Farmers in the East African Highlands, centred on Uganda, depend on bananas as a staple food crop and a source of income. The harvest, however, is threatened by many pests and diseases that also find bananas an attractive food. Two particular pests — banana weevils (Cosmopolites sordidus) and nematode worms (mostly Radophilus similis) — can devastate banana plots, causing a total loss of the harvest in the worst cases.
Weevils tunnel into the base of the banana plant, cutting off the supply of water and nutrients to the leaves. The result is usually miserable bunches of fruits and, in the case of a severe infestation, the death of the whole plant. In Central Uganda, once a renowned centre of banana cultivation, weevils have almost destroyed the crop entirely. Replanting is not a solution as the weevils can easily invade the new crop from infected plantations in the neighbourhood.

Nematodes also attack the base of the plant, primarily the roots, but farmers often don’t notice the damage and confuse it with the results of declining soil fertility. As equally devastating as weevils, nematodes can cut yields in half by denying the plant the water and nutrients from the soil as well as the felling of affected plants from weak roots.

Farmers can manage their crops in ways that reduce the damage. For example, farmers can control weevils to some extent if they routinely remove and split the banana “stems” (known as pseudostems because they consist of the tightly rolled leaf bases), destroying the weevils’ breeding grounds. Nematodes are more difficult to control. Chemical pesticides, for example, are expensive and out of the reach of most farmers, and can damage the environment. Biological control, using organisms that attack the nematodes, is generally ineffective.

In many cases, controlling these pests is labour intensive and requires the farmer to follow strictly a routine programme of management. Researchers have therefore been looking at strategies that are not so reliant on the active and ongoing efforts of farmers. One of the most promising is to breed banana varieties that are resistant to the pests. Unfortunately, there do not appear to be any varieties or wild bananas resistant to weevils. The situation is slightly better for nematodes, with some wild bananas immune to nematodes. In this case, however, the difficulty is that cultivated bananas are generally incapable of sexual reproduction, which makes it almost impossible, and very time-consuming and expensive, to transfer resistance from the wild bananas into cultivated bananas by conventional breeding.

For these reasons, for the past few years Bioversity International has partnered with Uganda’s National Agricultural Research Organisation (NARO) to transfer resistance from other sources into East African Highland bananas. One source of resistance is the bacterium Bacillus thuringiensis, which is used to combat many plant pests. A gene, known as Bt-Cry6a, which has been used to produce many pest-resistant crop varieties, was inserted into a local banana variety called Nakitembe, chosen because it matures early and is favoured as a good variety to eat green-cooked, in addition to being susceptible to weevils and nematodes. In trials, more than 20 different versions of Nakitembe carrying the resistance gene did indeed prove to be resistant to a deliberate infestation of weevils.

Nematode resistance came from Bt-Cry6a and another gene, CpCyst, which protects papaya plants from nematode infestations. In addition to Nakitembe, these genes were also put into a plantain variety called Gonja Nakatensese, aplantain type that is normally roasted. As with weevils, the varieties with the resistance genes were tested in the greenhouse, and proved very able to withstand a deliberate nematode infection. ‘…as NARO, we highly value this project because it is the only one that addresses highland banana-specific constraints- the banana weevil and the nematodes’, explained Dr Jerome Kubiriba, team leader, the NARO-Bioversity biotechnology project.

These are early days for these promising pest-resistant banana varieties. At the moment, they are being multiplied so that they can be evaluated in more detail in confined field trials, on research stations, before being tested in a scheme called ‘Early Evaluation Trials’, still on research stations. The promising varieties will then be evaluated in diverse agroecological zones of Uganda in Preliminary Yield Trials before final evaluation by farmers themselves. These trials will be conducted under the provisions of the National Biosafety Act, passed by Uganda’s Parliament in October 2017 and expected to come into force rapidly. The Act addresses the safety of biotechnology in general, and is not confined to agricultural uses. Banana farmers in Uganda as well as the east and central Africa region are looking forward to this technology and the project review team has strongly recommended the engagement of banana farmers both inside and the region. Professor Patrick Rubaihayo, Leader of the External Project Review team commented that “This project and its outputs are useful to the entire region’.

“It has been a long process, but we are very hopeful,” said Eldad Karamura, Bioversity International scientist and Regional Representative, Eastern and Southern Africa. “Of course, there is no silver bullet solution, as farmers need a wide portfolio of crop varieties at their disposal to help them meet the current challenges they are facing, with climate change and increases in pest and disease outbreaks. When these varieties have proved their worth, farmers will have additional options in their toolbox to protect their food security and livelihoods.

Bioversity International (formerly known as IPGRI) has a policy on banana improvement, according to which ‘…. Bioversity will work on and will support research to improve Musa varieties by genetic manipulation. It will do so because through its Banana Program (formerly the INIBAP and the Commodities for Livelihoods programme) the institute has a specific responsibility for the improvement of banana and plantain. Furthermore, banana and plantain, by virtue of their biology and their place in subsistence agriculture not only need genetic manipulation but also meet all of the requirements for the safe and equitable deployment of GMOs’

NARO-Uganda is a leading banana research institution in SSA, with advanced research facilities for genetic transformation, molecular analysis and evaluation of transgenes. NARO-Uganda was the first research organization to successfully develop highland banana embryonic cell suspensions and single-cell transformation of banana genotypes against banana xanthomonas wilt, weevils, nematodes and VAD. Without an enabling law, the development of these transgenes has stagnated for 4 years and the Confined Field Trial stage on station. Now that the biotechnology and biosafety bill has been enacted, the GMO material will be advanced to on-farm evaluation and eventual release.

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article thumbnailFarmers in the East African Highlands, centred on Uganda, depend on bananas as a staple food crop and a source of income. The harvest, however, is threatened by many pests and diseases that also...
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The vast majority of the bananas currently grown and consumed were not conventionally bred but are selections made over probably thousands of years from naturally occurring hybrids. Cultivated bananas are very nearly sterile and as a consequence are not propagated from seed but rather through vegetative propagation, primarily suckers as well as more recently micropropagated or tissue cultured bananas. These factors, very old selections, near sterility and vegetative propagation, mean that these bananas have not been genetically improved either for resistance or improved quality and are becoming increasing in affected by serious pests and diseases.

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