By Cristina Vuolo
The re-emergence of a Fusarium Wilt is threatening the banana industry once again, as a new strain overcomes the resistance of the cultivar that saved this billion-dollar industry from collapse just over half a century ago (Pérez-Vicente, Dita & Einar, 2014). During the mid-20th century, the commercial industry was dominated by the Gros Michel banana but today, the Cavendish banana populates supermarket shelves and accounts for around 47% of global production (FAO, 2020). In the 1950s and 60s, Gros Michel plantations were decimated by the soil-borne fungus Fusarium oxysporum f. sp. cubense (Foc). However, on discovering the resistance of Cavendish bananas to Foc Race 1, production was shifted to the resistant cultivars and the effects of the disease on the industry were overcome. Unfortunately, a new strain called Tropical Wilt 4 (TR4) has emerged and overcome Foc resistance in Cavendish plants. Worryingly, more than 80% of global banana production is susceptible, including many other cultivars which are relied on by smallholder farmers for local consumption and income generation (Pérez-Vicente, Dita & Einar, 2014).
TR4 is an aggressive strain of Fusarium Wilt that has caused production losses of hundreds of millions of dollars. It can survive in the soil for more than 20 years, travelling on planting material, water, soil particles, tools footwear and machineries (Pérez-Vicente, Dita & Einar, 2014). First observed on plantations in Malaysia and Indonesia in the late 1990s, TR4 quickly spread to China where it occurs widely. Within a decade, it had spread across South-East Asia and Australia, causing significant losses and impacting the family income of thousands of people. Measures to contain TR4 have only slowed its march across the globe, and in 2019 it was confirmed in Colombia. Six of the top ten producing countries are in Latin America and the Caribbean, so the introduction of TR4 to Cavendish plantations in America will have devastating social and economic impact (FAO, 2020).
Foc is an anamorphic fungus without a known sexual stage, producing three types of asexual spores for reproduction and dispersal: microconidia, macroconidia and chlamydospores (McTaggart, 2009). Chlamydospores are the most significant survival structure, persisting in previously colonised tissue, in soil or in host weeds for long periods until proximity to banana roots induces germination. Hyphae then attach to the roots and penetrate directly through the epidermis. Mycelia subsequently pass through the cortex and enter the xylem, where the fungus will remain, producing microconidia and toxins that colonize neighbouring vessels upstream and produce further fungal structures. Disease symptoms occur as a result of blockages of the xylem vessels by host defences and pathogen activity. Once the plant dies, the fungus is no longer confined to the xylem and spreads. In addition to recognising and invading host roots, Foc must also be able to evade antifungal compounds and produce virulence factors to enable the success of infection (Pérez-Vicente, Dita & Einar, 2014).
The external symptoms of Fusarium Wilt are yellowing of leaf borders and collapse of both yellowed and green leaves. In contrast to bacterial wilt, symptoms usually progress from old to young leaves. Within the plant there is yellowing of the root and rhizome vascular tissue, which develops into continuous yellow, red or brown strands in the pseudostem, the trunk-like part of a banana plant. A banana plant with Fusarium Wilt will rarely recover (Pérez-Vicente, Dita & Einar, 2014).
Modern bananas are triploid, so they cannot divide evenly to make sex cells, making them seedless and sterile. This feature has been key to their commercial success but is the source of their vulnerability. New banana plants are grown from cuttings of rhizomes and suckers and are therefore genetically identical, which produces consistent harvests, but means that all the plants of that cultivar are susceptible to infection (Thompson, 2020). For example, studies of TR4 infection of banana roots by Lin et al. showed that highly resistant cultivars can produce metabolites that inhibit germination and growth of Foc. However, all the clones of a susceptible cultivar will share a genotype that is unable to produce these inhibitory defences, leaving them all at risk of infection (Pérez-Vicente et al., 2014).
The triploid nature of banana plants means that traditional methods of cross breading to introduce new traits is not an option. Genetic modification may provide an answer, but this always encounters resistance and stringent regulation, so it is unlikely we will see a GM banana on our shelves in the near future. There are several other approaches which aim to produce banana hybrids with resistance to TR4. For example, crossing diploid and tetraploid varieties creates seedless triploid bananas. Alternatively, an infertile cross can be exposed to chemicals that induce doubling of the chromosomes, as was done with a hardy hybrid of wheat and rye called triticale, creating a fertile tetraploid banana that can be cross bred to introduce useful traits. However, none of these solutions tackle the problem at the core of this crisis (Thompson, 2020).
Hardly unique to bananas, the widespread planting of crops from a limited gene pool has created an intrinsically unstable system. Extensive monoculture clearly poses a risk but is often overlooked by farmers who prioritise the benefits of these high-yield systems (National Research Council, 1993). The role of supermarkets in creating this attitude must not be overlooked, as their “race to the bottom” forces prices continuously down, compromising the environment, the workers’ health and pay, and the banana crops around which this all pivots (Gray, 2020). The loss of the Gros Michel banana elucidates the potential for disaster created by genetic uniformity. As the re-emergence of Fusarium Wilt threatens bananas once again, the industry must look further than just another clone.
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