The Relevance of Cassava as a Food Security Crop in Africa

Posted by Adaora Anozie

On April 2, 2015

Cassava, is originally the fourth most important source of calories in the developing world after the cereal crops wheat, maize, and rice. Worldwide, it feeds an estimated 700 million people directly or indirectly. Cassava production has increased steadily for the last 50 years, with 242 MT harvested in 2012. The increase is likely to continue as farmers in more than 105 countries come to recognize the crop’s advantages. A semi-perennial root crop, cassava can stay in the ground for up to 3 years. This makes it an excellent food security crop: when all other crops have been exhausted, cassava roots can still be harvested. It is naturally drought resistant and resilient to climatic changes, high temperatures, and poor soils, and in addition, cassava responds extremely well to high CO2 concentrations, making it a very important crop for the 21st century.

Africa alone accounts for more than 55 percent of the world’s production, and cassava is the first food crop in fresh tonnage before maize and plantain in sub-Saharan Africa. Cassava is also an important source of income, especially for women in sub-Saharan Africa (SSA). Furthermore, cassava is the second most important source of starch in the world. Cassava is thus a highly valuable crop for the world today and in the future. It is critical that it should not be compromised by viral diseases.

Africa produces more than 50 percent of the world’s cassava, but the crop faces two formidable disease threats which are Cassava Mosaic Disease (CMD) and Cassava Brown Streak Disease (CBSD).

Cassava Mosaic Disease (CMD) is present in all cassava-growing countries in Africa and causes losses of at least 45 MT each year. The disease is triggered by the emergence and spread of at least eight species of geminiviruses transmitted by whiteflies (Bemisa tabaci) and disseminated through infected cuttings. It first appeared in 1894 in Tanzania, and several CMD epidemics in Africa have since been reported. The most recent outbreak—and by far the most economically important—began in Uganda in the late 1980s. The disease has subsequently invaded most of East and Central Africa (ECA). The pandemic of severe CMD has now affected 12 countries, including Nigeria and continues to spread. Its effects, however, are most devastating in countries and regions recently affected by the severe CMD pandemic. In south India and Sri Lanka CMD has caused significant losses; it is absent elsewhere in the world.

However, Cassava Brown Streak Disease (CBSD), represents the greatest threat for millions of cassava farmers. CBSD is caused by two species of ipomoviruses and  propagated through infected cuttings. It has a limited effect on the growth and appearance of plants, but can be catastrophic for production as the dry rot that it produces in tuberous roots can render entire crops unusable.

CMD and CBSD are now the two worst biological constraints to cassava production in Africa-both pandemics are being driven by unusually high populations of the whitefly vector. The increasing spread of super-abundant whiteflies raises justifiable fears that CMD and CBSD will spread further on the African continent and worldwide. This would have obvious major and unanticipated consequences for food security, economic development, and social stability in many countries, as much of the world’s cassava germplasm is highly susceptible to these viruses. Moreover, global warming is likely to exacerbate the situation because higher temperatures will favor the whitefly vector. This potential additional impact from pest and disease is all the more significant as cassava is one of the very few crops that may otherwise be relatively unscathed by future patterns of climate.

Considerable efforts are being directed at tackling the problem of cassava viruses in Africa. Yet these efforts are not fully coordinated, and the overall scale of mitigation programs is tiny compared with the scale of the threats to the world’s cassava crop. With this in mind, the Global Cassava Partnership for the 21st Century – GCP21, a recognized global organization within the cassava community, gathered representatives of 24 research and development (R&D) organizations and donor agencies. This established a Global Alliance to Declare a War on Cassava Viruses in Africa, to develop a strategic action plan. The resulting Road Map lays out three primary objectives: to prevent CBSD from reaching Central and West Africa (the largest cassava regions in Africa); to mitigate the effects of these constraints on cassava in parts of Africa already affected; and to prevent the spread of CMD and CBSD to the rest of the world.

The overall vision of the Alliance is that this set of actions must be undertaken immediately in a coordinated manner among the international research community, national research organizations and extension services and that these interventions must be sustained long term if they are to be effective and reach their specified goals.

Strategic approach

A “game-changing” vision for the control of cassava viruses in Africa and the world

So far, the international cassava R&D community has dealt with cassava virus epidemics individually but seldom with coordination among donors, scientists, and developers such as non-governmental organizations (NGOs). After more than 100 years of losses due to cassava viruses in Africa, the viral situation on cassava has never been worse. Furthermore, the explosion of the CBSD epidemic, triggered by the spread of super-abundant whiteflies, has greatly exacerbated virus problems.

Faced with the gravity of this situation, cassava R&D members know that a greatly strengthened and more effectively coordinated plan is needed to solve these problems. As stated above, the Global Alliance was created to develop a strategic action plan to “declare war on cassava viruses in Africa” in three principal directions. The Alliance’s founding document (the Road Map) provides the action framework for the strategy to complete its targets.

However, the Road Map includes a number of actions along the R&D continuum and cross-section of cassava stakeholders and actors. Policy makers, scientists, extension services, NGOs, private organizations, and farmer communities have vital roles to play and at a level of collaboration never previously developed. Only through concerted, careful coordination can the proposed actions be managed in space and time to effect substantive and lasting impacts on cassava viruses and, consequently, cassava productivity.

After the Road Map, the next line of action should be to engage many more participants at the international, regional, and national levels. It should be possible to implement all activities of the Road Map concurrently. Depending on the type of activities, however, they may last different lengths of time. Actions will certainly continue to be modified in response to changes and outcomes.

In Africa, the focus is to halt CBSD’s current spread from east to west and tackle cassava viruses and whiteflies in areas already affected by CMD and CBSD pandemics. Globally, addressing the threats of the spread of cassava viruses and super-abundant whiteflies across continents, chiefly South America and Asia, is the priority.

The Road Map represents a fundamental change in the attitude of cassava R&D professionals—from working a posteriori, to being pro-active. We are well aware of the crop’s tremendous future importance to both Africa and the rest of the world, as we strive to feed the planet under constantly changing climatic conditions. Cassava R&D practitioners are determined to undertake this strategic change in action because not only will it resolve the existing problems of CMD and CBSD in Africa, it will also help us to greatly improve responses to new emerging viral problems that are likely to develop as a consequence of the twin forces of climate change and increased world trade.

Viruses, surveillance, and diagnostics

Urgent action is required to prevent the continued spread of the pandemics of CMD and CBSD. In particular, all possible efforts should be made to ensure that CBSD does not spread from East to West Africa. The first priority is surveillance of areas where CBSD has not yet been reported but nevertheless border regions that are already affected. Meanwhile, ongoing monitoring programs are required in regions affected by the disease and where interventions have been targeted to reduce CBSD occurrence and impact.

Surveillance must be linked to a system of reporting and communication (networks) to ensure that infected plants are immediately eradicated when the disease is found in previously unaffected areas. Because new virus diseases can emerge as a result of long distance trade and transport, surveillance for CBSD should be implemented also in cassava-growing regions of Africa not directly connected to the regions of current distribution.

It should be undertaken in other cassava-growing parts of the world that have similar climatic conditions to tropical Africa (e.g., the tropics of South America, India, Thailand, Malaysia). Crucial to all activities is a thorough assessment of the human and technical resources available to partners in the field. Accurate virus diagnosis (detection and identification) requires a technical platform of appropriate methods and standardized procedures. These will help to ensure that results are obtained in a timely way and at sufficient resolution and statistical reliability to support decisions on interventions and disease management. Viruses present serious threats to crop cultivation. Newly emerging viruses —or the development of more aggressive strains of existing viruses —are difficult to predict. The ultimate goal will be to map all viruses infecting cassava in Africa by establishing a global system of surveillance and monitoring.

Immediate Actions on Cassava Virus Characterization and Surveillance:

  • Implement virus characterization studies from countries not yet surveyed to provide comprehensive sequence data for virus diagnostics development.
  • Conduct surveillance surveys of viruses causing CMD and CBSD in countries not yet surveyed and repeat surveys in surveyed countries to monitor disease change over time.
  • Run focused surveillance surveys in high-risk areas for the spread of CBSD.
  • Establish surveillance capabilities in national plant quarantine systems in important cassava-growing countries distant from, but threatened by, CBSD (e.g., Nigeria).

Virus diagnostic methods:

  • Develop standardized and robust protocols for detection of all species and strains of viruses causing CBSD and CMD.
  • Increase efficiency and reduce cost of existing diagnostics.
  • Produce novel diagnostics offering point-of-test functionality.
  • Extend diagnostic capabilities to national research and plant protection systems of all countries in Africa greatly threatened by CBSD.

Medium-term actions

  • Establish a database of information on all cassava viruses available at a “click of the button.”
  • Raise awareness amongst researchers, extension workers, plant protectionists, quarantine officers, farmers, and policymakers of cassava viruses and the threats they pose.
  • Strengthen capacity of all cassava stakeholders in understanding, responding to, and managing cassava virus diseases.

Long-term actions

  • Set up virus culture repositories to facilitate easy experimentation on virus resistance and virus-host-vector interactions.
  • Establish global virus alert networks to ensure effective communication among countries affected and threatened by cassava virus pandemics and to facilitate rapid responses in case of new outbreaks.

Advances in virus diagnosis will have immediate consequences for other priorities in the Road Map. For example, robust virus diagnosis is a prerequisite for cassava breeders to identify virus resistance or tolerance. It is necessary for seed systems to ensure virus-free propagation of planting material and eventually for improving the system of germplasm exchange.

Culled form Agro Nigeria