The
banana as we know it is in imminent danger
A genetic clone, the Cavendish banana that stocks supermarket
shelves could easily be wiped out by disease. Science offers hope of a
sustainable approach
‘Ultimately we need to increase the pool of genetic
diversity in cultivated bananas.’ Photograph: Clive Gee/PA
Tuesday 25 October
2016 08.00 EDTLast modified on Tuesday 25 October 2016 12.21 EDT
The banana is the world’s
most popular fruit crop, with over 100m metric tons produced annually in over
130 tropical and subtropical countries. Edible bananas are the result of a
genetic accident in nature that created the seedless fruit we enjoy today.
Virtually all the bananas sold across the western world belong to the so-called Cavendish subgroup of the
species and are genetically nearly identical. These bananas are sterile and
dependent on propagation via cloning, either by using suckers and cuttings
taken from the underground stem or through modern tissue culture.
The familiar bright yellow Cavendish banana is ubiquitous in
supermarkets and fruit bowls, but it is in imminent danger. The vast worldwide
monoculture of genetically identical plants leaves the Cavendish intensely
vulnerable to disease outbreaks. Fungal diseases severely devastated the banana
industry once in history and it could soon happen again if we do not resolve the
cause of these problems. Plant scientists, including us, are working out the
genetics of wild banana varieties and banana pathogens as we try to prevent a
Cavendish crash.
One of the most prominent examples of genetic vulnerability
comes from the banana itself. Up until the 1960s, Gros Michel, or “Big Mike,”
was the prime banana variety grown in commercial plantations. Big Mike was so
popular with consumers in the west that the banana industry established ever
larger monocultures of this variety. Thousands of hectares of tropical forests
in Latin America were converted into vast Gros Michel plantations.
But Big Mike’s popularity led to its doom, when a pandemic
whipped through these plantations during the 1950s and ‘60’s. A fungal disease
called Fusarium wilt or Panama disease nearly wiped out the Gros Michel and
brought the global banana export industry to the brink of collapse. A
soil-borne pathogen was to blame: the fungus infected the plants’ root and
vascular system. Unable to transport water and nutrients, the plants wilted and
died.
Fusarium wilt is very difficult to control – it spreads easily
in soil, water and infected planting material. Fungicide applications in soil
or in the plant’s stem are as of yet ineffective. Moreover, the fungus can
persist in the soil for several decades, thus prohibiting replanting of
susceptible banana plants.
Yearly,
it can take 50 or more applications of chemicals to control the disease
Cavendish bananas are resistant to those devastating Fusarium
wilt strains, so were able to replace the Gros Michel when it fell to the
disease. Despite being less rich in taste, and despite the logistical
challenges involved with merchandising this fruit to international markets at
an acceptable quality, Cavendish eventually replaced Gros Michel in commercial
banana plantations. The entire banana industry was restructured,
and to date, Cavendish accounts for 47% of the bananas grown worldwide and 99% of all bananas sold commercially for export to
developed countries.
But the Cavendish unfortunately has its own weaknesses – most
prominently susceptibility to a disease called Black Sigatoka. The fungus Pseudocercospora
fijiensis attacks the plants’ leaves, causing cell death that affects
photosynthesis and leads to a reduction in fruit production and quality. If
Black Sigatoka is left uncontrolled, banana yields can decline by 35-50%.
Cavendish growers currently manage Black Sigatoka through a
combination of pruning infected leaves and applying fungicides. Yearly, it can
take 50 or more applications of chemicals to control the disease. Such heavy
use of fungicides has negative impacts on the environment and the occupational
health of the banana workers, and increases the costs of production. It also
helps select for survival the strains of the fungus with higher levels of
resistance to these chemicals: as the resistant strains become more prevalent,
the disease gets harder to control over time.
To further aggravate the situation, Cavendish is also now under
attack from a recently emerged strain of Fusarium oxysporum, known as Tropical
race 4 (TR4). First identified in the early 1990s in Taiwan, Malaysia and
Indonesia, TR4 has since spread to many south-east Asian countries and on into
the Middle East and Africa. If TR4 makes it to Latin America and the Caribbean
region, the export banana industry in that part of the world could be in big
trouble.
Cavendish varieties have shown little if any resistance against
TR4. Growers are relying on temporary solutions – trying to prevent
it from entering new regions, using clean planting materials
and limiting the transfer of potentially infected soil between farms.
Black Sigatoka and Panama disease both cause serious production
losses and are difficult to control. With the right monitoring in place to
rapidly intervene and halt their spread, the risks and damage imposed by these
diseases can be considerably reduced, as has been recently
shown in Australia. But current practices don’t provide the durable
solution that’s urgently needed.
If there’s a lesson to be learned from the sad history of Gros
Michel, it’s that reliance on a large and genetically uniform monoculture is a
risky strategy that is prone to failure. To reduce the vulnerability to
diseases, we need more genetic diversity in our cultivated bananas.
Over a thousand species of banana have been recorded in the
wild. Although most do not have the desired agronomic characteristics – such as
high yields of seedless, non-acidic fruits with long shelf life – that would
make them a direct substitute for the Cavendish, they are an untapped genetic
resource. Scientists could search within them for resistance genes and other
desirable traits to use in engineering and breeding programs.
If there’s
a lesson to be learned, it’s that reliance on a large and genetically uniform
monoculture is a risky strategy
To date, though, there’s been little effort and insufficient
funding for collecting, protecting, characterizing and utilizing wild banana
genetic material. Consequently, while almost every other crop used for food
production has been significantly improved through plant breeding over the last
century, the banana industry has yet to benefit from genetics and plant
breeding.
But we have started taking the first steps. We now know the genome
sequences of the banana and the fungi that cause Fusarium wilt
and Sigatoka. These studies helped illuminate some of the molecular mechanisms
by which these fungal pathogens cause disease in the banana. That knowledge
provides a basis for identifying disease-resistant genes in wild and cultivated
bananas.
Researchers now have the tools to identify resistance genes in wild bananas
or other plant species. Then they can use classical plant breeding or genetic
engineering to transfer those genes into desired cultivars. Scientists can also
use these tools to further study the dynamics and evolution of banana pathogens
in the field, and monitor changes in their resistance to fungicides.
Availability of the latest tools and detailed genome sequences,
coupled with long-term visionary research in genetics, engineering and plant breeding,
can help us keep abreast of the pathogens that are currently menacing the
Cavendish banana. Ultimately we need to increase the pool of genetic diversity
in cultivated bananas so we’re not dependent on single clones such as the
Cavendish or the Gros Michel before it. Otherwise we remain at risk of history
repeating itself.
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