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 1 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, 2 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.
Trevor Williams, one of the key early figures in the movement to conserve crop diversity, has passed away at 76. Mike Jackson has done a great job of summarizing