Our favourite fruit, the banana, is threatened by fungal, viral and bacterial diseases. This was discussed on the BBC TV programme ‘The One Show’ on 13 May, to include an interview with Pat Heslop-Harrison by renowned journalist, food critic and presenter Jay Rayner. The bananas we eat in the West are almost all of the single variety called Cavendish, introduced to the world through the collections based at the country house Chatsworth in the 1820s, as shown in the programme. (For UK viewers and proxy-server users, available on iPlayer here until 20 May.)
Plants, just like people, are continuously threatened by new types of diseases. These diseases change every year so plants that used to be immune to the disease become susceptible or chemical controls (plant medicines) become ineffective. Our work in Leicester (pronounced ‘Lester’) is looking at the genetic diversity of all bananas and related plants (‘bird of paradise’ flower and ginger are examples) to find those with genetic properties that will allow bananas to meet these challenges, and make sure we can enjoy the healthy ‘five-a-day’ fruit, the 500 million people eating banana as a staple starch source keep their food, and the 30 countries where banana is a major cash crop can increase their prosperity and sustainability of agriculture.
As pointed out in the One Show, Cavendish banana is now being attacked by a new race of the Panama disease, Fusarium Tropical Race 4 (TR4 for short). The picture shows complete devastation of a plantation in Guangzhou, China. Another race of the same disease stopped production of the previous dominant variety, Gros Michel or Big Mike.
Many diseases can be controlled by careful growing of the plants, starting with healthy planting material grown in disease free soil – any gardener knows about the importance with another vegetatively propagated crop, potato. For a bacterial disease of banana called Xanthomonas wilt, stopping plant-to-plant spread means dipping the machete used to cut the fruit bunches and dead stems into a bucket of bleach between every plant. Stopping disease spread also needs careful crop management and cutting out of diseased plants: in parts of South India, banana-funeral-pyres are found every mile along roadsides, burning plants with banana bunchy-top virus. For another fungal disease, sigatoka, cutting leaves at the first sign of infection and removing or placing them upside down can slow spread of disease. Controlling plant diseases also means strict control of movement of plants – just like foot and mouth disease in the UK.
In Africa, political parties must stop using real banana leaves as their symbol at rallies or on buses, where they are driven around villages; Mexicans need to stop using banana leaves as cooling hats or to cover boxes of fresh fruit. For banana, the recognition of the new TR4 disease and its severity in the early 2000s has meant that the spread between continents and out of South East Asia has at least been slowed. The first press stories at the time – in the UK, including The Guardian and The Telegraph – certainly increased levels of biosecurity and care in transfer of plants.
Another approach to disease control is sprays. Like antibiotics in human medicine, plant diseases can become resistant to them, or the side-effects and toxicity of the drugs are realised to be so great that they are withdrawn – so both the costs and lack of sustainability means that the spray approach is not sustainable.
Fortunately, within banana, there is a lot of genetic variation, and the different wild and cultivated lines have differences in resistance. Our work is looking at diversity in banana, and hoping to find the genes which can be used to meet the disease challenges faced by the crop.
Banana plants look rather similar, and it is difficult to measure the differences, not least because their appearance is a function of both the genes and the environment – known as the genotype x environment interaction. Growing plants to look at disease responses is also difficult – and you would certainly not want to move a disease from one banana growing country to another.
In fact, here in Leicester we have bananas – with their diseases – from all over the world. One plan we have is to grow plants in our confined greenhouses where infection can be measured, and there is no chance of spreading disease to plantations. Resistance trials are essential, and we can then find the genes which give resistances, and survey even more varieties by directly reading their DNA sequences.
The final part of the puzzle to ensure the future of banana is using the diversity. Almost all bananas that are eaten have three genetic parents, not just two which most species and wild bananas have. This is part of the reason it makes fruits without seed – in human terms, like a placenta without a baby. We need new crossing and ways to use the genes to bring the resistances into banana, another part of the projects here in Leicester with both theoretical and practical aspects.
So overall, I am very optimistic about the future of banana as a fruit in the UK, and as a staple food in the tropics, and as a source of much-needed and sustainable income. But the future varieties will be different from those we eat now, and hopefully the genetic improvements will increase sustainability of production throughout the world, while delivering the nutrition required by a population increasing to 9 billion.
For those looking for more information, several other AoBBlog.com posts have written about bananas:
- Molecular tools and quality improvement in vegetatively propagated crops
- The origin of hybrid edible bananas (Viewpoint)
- The roadside botanist, bananas and cassava biodiversity
- Banana sustains … for nutrition, for livelihoods and for the environment
Two papers in Annals of Botany, freely available (although written with a technical approach and formal writing style), also give much information about banana research: Did backcrossing contribute to the origin of hybrid edible bananas? by Edmond De Langhe, Eva Hribová, Sebastien Carpentier, Jaroslav Dolezel and Rony Swennen (http://dx.doi.org/10.1093/aob/mcq187 )
and my own: Heslop-Harrison JS, Schwarzacher T. 2007. Domestication, genomics and the future for banana. Annals of Botany 100(5):1073-1084. (http://dx.doi.org/10.1093/aob/mcm191 )
Several powerpoint talks of mine are on the web, accessible through my labs homepage at http://www.molcyt.com For any University Students suffering from Death-by-Powerpoint, there is a non-powerpoint talk on Prezi.