The year of TR4

Anne Vézina Thursday, 19 December 2013

Ten years ago the export banana was given until 2013 before being wiped out by Tropical Race 4 (TR4), the particularly virulent Fusarium strain that has been cutting a swath through Southeast Asia’s commercial plantations of Cavendish bananas. 2013 will not go down in history as the year the iconic banana disappeared from supermarket shelves, but it will be remembered nonetheless; as the year the TR4 fungus was first reported in Africa. The news that TR4 had been confirmed in a Cavendish plantation of northern Mozambique came on the heels of the revelation that the fungus has been in Jordan since 2006. It later transpired that TR4 might also have been in Mozambique for a while, as much as two to three years, according to a news piece in the journal Nature.

As before, the main preoccupation of the media in banana-importing countries is with the export banana (although this time the reaction has been more subdued), whereas the threat the fungus poses to the livelihoods and food security of the millions of Africans who grow and eat cooking bananas hardly got a mention. It’s impossible to predict how the situation in Africa will evolve. The commercial banana farm where TR4 was found has taken measures to isolate the infected areas and no other outbreak has been reported, but then again Cavendish cultivars are not widely grown in East Africa[1]. The region’s main source of bananas, 79% of production, comes from East African highland bananas and other cooking types.

With so few Cavendish around in Africa, some of the movements of the TR4 fungus could go undetected because of the presence in soils of other Fusarium oxysporum f. sp. cubense strains, like the race 1 strains to which Gros Michel and Pisang Awak cultivars are susceptible. Even though TR4 also attacks these groups, a ‘Bogoya’ or a ‘Kayinja’ showing symptoms of Fusarium wilt (or Panama disease as many people say) would not sound the alarm since it would be assumed that the plant is infected with an already established race 1 strain. On the other hand, sick East African highland bananas and Plantains (which account for 70% of bananas grown in West and Central Africa) would be investigated since the cultivars in these groups are largely resistant to the race 1 strains present in Africa. Of course, the hope is that they will also be resistant to TR4. Some readers may remember that a handful of these African cultivars were sent to the Philippines and China to evaluate their reaction to TR4. There are plans to replicate the study with more cultivars over a longer period of time, but preliminary results suggest that most of the screened cultivars have encouraging levels of resistance.

Confirmation of these findings would be good news for African banana farmers, not only because of the continent’s patchy track record in containing pests and pathogens, but also because these cooking bananas are sometimes intensively cultivated. TR4 is usually presented as an industry problem because of the way export bananas are grown in monocultures. For example, after noting that only 13% of the 150 million or so tonnes of bananas produced annually are exported, the Nature news went on to say that TR4 poses less of a threat to the bulk of banana production. But the bananas produced for domestic markets are not necessarily grown in the kind of mixed production systems that would make them less vulnerable to the soil-dwelling fungus. In Uganda, for example, the demand for East African highland bananas is in great part met by smallholder farmers growing primarily bananas over large areas (photo), even if the individual plots are relatively small.

The world of banana farming has changed since race 1 strains put an end to the Gros Michel-dominated export trade. For one, the diversity of banana cultivars in smallholder fields has declined, because of pests and diseases, but also because of market forces which tend to favour a narrower range of cultivars. Another difference is that the Cavendish cultivars which dominate the export trade now account for a substantial share of global production, some 40%, according to the statistics published in FruiTrop. India alone, the world’s largest producer of bananas – few of which are exported – produces 7 million tonnes of Cavendish bananas, more than the quantity produced in Central America. Round two of Fusarium wilt has the potential of being more devastating than round one.

TR4’s lethal impact, persistence in the environment and wide host range are among the attributes that resulted in the fungal strain being ranked as the greatest threat to banana production, hence the emphasis on preventing the spread of the fungus. Australia may be in a league of its own when it comes to this issue, but its ability to contain TR4 in the Northern Territory – where it showed up in 1997 – provides evidence that quarantine regulations can work, or at the very least buy precious time, when they are strictly implemented. The country’s state of preparedness has also been increased with the introduction in 2005 of the Emergency Plant Pest Response Deed, a legally binding agreement between various levels of government and the national plant industry body that is drawn up in advance to minimise uncertainty over management and funding arrangements when a pest eventually strikes.

We can expect to hear more over the coming years about action plans, risk assessments, diagnostics tools, resistant bananas (including GM ones) and other management options. New research may also provide answers to questions that were dropped from the agenda when the ‘Fusarium problem’ was considered solved following the replacement of susceptible cultivars with Cavendish ones. It wasn’t. As a British blogger said: scientists get to work.

1. Cavendish cultivars represent about 10% of the region’s production according to the statistics compiled by CIRAD’s Thierry Lescot and published in FruiTrop.