Towards a healthy banana TC industry

Thomas Dubois, t.dubois@cgiar.org

Tissue culture banana
Banana in smallholder farmer systems is traditionally propagated by means of suckers. These contain soil-borne pests and diseases, and by using them, farmers unknowingly distribute and perpetuate pest and disease problems.

Plants produced by tissue culture (TC), because they are produced axenically in the laboratory, are material that is free from pests and diseases with the exception of fastidious bacteria and viruses.

Young tissue culture plantation in Nairobi, Kenya. Photo by T. Dubois, IITA.
Young tissue culture plantation in Nairobi, Kenya. Photo by T. Dubois, IITA.

There are many added benefits to using TC plants: (1) they are more vigorous, allowing for faster and superior yields; (2) more uniform, allowing for better marketing; and (3) can be produced in huge quantities in short periods of time, allowing for faster and better distribution of existing and new cultivars, including genetically modified banana. In other words, the TC technology can help banana farmers to make the transition from subsistence to income generation.

However, TC plantlets are relatively fragile and require appropriate management practices to fully harness their potential, especially during the initial growth stages shortly after being transplanted to the field. In East Africa, TC plantlets are often planted in fields burdened with biotic pest pressures and abiotic constraints.

A SWOT analysis
The importance of the private sector
The adoption of TC technology is still relatively low in East Africa. In Kenya, coverage of TC banana is estimated at 5–7% of the total banana acreage; adoption rates are significantly lower in countries such as Uganda, Burundi, and Tanzania, although reliable data do not exist.

Cumulative yield (t/ha/cycle) of a plantation derived from tissue culture (orange bars) compared to one derived from conventional planting material (blue bars), over 5 cropping cycles under two management regimes (low input and high input). Every little block represents one crop cycle. Data based on 1,600 plants total.
Cumulative yield (t/ha/cycle) of a plantation derived from tissue culture (orange bars) compared to one derived from conventional planting material (blue bars), over 5 cropping cycles under two management regimes (low input and high input). Every little block represents one crop cycle. Data based on 1,600 plants total.

In East Africa, the technology is booming under the impetus from the private sector. At least 10 commercial private laboratories have sprung up in the last decade in Burundi, Kenya, Uganda, and Tanzania. Collectively, they produce at least 2 million plants/year, although exact numbers seem to fluctuate widely and are hard to come by. Most of these companies manage the entire production chain, from sourcing the mother plants to weaning the TC plantlets. Despite the steep entry barrier, the TC business is very lucrative for the entrepreneur who engages in plantlet production. In some countries, universities and research organizations are also involved in the commercial production of TC banana.

Lack of quality standards and virus indexing
One of the biggest dangers for sustainable commercial production of TC plants is the lack of several essentials: (1) standards for quality management during the production process, (2) plant health certification, and (3) regulatory procedures. Such conditions are especially important to avoid spread of viruses, which are easily transmitted through TC plantlets.

For instance, Banana bunchy top virus (BBTV) is on the list of the 100 most dangerous invasive species worldwide. It is widely distributed in Central Africa and also in Burundi and Rwanda in East Africa, yet implementation of virus indexing schemes is largely absent in East Africa. It is important to put in place standard procedures for ensuring the production and distribution of high-quality, virus-free planting material, and to establish independent agencies that set and implement standards and improve the skills of personnel. In East Africa, certification schemes need to be regionally harmonized, especially with the transnational movement of plants between the countries, so that there is no weak link in the region.

Unregulation—a potential danger to the spread of diseases
At present, the commercial production of TC banana plantlets is largely unregulated. Not only are TC banana plantlets being moved in very large quantities across borders; uncertified mother material is also crossing borders. This practice is potentially risky, and could perpetuate infected sources and cause new outbreaks of disease.

In the ideal situation, there need to be certification standards for the quality and health of TC plants and the monitoring of TC producer operations. These are largely ignored because of poor awareness, and the lack of capacity and regulations required for the implementation of such standards. To transform the system, governments and/or the TC industry could consider common facilities to implement certification schemes. For instance, an accredited governmental or independent virus indexing laboratory, established as a commercial service for TC operators, would leverage costs and improve TC standards.

Another important requirement for TC producers is sustainable access to virus-free and true-to-type mother plants and this is currently lacking. The establishment of certified mother plant gardens as a common resource, either by governmental agencies or a consortium of commercial TC producers, would provide this essential requirement.

test-tubes-to-farms

Contrary to a general perception, especially among donors, it is not merely the standards themselves that are a constraint, but also a lack of knowledge on how procedures are actually implemented along the value chain, through certification schemes. The equipment for virus indexing has become relatively cheap and technical skills are quite easily acquired. Their costs can be offset, e.g., through a service-based fee to private sector stakeholders.

Also, emphasis could equally be placed on certifying general operational procedures in a private TC laboratory. Currently, the quality of TC plantlets varies significantly, and several producers are struggling with off-types and accidental mixtures of varieties that become apparent only after being planted in the field, resulting in negative perceptions about TC.

Certification schemes need to be implemented in such a way that they do not become burdensome to producers or create bureaucratic barriers. Several quality certification schemes used for clonal crops, including banana, from other regions can be considered to develop an appropriate scheme for East Africa. Ultimately, it is not only the commercial sector that should self-regulate; governmental bodies need to take responsibility.

Nurseries
Nurseries for TC plants are essential, as they act as a distribution hub connecting producers to the farmers. They also act as focus centers for farmers and farmers’ groups, and are therefore an easily approachable venue for training and other interventions. The survey by IITA and University of Hohenheim of all TC nurseries in Burundi, Kenya, and Uganda, found that nurseries in East Africa face an array of problems. Relationships between producers and nurseries, especially those related to timing, quality, and quantity of plantlet supply, are often suboptimal.

At the nursery level, there are three main operational issues: access to water, credit, and the transport of plantlets. The location of the nurseries is also crucial. Nurseries need to be close to the producer and to the market, otherwise they might fail. Clear drivers for the success of a nursery are good agricultural practices and, interestingly, a diversification into crops outside banana.

Plantain for sale in market. Photo by IITA.
Plantain for sale in market. Photo by IITA.

In TC banana value chains, nurseries have different roles across countries in East Africa. In Uganda, nurseries are run as businesses independent of the TC operators and of the farmers. In Burundi, the nurseries are owned and managed by the producers. In Kenya, nurseries are run as entities separate from the producers, and most of them are owned by farmers’ groups that act as the customers for these nurseries. The business model in Kenya seems to hold the secret for a sustainable and vigorous link between producers and farmers.

Distorted value chains
One danger for a healthy commercial TC sector is the lack of sustainable market pathways to deliver the plants to the farmers. Especially in Burundi and Uganda, outlet markets for TC plantlets are mainly governmental and nongovernmental organizations, a situation which seems unsustainable in the long term.

The sustainability of the banana TC industry is especially worrisome in Burundi, where the entire value chain is subsidized. Virtually all TC plantlets are being bought by developmental agencies, which then pass on these plantlets to often untrained farmers, free of charge, and without embedding this transfer in an encompassing training program or input package (e.g., fertilizers).

Empowerment of farmers in the value chain through farmers’ groups
Organizing banana farmers into groups has long been considered advantageous, because of increased buying and selling power, reduced economic and social risk, increased economies of scale, and improved access to credit and inputs by formally certified groups.

The study by IITA and the University of Hohenheim of the farmer-to-market linkage in Uganda demonstrated that farmers in marketing groups obtain higher prices than their ungrouped colleagues. The certification of farmers’ groups implemented by IITA’s national partners, ISABU (L’Institut des Sciences Agronomiques du Burundi) in Burundi and VEDCO (Volunteer Efforts and Development Concerns) in Uganda, has made them eligible for savings and credit schemes. Some have even engaged in other commercial activities, such as the start-up of a catering service.

The importance of a training package
In East Africa, the distribution of superior planting material alone will not ensure a good crop. Commercial farmers are skilled in juggling the inputs and effort needed to produce crops and make a profit but smallholder farmers are constrained by factors such as a lack of land and capital, access to technology, and a good marketing infrastructure. Therefore, efficient distribution systems will be needed to deliver the TC plants as part of a package, including training and access to microcredit.

Training of farmers' group on business skills in Uganda. Photo by M. Lule.
Training of farmers' group on business skills in Uganda. Photo by M. Lule.

IITA and its national partners, ISABU, JKUAT (Jomo Kenyatta University of Agriculture and Technology), and VEDCO, have been implementing hands-on, comprehensive training schemes for farmers as well as the operators of TC banana nurseries. Training schemes encompass modules in agronomy, marketing, business and financing, and for farmers, group formation and group dynamics. Participants were followed for over a year, and their ability to implement the skills learned during the training program was monitored. So far, a total of 851 separate training events have been implemented in Burundi, Kenya, and Uganda, and through the partnership, 10 new farmers’ groups and 5 new nurseries have been established.

Location, location, location
TC banana plantlets come at a cost, and might not be economically beneficial throughout all banana-producing areas in East Africa. Location is everything.

IITA, in collaboration with Makerere University, conducted a cost-benefit analysis of the technology based on a comprehensive quantitative questionnaire with 240 farmers across four districts in Uganda, and compared it with the use of conventional planting material.

Gross margins (in Ugandan shillings)/ha/year of banana plantations derived from tissue culture (yellow bars) compared to conventional planting material (orange bars) in Uganda, the further away from the main banana market.
Gross margins (in Ugandan shillings)/ha/year of banana plantations derived from tissue culture (yellow bars) compared to conventional planting material (orange bars) in Uganda, the further away from the main banana market.

Both production costs and revenues were consistently higher for TC-derived material than for suckers. However, banana prices varied greatly with district and declined significantly with increasing distance from the main market (see graph). Also, production costs decreased significantly the further away the farms were from Kampala due to better agroecological conditions and the much reduced pressure from pests and diseases. As a result, although both TC plantlets and suckers were profitable to the farmer, TC material was more profitable than suckers closer to the main banana market.

In districts with low banana prices and at a greater distance from the main banana market, farmers could receive similar gains by planting suckers rather than TC plants. For a farmer in Uganda, it makes economical sense to grow TC banana close to the main urban market.

An objective ex-post assessment
Despite a booming commercial sector, there is only anecdotal evidence that farmers who have adopted TC banana benefit tremendously in terms of higher yields and household incomes. Sound socioeconomic analyses are crucial to guide policy strategies, learn from successes already achieved, and identify important constraints for a wider dissemination of TC banana in the region.

Earlier studies on the impacts of TC banana in the region have either employed ex ante methods before any meaningful adoption was actually observable, or they have used relatively simple and ad hoc qualitative methodological tools, which do not allow robust and representative statements. The large body of subjective ‘gray’ literature, sometimes unconditionally and unilaterally promoting the benefits of TC banana, without considering the quality of the plant material, input package, and market access, risks having an adverse effect on the adoption of the technology in the long term.

Banana market in Ikire, Nigeria. Photo by O. Adebayo, IITA.
Banana market in Ikire, Nigeria. Photo by O. Adebayo, IITA.

The University of Göttingen, in collaboration with IITA, is currently answering the following main research questions: (1) What are the determinants of TC banana adoption among farmers? (2) What are the impacts of this technology on on-farm productivity, household income and income distribution, and poverty and food security? (3) How do institutional factors in technology delivery and product marketing influence adoption and impact?

Some of these research questions have been answered. In Kenya, a substantial share of the population has heard about TC banana and is, therefore, generally aware of the technology’s existence, although only a few have had a chance to fully understand its performance and requirements. This study finds that farmers’ education, access to agricultural information, knowledge of the location of a TC nursery within a reasonable distance, and affiliation to social groups significantly increase the likelihood of the TC technology being adopted.

This study also highlights the positive role of access to credit and of gender in the adoption of TC material. Farmers with access to credit and female-headed households are more likely to adopt TC plants. The latter finding is particularly interesting from a policy perspective, because it shows that, when there is an equal chance for both men and women to acquire sufficient knowledge about an innovation, women are more likely to adopt it.

Why manage noncrop biodiversity

Muris Korkaric, m.korkaric@iita-uganda.org and Fen Beed, f.beed@cgiar.org

When it comes to the diversity of nonplant taxa, the numbers alone are highly impressive. There are an estimated 5–30 million species of microorganisms globally but only two million have been formally described. In 1 g of soil, over a billion bacteria cells can be found, but fewer than 5% of the species have been named or can be grown on artificial media. For fungi, about 1.5 million species are estimated to exist and yet only 5% have been characterized taxonomically.

Disease symptom caused by Colletotrichum fuscum on lettuce leaf. Photo by F. Beed, IITA.
Disease symptom caused by Colletotrichum fuscum on lettuce leaf. Photo by F. Beed, IITA.

Nematodes remain particularly poorly described with only a fraction of the suspected half million found in nature known to man. For insects, arachnids, and myriapods only 1.1 million have been named from a potential 9 million. These numbers compare with an estimated 420,000 seed plants of which most have been described.

Knowledge of biodiversity is uneven, with strong biases towards the species level, large animals, temperate systems, and the components of biodiversity used by people. Although biodiversity underlies all ecosystem processes, modern agriculture is based on a very limited genetic pool of crops and an even more limited exploitation of the genetic resources of nonplant taxa.

This is surprising, considering that as a consequence of their diversity microorganisms and insects play pivotal roles across ecosystems that far exceed those of plants. They provide critical functions and services for food and agriculture. They are indivisibly connected with ecosystem resilience, crop health, soil fertility, and the productivity and quality of food. Modern agriculture in the developed and especially the developing world uses only a small fraction from this rich pool of genetic resources.

Conserving and using nonplant taxa
One of the vital pillars in the work of the Consultative Group on International Agricultural Research (CGIAR) is the conservation and use of agrobiodiversity and related knowledge. Over 650,000 accessions of crop, forage, and agroforestry genetic resources are stored and maintained through the centers’ genebank system and distributed to researchers and breeders throughout the world.

However, scientists from different CGIAR centers are also involved in collection, conservation, and sustainable use of insects and mites, fungi, bacteria, viruses, and nematodes that are either beneficial or antagonistic to crops. These research collections are used in two main areas: (1) crop health and productivity, where the collection supports screening for resistance in breeding programs, pathogen diagnostics, and the development of biological control technologies, and (2) soil health, fertility and ecosystem resilience where for example, collections support the development of biofertilizers.

IITA’s main collections of nonplant taxa are housed at the stations in Ibadan (Nigeria) and Cotonou (Bénin). At the headquarters in Ibadan, the collection and study of plant pathogenic fungi, bacteria, and viruses of important crops are coordinated and collections are maintained. Examples are those for yam and cassava anthracnose, cassava bacterial blight, and soybean rust pathogens.

Aflatoxin-producing fungus Aspergillus flavus growing out of maize grains in a culture medium. Photo by J. Atehnkeng.
Aflatoxin-producing fungus Aspergillus flavus growing out of maize grains in a culture medium. Photo by J. Atehnkeng.

Some of the collections contain large numbers of isolates of the same species which are often unique, not being found elsewhere in the world. International repositories might hold many different species, but tend to store fewer isolates per species and rarely prospect across the developing world. A diverse range of isolates gives a more complete representation of the genetic diversity which can be crucial for understanding evolutionary patterns, pathogen variation, and population dynamics. It helps breeding programs to identify targets for resistance selection.

Collections of isolates of the same species can be used to develop appropriate biocontrol technologies. One such example is IITA’s collection of Aspergillus flavus, a fungus that normally produces aflatoxin, a compound that is toxic to humans and animals. Over 4,500 strains have been collected from Nigeria alone and screened for toxin production and their ability to outcompete other strains when found simultaneously on foodstuffs. The atoxigenic and most competitive strains have been used to formulate aflasafe®, a biocontrol product (see R4D Review September 2009 issue).

Also in Ibadan, collections of beneficial soil microorganisms are studied and maintained. These organisms (such as Rhizobia spp. and mycorrhizae) enhance the nutrient uptake of leguminous crops and can be used as biofertilizers.

At IITA-Bénin, microorganisms and arthropods have been characterized and preserved for use in biological control programs to manage invasive crop pests and weeds. Plant pathogens have been identified and stored since the deployment of appropriate control measures first requires definitive identification of the causal agent of the disease. The biodiversity center maintains over 360,000 insect and mite specimens and is one of the largest reference collections in West Africa (see R4D Review September 2009).

Other IITA stations keep smaller working collections of nonplant taxa. At IITA-Uganda, collections of nematodes, bacteria, and fungi are maintained—mainly those associated with banana production. Certain Fusarium strains, for example, are used for endophyte-improved banana tissue culture for enhanced pest and disease resistance.

Looking like strung beads, these are part of a sample of insects received by the IITA biodiversity center in 12 months. Photo by G. Goergen, IITA.
Looking like strung beads, these are part of a sample of insects received by the IITA biodiversity center in 12 months. Photo by G. Goergen, IITA.

IITA is a lead organization for the conservation and use of nonplant taxa across sub-Saharan Africa. It is now characterizing nonplant taxa collections across the CGIAR as part of the World Bank-funded GPG2 project (Phase II of the Collective Action for the Rehabilitation of Global Public Goods in the CGIAR Genetic Resources System). This is the first system-wide inventory and collation of the existing global, nonplant taxa collections. The aim is to provide a coordinated and harmonized service for research and use of noncrop taxa to support durable farming systems in the developing world.

Future challenges and opportunities
There is a growing appreciation of the fact that farming occurs in an ecological context with complex interactions between crop and nonplant taxa that can be beneficial or antagonistic. There is also increasing demand for sustainable and environment-friendly solutions to manage pests and diseases, with the expectation that the biopesticide market share will increase to over 4.2% by 2010 and, for the first time, reach a market of over US$1 billion. Due to the rate of population increase the World Bank estimates that the global demand for food will double within the next 50 years. At the same time, the amount of arable land is decreasing from pressure from nonfarming activities and the unsustainable farming practices that are causing losses in soil fertility. This scenario is exacerbated by the fact that 40% of what is grown in the world is lost to weeds, pests, and diseases. In developing countries it is common for up to 70% of the yield to be lost due to attacks from insects and microbial diseases.

Therefore, agricultural production needs to be intensified and more marginal land used to produce sufficient food. This requires the deployment of improved land management techniques combined with the selection and distribution of appropriate crop and noncrop germplasm to exploit interactions with beneficial nonplant taxa and resist increased pressure from antagonistic nonplant taxa. Other factors such as climate change are likely to add new layers of complexity to these challenges. To predict risk and develop appropriate adaptation strategies, CGIAR and governments will become increasingly reliant on knowledge of and access to nonplant taxa genetic resources for food and agriculture. This will be used for research, training, or direct use in agriculture and originate, or be found, in a range of countries or centers.

Collections form the mechanism through which information and access to nonplant taxa can be obtained, but the survival of these collections is under threat from funding constraints. Appropriate policies, investments, and collaborations among CGIAR centers and with international collections are urgently needed to recognize noncrop taxa as global public goods. This would facilitate the conservation of collections, increase their visibility, and maximize their use for the benefit of sustainable farming systems. Especially in Africa, where the biodiversity is high, but the taxonomic and technological capacity is limited, work is needed to manage the full potential of nonplant taxa for food and agriculture.

The cassava scourge

James Legg, j.legg@cgiar.org

Close up of Bemisia tabaci adults. Photo by CIAT
Close up of Bemisia tabaci adults. Photo by CIAT

Who would think that delicate and exquisite little insects such as whiteflies could pose an ongoing and global challenge to humankind’s need to meet its food requirements?

Whiteflies are one of the top 10 most serious pest threats to agriculture. Although whiteflies, in the taxonomic family Aleyrodidae, are a diverse group of insects of more than 1,200 species, only a few of these are economically important. Among this small group, Bemisia tabaci (Genn.) is by far the most important single species.

B. tabaci was first described from tobacco in Greece, towards the end of the 19th century. Its progress has closely matched developments seen in agriculture in subsequent years, and it now occurs virtually throughout the crop-growing parts of the globe. Its preference for warm weather means that it is particularly prevalent in the tropics, although it has also been able to exploit protected agricultural environments in temperate regions.

Deadly partnerships
If B. tabaci contented itself with doing its own thing and sucking small quantities of sap from the plants that it feeds on, it would probably have fallen under the radar of those whose job it is to protect crops. But it did not. Over time, it evolved a relationship with plant viruses, a relationship that allowed the whitefly to pick up viruses when feeding on plants, harbor them for some time, before introducing them to another plant during feeding, thereby giving rise to a new infection. This enabled the viruses transmitted to expand their ranges as B. tabaci populations grew and spread. These deadly partnerships thus gave rise to plant disease epidemics that had devastating impacts on the crops affected, and on the people growing them.

Large population of B. tabaci adults feeding on the underside of a young cassava leaf
Large population of B. tabaci adults feeding on the underside of a young cassava leaf. Photo by IITA

B. tabaci transmits many hundreds of virus species, a number that keep rising as more viruses are described and research efforts on the B. tabaci vector are also broadened. The viruses transmitted fall into four virus genera: Begomovirus (family Geminiviridae), Ipomovirus (Potyviridae), Crinivirus, and Carlavirus (Closteroviridae). More than 90% of the more than 100 species transmitted, however, are in the Begomovirus group. One of Africa’s most economically destructive diseases, cassava mosaic disease (CMD), is caused by a group of viruses in the Begomovirus genus. Collectively, these are usually referred to as the cassava mosaic geminiviruses. Evidence also points to B. tabaci being the vector of cassava’s other major expanding disease threat, cassava brown streak disease (CBSD) caused by the Ipomovirus, cassava brown streak virus.

Cassava has always been at the heart of IITA’s research-for-development agenda. Thus, diseases such as CMD and CBSD, and the agents that promote their spread, have long been the focus of research efforts. From its earliest beginnings, IITA was fortunate to receive cassava germplasm, developed in East Africa through the Amani breeding program that most importantly was endowed with resistance to CMD.

It may have been an unfortunate spin-off of the tremendous success of the CMD-resistant varieties, but B. tabaci, the humble vector of the CMGs, received very little research attention before the 1990s. Things were to change abruptly in the mid-1990s, however. It became increasingly clear that unusually large whitefly populations were propelling the expansion of a new, highly virulent form of CMD in Uganda.

Studying the pandemic
IITA initiated a wide-ranging research program with the dual aims of enhancing scientific understanding of the deadly virus-vector combination as well as working with national partners to manage the pandemic.

The genetics and epidemiology of CMGs associated with the pandemic were extensively documented over the decade following the initial explosion of interest. Although less research attention was focused on the whitefly vector, a number of important advances were made in understanding the nature and role of B. tabaci. Perhaps most significantly, it was demonstrated that superabundance of B. tabaci was a key factor driving the pandemic’s so-called ”front”, and, furthermore, that the front could be pushed forwards by up to 100 km/year in this way. Although whiteflies are weak fliers, single B. tabaci individuals have been shown elsewhere to fly for up to 7 km, assisted by the wind, and given a generation time of slightly less than one month, it is easy to see how such a long distance spread could be achieved.

Superabundant <em srcset=B. tabaci and the CMD pandemic (Yellow shaded area is the approximate region affected by the CMD pandemic by 2009. Arrows indicate the direction of pandemic expansion. White explosions indicate areas in which B. tabaci superabundance has been most prominent, together with associated physical damage to cassava crops.)” width=”250″ height=”181″ />
Superabundant B. tabaci and the CMD pandemic (Yellow shaded area is the approximate region affected by the CMD pandemic by 2009. Arrows indicate the direction of pandemic expansion. White explosions indicate areas in which B. tabaci superabundance has been most prominent, together with associated physical damage to cassava crops.). by IITA

Extensive and regular disease surveys conducted by IITA and its NARS partners from 1997 up to the present have helped build up a comprehensive picture of the pandemic’s expansion into 11 countries of East and Central Africa and the interrelationships with vector populations. Moreover, these data have been used to provide risk assessments of future patterns of spread which have supported disease management initiatives.

Superabundant B. tabaci populations are typically 100-fold greater than those outside the pandemic zone. As well as delivering a sharply increased level of virus transmission, these cause physical damage to cassava plants. Experimental studies conducted at IITA-Uganda showed that yield losses from whitefly damage alone can be as much as 50%, and that these losses are particularly severe for some of the recent releases of CMD-resistant varieties. A gray-black sooty mold covering the lower leaves that develops on the sugary excreta produced by whitefly nymphs is a characteristic symptom of heavy whitefly infestation. These symptoms have been observed in various parts of East and Central Africa, and always occur in areas affected by the CMD pandemic.

Research priority
The obvious research question that has been thrown up from these sets of circumstances is: ”what causes superabundance in B. tabaci?” There are two principal hypotheses. One suggests that superabundance is a result of the spread of a novel ‘fitter’ B. tabaci biotype, and the second, that superabundance is the consequence of a synergistic interaction between B. tabaci and CMD-infected cassava plants.

To examine the first hypothesis, IITA has been working with the University of Arizona, USA, to develop molecular markers to allow discrimination between cassava-colonizing B. tabaci populations. The earliest work made use of sequence portions of the cytochrome oxidase 1 gene of mitochondrial DNA (mtCO1). MtC01 sequences were obtained from whiteflies collected along transects straddling the pandemic ”front” in Uganda. Analysis of sequence homologies showed that there were two major genotype clusters, and that one of these, the so-called “invader” was strongly associated with the pandemic-affected zone. Subsequent collections made after the pandemic had covered the whole of the cassava-growing area of Uganda, however, provided an apparently contradictory outcome, as the ”invader” genotype cluster appeared only infrequently. This is not altogether surprising, however, as B. tabaci cassava biotypes from different countries, and even different continents, have been shown to be able to interbreed successfully.

Finding novel solutions
Current efforts are therefore focusing on developing microsatellite markers that provide a much wider coverage of the B. tabaci genome and will make it more likely that we can discriminate between putative superabundant and normal B. tabaci biotypes. To investigate the intrinsic biological characteristics of different cassava B. tabaci populations, their associated genetics and the biology of offspring produced through inter- and intra-population mating, core funds are currently being used to run a PhD program in Tanzania. This study will also be used to examine the hypothesis of B. tabaci-CMD infected cassava synergism. Preliminary results from cage trials conducted at NRI using a single variety have shown increased B. tabaci abundance on CMD-infected plants, when compared with uninfected material.

Chlorosis on shoot tip and sooty mold on lower leaves caused by heavy <em srcset=B. tabaci infestation” width=”250″ height=”188″ />
Chlorosis on shoot tip and sooty mold on lower leaves caused by heavy B. tabaci infestation. Photo by IITA

The idea that diseased cassava makes for a better food source for B. tabaci has parallels in studies conducted with B. tabaci on other host plants, where virus infection has led to increased whitefly populations. In the cassava system there are some contradictions, however. It is significant that the greatest abundances of B. tabaci in pandemic-affected areas are actually observed on CMD-free resistant varieties. Further research is clearly required before a clear-cut explanation can be given for the superabundance enigma.

With whitefly numbers at record levels, and physical damage exacerbating the already grave problems posed by CMD, it has been increasingly recognized that effective measures for whitefly control need to be identified. Two main options appeared to offer greatest potential: resistance and biocontrol. Pesticides, although widely used in northern commercial agricultural systems, are easily dismissed for use on cassava in SSA, because of the extreme cost and the environmental hazard that they pose.

Is biocontrol the answer?
IITA had great success in its classical biological control programs for managing cassava mealybug and cassava green mite. Why not do a similar thing for whiteflies? Sadly, B. tabaci poses a greater challenge since it is considered to be African in origin, and therefore should already be benefiting from the presence of indigenous natural enemies. Significant work was nevertheless undertaken at IITA-Uganda to characterize the natural enemies of B. tabaci on cassava and to investigate the potential for augmentation.

A combination of surveys, life table studies, mortality measurements, and behavioral assessments conducted over a 10-year period—from 1999 to 2008—revealed that although natural enemies accounted for significant mortality in B. tabaci populations, under normal circumstances this was not sufficient to keep B. tabaci populations at levels below those causing significant economic damage.

To change this balance, it was concluded that complementary B. tabaci control measures would be required, such as the introduction of climate-matched exotic B. tabaci parasitoids or the use of cassava varieties either less favorable to whiteflies or more favorable to parasitoids. Although no attempt has yet been made to introduce exotic B. tabaci parasitoids to East Africa, a significant amount of effort has been made to enhance whitefly resistance in cassava germplasm. IITA partnered with CIAT, NRI, and NARO (Uganda), under the SP-IPM’s Tropical Whitefly Project, to pioneer efforts to introduce to East Africa strong sources of whitefly resistance developed in Latin America by CIAT (albeit to different whitefly species).

The NARO team have had some success in identifying Latin American germplasm that appears to have partial resistance to African B. tabaci, but the challenge still remains to combine these sources of resistance with the other key traits that are required by cassava in the East African farming environment. To achieve this, whitefly resistance markers will need to be built in to marker-assisted selection approaches. Much untapped potential may yet exist, however, in African germplasm, and beyond that, within wild relatives. These are important areas of future research.

Women bringing cassava to market
Women bringing cassava to market. Photo by IITA

Need more studies on whitefly
The recent upsurge in the importance of CBSV in the Great Lakes region of East/Central Africa poses yet more challenges to the cassava research community. Although published reports identify B. tabaci as the vector, researchers remain divided on the accuracy of this claim. As such, IITA, working closely with NRI, is actively addressing this question systematically, by combining field epidemiological studies with cage-based transmission experiments, both of which are being facilitated by newly improved virus diagnostic techniques.

Preliminary results seem to support the earlier claim that B. tabaci is the vector, as the level of CBSV infection in whitefly-protected experimental plots was approximately half that in whitefly-infested plots. These preliminary data will need to be confirmed by repeat trial plantings and cage trial results before any more definitive outcome can be claimed.

Whiteflies have been recognized as an important threat to cassava production for more than a century, but at the outset of the 21st century, that threat appears to be greater than ever. It appears likely that B. tabaci is driving a dual pandemic of CMD and CBSD through the cassava-growing heartlands of Africa.

Recognition of the importance of the twin threats to cassava is at an all-time high, with record levels of funding available to tackle them. By contrast, the role of the vector in the cassava crisis has received much less recognition. This fact will need to be addressed by IITA and its partners in developing future cassava-oriented R4D projects and programs.