Australian banana farming changed forever last month. That’s because TR4 was detected on two farms in north Queensland, representing probably the greatest ever threat to the A$ 400 million industry.
Tropical race 4 (TR4) is the name given to the fungal strains of Fusarium oxysporum f. sp. cubense (Foc) that cause Fusarium wilt (popularly known as Panama disease) in Cavendish cultivars grown in tropical conditions.
Remember that Cavendish dominates the international banana trade. The spread of TR4 is seriously bad news, as recognized even by FAO.
Following the Australian report, someone asked on the ProMusa mailing list ((Which is well worth subscribing too, BTW.)) whether the wild species might be a source of resistance. While admitting that he wasn’t fully up to date on developments, and pointing to some very recent literature, ((Li WM, Dita M, Wu W, Hu GB, Xie JH, Ge XJ (2015) Resistance sources to Fusarium oxysporum f. sp. cubense tropical race 4 in banana wild relatives. Plant Pathology.)) Australian expert Dr David Jones had this to say:
As far as I am aware, wild Musa species have not been included in tropical race 4 screening trials except in the Northern Territory of Australia and then it was limited to six accessions of M. acuminata subspecies malaccensis. Of these six accessions, three were resistant and three susceptible. M. a. malaccensis grows wild in areas of Malaysia and Indonesia where it is believed Foc TR4 evolved.
More wild M. acuminata subspecies, other Musa species and pollen-producing, banana cultivars need to be screened as part of the global programme that is being developed to combat TR4. Germplasm with resistance could then be incorporated into conventional breeding projects. Others may have more information on proposed or active screening trials not available to me.
On the local front here in Australia, TR4 has been detected on another farm in North Queensland. The big worry is that it is on the Atherton Tableland in a completely different banana-growing district some 180 km by road from the Tully Valley, the scene of the first outbreak. The prognosis is not as good as it was, but we will have to wait and see what happens next.
Those involved with genetic engineering now have a golden opportunity to push their breeding techniques as the only way to save the global banana industry based on TR4 susceptible cultivars, notably Cavendish, even though no gene ready for insertion is guaranteed to work. If by some lucky chance a banana could be developed that has resistance, I have grave doubts about whether discerning consumers would be willing to eat the fruit. I think the big export fruit companies know this.
It has also often been said by proponents of GM bananas that there is no possibility of genes engineered into GM bananas ‘escaping’ into the environment because commercial cultivars are propagated asexually. I am wondering what would happen if GM bananas were grown in countries with wild bananas present in the adjacent bush, like Malaysia, Indonesia, PNG etc.? Isn’t there a risk that if pollen were released from GM plants it could fertilise nearby wild species?
So anyway, the wild might be a source of resistance, but so also are the Eastern African Highland bananas and plantains, apparently. Some will ask whether the Cavendish is worth saving at all, whether by biotechnological or conventional means, whether using wild or domesticated sources. But that’s another story.
TR4 is defined by the fact that it attacks Cavendish varieties, but its host range is much wider. It also causes Fusarium wilt in most of the varieties affected by races 1 and 2, as well as in varieties that up to now were not affected by the disease. That’s why preventing the spread of TR4 is so important. It’s not just to protect the Cavendish-based export trade.
The general public is being lulled by the media into believing that the disease only affects one variety out of 1,000 or so. The problem with this simplification is that it misrepresents how the crop’s overall genetic diversity is distributed. The greatest differences are not between varieties but between groups of varieties, of which there are not many, maybe 25 or so. Cavendish is one such group. It has dozens of varieties, but since they are somatic mutants of each other, they are essentially variations on the same basic pattern, including their reaction to pests and diseases. Since resistance tends to run in groups, chances are that the 200 or so varieties of Plantain and nearly 100 varieties of East African highland bananas will be resistant to TR4 if the preliminary results are confirmed. This would be good news for African farmers depending on these two types of cooking bananas. But it wouldn’t help farmers growing susceptible dessert types, for example. They are also unlikely to benefit from breeding efforts using Plantains or EAHBs as sources of resistance. Banana breeding doesn’t work that way. Genetic engineering on the other hand…
Papua New Guinea, Australia’s neighbour has some of the banana biodiversity that are course of concern to TR4. Apart from soil and live plant material, what are the other modes of transmitting the diseases?