Scientists have announced a breakthrough in the search for banana varieties that are resistant to the lethal bacterial banana wilt disease.

The disease, which causes premature ripening and rotting of fruits, wilting, and eventually death of the plant, has drastically affected the production of the highland cooking banana in East and Central Africa and hence adversely affected the food security situation and livelihoods of millions of banana farmers in the region.

Until this new development, it was believed that all banana varieties in the region, except for a wild-seeded banana called Musa balbisiana, were susceptible to the disease, which originated from Ethiopia and has now invaded and infested all banana growing areas in the highlands of eastern and central Africa.

The research team led by the head of banana breeding at International Institute of Tropical Agriculture (IITA) Prof Rony Swennen, Dr George Mahuku, the senior plant pathologist for Eastern, Southern and Central Africa, and Dr Valentine Nakato, a plant pathologist, evaluated 72 bananas representing the Musa banana genetic diversity, in an outdoor confined trial.

The midribs of the youngest leaf of three-month-old banana plants were inoculated with Xcm, the main transmission agent of the disease, and their symptom development assessed weekly for four months, with the results confirming that Musa balbisiana genotypes are indeed resistant to lethal bacterial banana wilt disease. The others were not.

Challenge

The challenge was that the Musa balbisiana variety is not preferred for breeding since it belongs to the BB subgroup, which includes the superstition-surrounded, Tani banana variety, whose edible cultivars do not exist and is mainly cultivated for commercial purposes for its by-products such as leaves and animal feed-making components.

Most of the edible bananas are of the A genome.

Lethal bacterial banana wilt disease is caused by Xcm bacteria and its symptoms include yellowing and wilting of leaves, a cream to pale yellow bacteria-laden sap when the plant is cut, shrivelling of the male bud, premature ripening, internal discolouration of fruits, and finally death of the infected plants.

Its transmission is fast and mainly through use of contaminated tools, insect vectors, and planting material, and hence major investments by national programmes, donors, and scientists have been geared towards thorough monitoring of banana fields where it is suspected, removal of diseased plants, and decontaminating farm tools.

"The discovery is very important for the millions of smallholder banana farmers in the region, because one of the most effective ways to control any disease is developing varieties that are resistant to it,” said Dr Nakato, who is based at IITA, Uganda.

The development thus affords researchers potential to utilise genomes from the resistant, but inedible varieties, to develop edible banana and plantain varieties with tolerance to Xcm, according to the scientists.

Banana wilt disease

In Uganda, bananas provide up to one-fifth of the total calorie consumption per capita. Xcm, the key agent of the crop’s main destroyer - lethal bacterial banana wilt disease, has only been found in Burundi, Democratic Republic of Congo, Kenya, Rwanda, Tanzania, and Uganda; countries which annually produce up to 21 million tonnes of banana with a value of up to Sh433billion, with the disease now threatening this sector.

In their quests and developments for the resistant varieties, the research team systematically screened the entire banana collection at their IITA station in Uganda, and identified 13 varieties, with potential of resistance next to that found in the inedible wild-seeded banana variety, Musa balbisiana.

Dr Nakato identified several diploids derived from Musa acuminata, another wild banana variety, which is already part of the existing highland breeding program of IITA and NARO (National Agricultural Research Organisation), Uganda.

These, she said, could prove instrumental in developing edible bananas unaffected by the disease.

"The findings of this study are very significant for the banana breeding community and through these outcomes, we will intensify our efforts in developing banana varieties with resistance to the disease,” pointed out Prof Swennen.

The two organisations have in the recent past advanced superior high-yielding matoke hybrids called NARITA, which were developed by crossing fertile female cultivars of the East African highland bananas and the Calcutta 4, from Musa acuminata varieties.

The NARITA variety, according to the scientists, could be screened for bacterial wilt resistance and be used as part of future breeding schemes to develop matoke varieties resistant to the disease.

Agencies