Fighting a two-pronged attack on African bananas

Flesh of banana fruit infected by BXW. Photo by Piet van Asten.
Flesh of banana fruit infected by BXW. Photo by Piet van Asten.

Among the many diseases that affect bananas and p exciting new major f use of Ralph Lauren Outlet Australia product ion lantains in Africa, the two greatest threats are Banana Xanthomonas Wilt (BXW) and Banana Bunchy Top Disease (BBTD). Combined, these diseases have the potential to wipe out these economically- and food security-vital crops from the continent. Their rapid spread in recent years has alarm bells ringing in banana-producing countries across Africa Hüpfburgen. BXW, which in the past had only been prevalent in Ethiopia, has been highly active in East Africa this past decade. On the other hand, BBTD, which was first reported in the 1920s and then the 1960s in Egypt and DR Congo, respectively, has been spreading rapidly in the East African highlands and in Central and Southern Africa. In the face of this two-pronged threat, we have been actively engaged in a number of complementary disease-management research. These include developing specific diagnostic assays, initiating regional surveillance to map current disease distribution and future spread, developing management tools to minimize establishment, spread, and impact, and working on host-plant resistance through germplasm screening and biotechnology approaches.

Banana plants showing classic symptoms of BBTD. Photo by Fen Beed.
Banana plants showing classic symptoms of BBTD. Photo by Fen Beed.

As the BXW pathogen is closely related to other Xanthomonas pathogens that affect maize, sorghum, and sugarcane, we, together with advanced laboratories in the USA, developed a highly specific assay to identify the banana variant using genomic tools. We also developed a sensitive assay for the banana bunchy top virus (BBTV), the causal agent of BBTD. These two assays are now being used for verification of suspected BXW and BBTD cases in East and southern Africa in cooperation with national and private sector partners. The development of sensitive diagnostics for BXW and BBTD permits epidemiological studies in plants that do not show symptoms during the latent period, rapid deployment of control measures, and effective detection of the pathogens by quarantine officers along borders. We also conducted surveys with national partners in southern and Central Africa to map the extent of spread of BBTV, and to determine the abundance and distribution of the banana aphid, the only known insect vector of BBTV.

Map showing distribution of BBTD and BXW in Africa. Image provided by Fen Beed, IITA.
Map showing distribution of BBTD and BXW in Africa. Image provided by Fen Beed, IITA.

We recognize that insects play an important role in the spread of banana diseases, so we initiated studies on host-plant resistance to the banana aphid that spreads BBTV. We also plan to explore for natural enemies of the banana aphid in its putative area of origin in 2010. In Malawi, we established a field trial of various banana cultivars to study host reaction to BBTV and assess virus concentration. Many insects have been implicated in the medium-distance, farm-to-farm spread of BXW. To help us develop management guidelines, we have been conducting studies in  th Ralph Lauren Mens Polo Shirts Australia do you an isolated and controlled site in a forest reserve to further understand how insects spread the BXW pathogen. Together with the Southern Africa Development Community,  the Association for Strengthening Agricultural Research in Eastern and Central Africa, FAO, Bioversity International, and other partners, we co-organized an international workshop in Arusha, Tanzania in August to integrate recent information on these diseases and develop control strategies. The workshop recommended measures to slow the spread of these diseases into new regions and offset their impact in already-affected areas. These included large-scale awareness and surveillance campaigns, community-level cooperative actions, establishment of reporting, communication, and monitoring systems, improved “seed” systems, development of national contingency plans, and long-term programs for eradication and/or management of BXW and BBTD. A follow-up meeting in 2010 is being planned to establish the framework of a region-wide disease-management and production strategy.

Looking into the dynamics of Musa production in Africa

A banana valley near Ruhengeri, Rwanda. Photo by Piet van Asten, IITA.
A banana valley near Ruhengeri, Rwanda. Photo by Piet van Asten, IITA.

This year, we undertook research to further und Ralph Lauren Mens Polo Shirts Australia off started with  erstand the dynamics of the relationships among factors affecting banana and plantain production in Africa such as pest and diseases, biotic and abiotic stresses, and farmers’ preferences. This is to establish some of the underlying causes why bananas and plantains in Africa are as they are. More importantly, this would help us plot a more effective course for our Musa research-for-development efforts.

In East Africa, we conducted large-scale diagnostic surveys with our partners particularly in the major production areas of Uganda, Rwanda, Burundi, Eastern DRC, and central Kenya. We mapped yield levels, crop management practices, pest and disease pressure, nutrient deficiencies, and ecological parameters such as rainfall and altitude.

Our surveys came up with some surprising facts:

  • Yield levels (t/ha), taken from measurements of hundreds of farmers fields, were more than double the figures reported by national statistics and cited by FAO;
  • Uganda, which has been traditionally regarded as the regional champion of banana production, actually had lower average yields (around 15 t/ha) than neighboring Rwanda, Burundi, and East DR Congo (more than 20 t/ha);
  • Sigatoka disease pressure, which had been the primary focus of breeders, was generally low, especially in the higher altitudes;
  • Nematode and weevil pressure was still important in the lower parts of the highlands (less than 1200m above sea level), but were not a primary yield constraint in most production areas;
  • Nutrient deficiencies were widespread. With the exception of young and volcanic soils near the Albertine rift, the dominant Acricols and Ferralsols were low in nutrient stocks.; and
  • Soil organic matter management is a key factor, and often explained the large production differences observed when moving 50 meters away from the relatively fertile soil adjacent to houses to banana plots farther away and less likely to receive discarded kitchen waste.
Banana being transported via truck in Uganda. The farther the farms are from markets, the lesser the incomes farmers get. Photo by Piet van Asten, IITA.
Banana being transported via truck in Uganda. The farther the farms are from markets, the lesser the incomes farmers get. Photo by Piet van Asten, IITA.

Our on-farm fertilizer trials across Uganda showed that modest fertilizer doses (average 71N, 8P, 32K kg ha-1 yr-1) doubled yields from 10 to 20 t/ha per year in areas such as Central Uganda. Fertilizer use proved highly profitable near large urban centers such as Kampala, but at farther distances (>150km) from the market, the increased transport cost reduced farm gate prices to levels that would make fertilizer investments too risky (marginal rates of return <100%).

Besides soil fertility, regional production gradients seemed also strongly correlated to rainfall gradients. To prove this, we explored data from past field trials, relating inter-annual yield variations to rainfall variations. Drought proved to be one of the biggest yield constraints, with an estimated 50% yield loss in large production areas in the highlands that received ”only” 1000 mm of rainfall per year. Pot trials confirmed that even moderate drought stress (pF 2.8) resulted in strongly reduced growth (>63%) compared to pla stole helene de shopbust baugigny  nts that remained well watered (pF 1.8). Drought stress does not result in obvious visual stress symptoms, explaining why farmers and researchers in the East African highlands had not given it much attention.

We are planning to conduct similar diagnostic surveys for the plantain systems in West and Central Africa, as well as setting up irrigation trials in West and East Africa, in 2010.

Our plant health researchers are also conducting studies to probe deeper into the complex relationships between pest and disease resistance and abiotic and biotic stresses, and develop appropriate solutions to optimize Musa production in Africa.