In the UK we are used to seeing full shelves of bananas in our supermarkets, but their apparent abundance conceals the very real threat that faces global banana production. That threat takes the form of Panama Disease – this disease is caused by a soil-borne fungus called Fusarium oxysporum f.sp. cubense (Foc) and its most recent strain, Tropical Race 4 is devastating the industry. To date there is no known way to combat the disease and it will affect 80-85% of all banana varieties grown.
Bananas are the fourth most important crop in the world (after rice, wheat and maize), with annual global production exceeding 114 million tonnes from 5.4 million hectares cultivated (FAOStat, 2017). Although they are grown in more than 100 countries in a region between 30°N and 30°S latitudes, India is the world’s largest producing nation with more than 29 million tonnes. Approximately 85% of bananas produced are consumed locally and they provide an essential component of the diet for more than 400 million people reliant upon them for 15-27% daily calorie intake. Whilst a decline in the production of bananas for export would be inconvenient for the developed world, the threat that disease poses to the tropical regions in which they are grown presents a very real food security risk which could prove calamitous not only economically but also socially and environmentally as well.
Yet it is ironic that the industry has faced the threat of natural disaster in the form of a devastating Fusarium wilt before. The selection of Cavendish bananas as the preferred variety for export arose as a direct result of the crop failure of a previous monocultured variety – Gros Michel which was attacked by the same fungus – Fusarium oxysporum f.sp. cubense Tropical Race 1 in the early 20th Century. The disease spread throughout the region and worldwide and within fifty years the Gros Michel variety was virtually wiped out at a cost equating to US $18.2 billion today (Wageningen Univserity & Research, 2017). There was no cure for the disease and the only way that production could continue was to abandon the affected plantations and move to new soil every decade which had massive environmental, economic and ethical consequences. The Cavendish banana was as a direct consequence of Panama disease – Tropical Race 1 because it was resilient to infection, however the trade-off for consumers was the acceptance of a more bland, less sweet and less creamy texture (Koeppel, 2009).
Because the Cavendish banana has become so successful, it is now also monocultured and therefore there is no genetic resilience to the spread of Tropical Race 4. The impact of this strain has already cost more than US $400m in the Philippines alone (Wageningen Univserity & Research, 2017). Figure 1 shows the current status of the disease highlighting global cases that have been reported and/or confirmed.
Figure 1: Global reported (red) and reported and confirmed (yellow) cases of TR4 (Wageningen Univserity & Research, 2017)
Because the disease is soil borne it is slow to spread, but it also has a long latency period and persistence. Quarantine measures have been developed but the spread of the disease across Asia shows an exact genetic match (Ploetz, et al., 2015) and therefore although they may slow the spread, this is not a long term solution. Quarantine is also dependent on quick, accurate testing based on genetic sampling of the disease rather than just a visual assessment and the development of DNA diagnostic techniques has been a key research focus (Bentley, et al., 1998) (Dita, et al., 2010). However the quarantine measures can only be afforded by wealthier producers and the measures required are unachievable for small scale farmers because they do not have the resources to implement.
Solutions to the problems posed by pest and disease that have been implemented to date have been new land, chemical treatment through pesticide applications and conventional breeding. However given the rising cost and reduced social acceptance of pesticides and land clearance and the complexity of conventional breeding programmes these alone are no longer viable options in finding solutions to the threats posed by the diseases commonplace in the industry in the 21st Century. The development of genetic improvements in banana has become something of a research hotspot with the mapping of the banana genome and identification of genes that offer resistance to disease.
The opportunity to genetically manipulate the banana genome to offer resistance to Tropical Race 4 could provide food security to billions of people worldwide, but in order for any new genetically modified varieties to become widely produced, consumer attitude to genetic modification must become more accepting. Yet a genetic solution alone could result in history repeating itself again in the future. In the same way as the Gros Michel and now Cavendish varieties have succumbed to a virulent disease, if any new variety is monocultured then the same threat could lurk in the future. Increasing the genetic diversity of commercially produced banana crops is the only way to stop this cycle.
Nevertheless until a solution can be found a very large proportion of the developing world will remain reliant on a highly vulnerable crop.