the other of the ‘big two’ banana pathogens. With support from banana giant Chiquita, Kema’s team has made rapid progress: the first potential replacements of Cavendish are due in 2028.
There are still many knowledge gaps, Kema admits. Having started his career working with wheat, he finds the difference in resources and data for banana research stark.
“For wheat, there is a global community. There are many research programs. There is a thick book that describes all the validated genes in wheat. In banana, you can count them on two hands, that’s it,” he explained. “We are way, way behind compared to the leading staple crops, but banana is a very important staple, particularly in developing economies.”
Time for an edited ‘Cavendish 2.0’?
The other route for using genetics to save banana production is through gene-editing the Cavendish banana to continue its cultivation. This is the option explored by Prof James Dale’s team at the Centre for Tropical Crops and Biocommodities in Australia.
After showing the potential of inserting genetic material from wild type bananas and nematodes into Cavendish bananas to bring about TR4 resistance in 2017, the work has developed further to include gene-editing using CRISPR-Cas9 technology. Research using the ‘Williams’ cultivar of Cavendish found targeting the banana’s Phytoene desaturase gene to be “highly effective”.
This later prompted an international team of researchers to calculate the economic damage which could be caused by delays in rolling out such a potentially efficacious approach, citing strong global consumer benefits and pointing to exemplar bottlenecks such as Europe’s current reticence to unleash gene-edited crops. According to their work, a five-year delay in getting gene-edited bananas to market could result in billions of dollars in global industry losses.
With those sorts of figures in mind, there have been moves towards making the gene-edited banana mainstream of late. Australia and New Zealand have approved the commercialisation of the resistant QCAV-4 line —which is likely to prove the first of many such approvals around the world in due course.
Microbial matters
Despite breeding and modifying bananas being very much at the forefront of the scientific effort to reign in the disease, they cannot sit in isolation to a broader consideration of the soil where the fungal diseases reign. The microbiome associated with bananas offers a particularly rich area for exploration.
“Fusarium is a soil borne fungus, and so you have to deal with all that complexity around the rhizosphere, and how can you engineer that towards a more sustainable production,” said Kema. “I think that is very promising. It still requires a lot of research, but the speed of discovery is incredible.”
One option, described as “a paradigm shift” by a team of Indian experts, is to ‘harden’ commercial banana strains using beneficial microbes from wild types. Specifically, the microbes are endophytes, which live within plant tissue without causing harm, and the process involves inoculating banana plants during propagation.
Studies have shown that such inoculation with Bacillus subtilis strains and Pseudomonas fluorescens can work against Fusarium wilt in the field and can have added beneficial effects both in terms of plant growth and action against other diseases. Despite the great progress, though, researchers have warned that there is still much unknown about the exact mechanisms of endophyte-induced resistance to the disease.
“Comprehending these defence mechanisms, which offer resistance against root-infecting vascular phytopathogens, presents an opportunity for improved wilt disease control without disrupting the natural defence-related pathways,” the Indian team wrote in the journal The Microbe.
The soil in which bananas are grown appears to be key in defining the role of the microbiome in the fight against the disease. Work comparing banana plantations with and without added phosphorus have shown that the soil nutrient makes a big difference, with higher phosphorus meaning lower disease prevalence and a more active microbiome.
Biopesticides and soil shifts
To potentially add to future integrated control strategies, there have also been some promising findings from research exploring compounds outside the realm of conventional fungicides.
Researchers have demonstrated that specially engineered guanidine salts can suppress F. oxysporum f. sp. Cubense over the long term in soil, with low environmental toxicity. Silver nanoparticles, an even more advanced potential addition for below ground-level, have fared well against Ecuadorian strains of Fusarium in initial testing.
Elsewhere, other scientists have explored the potential of plants used in Chinese herbal medicine. They found that four stand out from the bunch and in particular, Chinese rhubarb (Rheum palmatum) shows the strongest activity against TR4.
Multiple measures are likely to be applied together against the disease, and work in China has shown that the bacterium Bacillus velezensis EB1 suppresses the fungal pathogen effectively in tandem with the compound potassium sorbate. For a potential solution from a perhaps-unlikely location, scientists have also found a number of actinobacteria from Antarctic soil which have potential in F. oxysporum f. sp. Cubense control. Answers from under the ice? Watch this space.
Similarly to genetic technologies, biological protection products are nearing commercial viability, including what is described as a two-in-one “bio vaccine” against TR4 and black Sigatoka. There are certainly some decent performance claims on the back of product testing, but the coming years will be the real test of its efficacy as it rolls out in the field. Interestingly, the company behind it is also launching their own mutant Cavendish variety with claims of TR4 resistance and fast growth.
Supporting banana science
Breakthroughs are coming thick and fast. Chinese researchers have just identified a new banana defence mechanism against Fusarium wilt regulated by a gene known as MaHOS15. However, with this being emerging knowledge, it is likely to be a while before any applied approaches using this genetic target are developed.
Whether there’s a new banana on the block or the Cavendish gets a new lease of life, new weapons for banana growers against a familiar threat appear to be just around the corner. Yet to truly get on top of present and future disease threats to banana production, more support is needed, Kema stressed.
“There have been investments in research, but it’s still not enough,” he added. “I would really plea for more attention for this problem in this very important crop, to make sure that there will be sufficient funding to really address, not necessarily the ‘nice to knows’, there are many of them, but the very crucial ‘need to knows’ that we still have to address.”
