Rearing its ugly head - bunchy top disease
The outbreak of a disease is not normally a cue for laughter. But when banana bunchy top disease (BBTD) arrives in a village for the first time, affected plants may initially be enjoyed for their ornamental novelty. However, any hilarity is short-lived: in place of large, green, drooping leaves, the stunted plants sport an unkempt shock of erect, narrow spikes with yellow edges. Fruit are scarce, perhaps just a few twisted, deformed fingers. Life expectancy of infected plants is low, the disease spreading rapidly, aided by an insect vector, the black banana aphid, and the use of infected planting material. Within a few crop cycles, the village's banana production has collapsed.
In Malawi, BBTD causes 40 per cent of annual production losses in banana, with nearly half a million hectares devastated by the disease. The Democratic Republic of Congo, Angola and Zambia have also recently experienced severe outbreaks, and the disease has been reported as far north as Cameroon for the first time, in a survey done by IITA and national partners.
The causes for the recent spread of this virus, which was first reported in Africa in Gabon in the mid-1950s, are not well understood. Various potential culprits are being investigated, from increased vector pressure, changes in climate and cultivar choice to changes in crop production practices. What is certain is that in the last 20 years BBTD has become one of the most serious pest and disease problems facing smallholder banana farmers on the African continent; as such, it is deservedly attracting attention from plant scientists.
Sharing experience - globally and locally
Australia has a good track record in dealing with BBTD. Following an initial outbreak in 1922, the disease devastated banana orchards in Queensland during the 1930s, where its spread was contained through a three-pronged approach: restricted movements on planting materials from infected zones; eradication of infected plants and spraying of black banana aphid, the virus vector; and production of clean banana suckers. That experience is now being shared: staff from the Queensland University of Technology have established a three-year project with Africa Harvest to identify viruses affecting East African highland banana, with the ultimate aim of propagating virus-free planting material of nutrient-rich varieties.
On a larger scale, ten BBTD-threatened countries in Central and Southern Africa are collaborating in a consortium including Bioversity International, IITA and CARBAP to develop and implement disease management strategies. Pilot work is being done in the Rift Valley of Burundi, including an initial survey on disease spread and farmer coping strategies. Working with farmer groups, a team will develop and test strategies for rehabilitating infected fields based on a low-cost clean seed approach.
Additional zones are projected for Kisangani and Bas Congo in the Democratic Republic of Congo. Proposals are also being formulated for Central African Republic and Congo Brazzaville. According to Charles Staver, who coordinates this work in Africa for Bioversity International, "This disease represents a true challenge to the research and development communities, because it calls for both technical and institutional development."
Challenges to managing the disease are many. Nearly all bananas and plantains in sub-Saharan Africa originate from locally-produced suckers, often exchanged with friends and neighbours. This informal system is unfortunately ideal for rapid disease spread. And while farm level methods are available for control of nematodes, weevils and bacterial wilts, there is no on-farm system for disinfecting suckers of BBTD. Some farmers have shifted to banana types - such as Sabah and Gros Michel - which have some tolerance to the disease. However, the tolerant plants simply become a store for the virus, increasing the threat to more marketable but susceptible varieties. In addition, facilities for virus-screening are scarce in the region, as are trained virologists. Most tissue culture laboratories are therefore unable to test material for viruses before multiplying plantlets, and effective quarantining between regions is almost impossible.
Developing innovative systems to produce and distribute clean banana suckers is a key focus of the partnership's work. Maintaining banana biodiversity is important, but producing sufficient suckers from a diversity of cultivars will be a daunting challenge. Large scale production of a few selected varieties is already serving the banana export industry, but there are currently no commercial tissue-culture laboratories that offer a wider range of banana or plantain cultivars. Depending on the virus pressure in a region, the use of virus-free tissue culture plants for group sucker production by farmer organisations may be an alternative.
International partners include Natural Resources International (UK), CIRAD (France), and the Queensland Department of Primary Industries (Australia). Africa-based research partners include IITA, CARBAP, the University of Pretoria and DuRoi Laboratories, South Africa.
Date published: January 2009
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