The world's banana crop is under threat from a virulent fungal disease, which is spreading rapidly through the major centres of production.

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Bananas are worth over £25 billion, and the fruit forms a key part of the diet of over 400 million people worldwide. However, as pests and disease threaten production, the industry's over-reliance on genetically similar varieties has been laid bare.  

Major banana exporters have said there is currently no threat to export crops from disease, but expert scientists have warned that banana crops around the world are extremely vulnerable, most prominently to a strain of fusarium fungus called Tropical Race 4, for which there is no known treatment. The UN Food and Agriculture Organisation is set to issue a warning about the risks presented by TR4 this week.

The disease originated in South-East Asia, but jumped to Mozambique and Jordan in late 2013 and scientists have recently said there is good reason to believe it is now present in South America, too. The disease affects the Cavendish cultivar banana, which accounts for the vast majority of bananas grown for the worldwide export trade.

Though many bananas are grown by small-scale farmers, who use many different cultivars, the Cavendish banana accounts for almost 50 percent of global production and makes up 95 percent of the fruit grown for export. Cavendish bananas rose to prominence in the 1950s, when the strain was developed and quickly adopted due to its partial resistance to an earlier fusarium threat known as Panama Disease. Panama Disease wiped out the Gros Michel cultivar, which had been the major export variety since the 1800s.

The rise and spread of Tropical Race 4 and its potential impacts on the export banana trade serve to illustrate the danger of weakened agricultural biodiversity. If, as is increasingly the case, a very similar stock accounts for a large proportion of a given crop-type grown around the world, then diseases risk spreading more quickly and having a more marked effect, as genetically similar crops will be more susceptible to the same disease than more diverse ones.

According to the UN FAO, over three quarters of the world's agricultural biodiversity was lost over the course of the 20th century, as the dominant agricultural paradigm demanded uniformity, predictability and compatibility with the desires of processors and retailers.

Although the FAO will this week warn that the threat of disease to Cavendish bananas may also affect many cultivars grown by small-scale farmers, which provide local food security, experts have suggested that many such varieties could display resistance to the disease. Some farmers have already begun to alter production methods and switched to partially-resistant banana varieties, in a bid to beat TR4, but losses and higher costs associated with the switch are threatening the viability of these plantations.