Epidemiological Modelling of Simultaneous Control of Multiple Cassava Virus Diseases

In this project, we will collaborate with researchers from six West African countries (Nigeria, Benin, Togo, Ghana, Côte d'Ivoire and Burkina Faso), which are part of the Bill and Melinda Gates Foundation and the Department for International Development project WAVE (West Africa Virus Epidemiology for Root and Tuber Crops), to design effective control and management strategies for these cassava diseases. Our research aims to assess disease control methods that could maximise yield in a cost-effective manner. Current potential control methods for CMD and CBSD include using resistant or tolerant cultivars, removing infected plants and restricting trade. From our previous work on the control of cassava diseases, we know that implementing these measures may not always be straightforward. For example, trade restrictions limit the dispersal of the disease, but also slow the dispersal of new varieties through the informal trade sector. This suggests that control through a combination of strategies requires careful planning.

Recently the Bill and Melinda Gates Foundation and the Department for International Development have awarded the project "West African Virus Epidemiology for Root and Tuber Crops" (WAVE). The WAVE project aims to collect data to underpin the development of disease control strategies. We currently support the WAVE project with sampling guidance, however, within WAVE there is no capacity to use the data to develop models and produce a set of effective control options for multiple diseases simultaneously. Our proposal aims to identify, using modelling in combination with the data from the WAVE project, how best to coordinate a combined approach to controlling these diseases based on the use of resistant and tolerant planting material, which will help decision-makers across the region to plan how best to implement disease control strategies to alleviate the yield loses caused.

In order to assess the most effective cassava disease control strategies, we will begin our work by modelling the distribution of cassava in the region, using the most recent satellite population, cropland distribution and cassava production data. We will use this host distribution map to advise on sampling strategies, as well as offering statistical and data management support throughout. We will then develop a model for the spread of CMD and explore factors that deliver robust control of the pathogen. We will adapt a previous model on the spread of CBSD to a West-African context, and will determine both the risk of introduction and the likely rate of spread should it reach West Africa. We will then identify factors that are effective in rapid containment and eradication of the disease. Finally, we will combine the models to consider the dispersal and control of both diseases simultaneously. We will use this to advise WAVE collaborators on the best use of control strategies in order to increase the likelihood of successfully managing CMD while retaining the ability to eradicate CBSD incursions. This will lead to significant reductions in yield losses attributed to both diseases for cassava growers across the region and a subsequent increase in population welfare.

Cassava mosaic disease and cassava brown streak disease are key restraints on crop production in sub-Saharan Africa. The former is near-ubiquitous, while the latter has recently begun to spread from coastal regions of East Africa inland, with real fears that it could spread to vital cassava-growing regions in West Africa. Research groups across the region are working on these threats, but have little statistical or modelling support.

This support is required in order to understand dispersal of the pathogens and to realise effective control measures. Control measures must take account of multiple different control strategies (such as roguing, trade restrictions and the distribution of resistant or tolerant material) and diseases in order to ensure that strategies complement one another and simultaneously combat all diseases present, or at the least do not manage one while worsening another.

As control measures such as improved material and trade restrictions are currently considered separately, and data have yet to emerge on their effectiveness, we will use statistical and mathematical modelling to integrate current knowledge about the diseases and their control in order to compare the effectiveness and risks of failure of different strategies. We will use the models to produce guidance to be disseminated through WAVE to stakeholders to improve the success of disease control measures.

The outputs of this project will be fourfold; a fine-scale cassava distribution map for West and Central Africa, a novel model of dispersal of cassava mosaic and cassava brown streak diseases for testing scenarios, continuous statistical and data management support for our collaborators and a set of guidelines for strategies to increase the likelihood of successfully (i) controlling CMD, (ii) containing and eradicating CBSD incursions and (iii) managing both diseases in an integrated manner. The implementation of these guidelines will be primarily through our West African Virus Epidemiology for Root and Tuber Crops (WAVE) project partners. The WAVE project is focused on understanding the viral threats to root and tuber crops across six countries in West Africa, as well as the establishment of national and regional capacities to respond to these threats. As such, our work outputs will be integrated by WAVE into their strategic programme.

Who will benefit and how

This project will have a number of academic beneficiaries, including researchers working on cassava and the control of its viral diseases across the whole of Africa. Those researchers looking to model or sample for cassava diseases in West Africa specifically, as well as those hoping to study markets, food security and nutrition, will also benefit. In addition, the work will positively impact on researchers, primarily modellers, studying the control of multiple diseases at different stages of establishment using a combination of control strategies in other plant pathosystems. Finally, researchers working on the project will themselves develop collaborative and modelling skills, which could be applied to similar problem sets.

Our research will indirectly benefit decision-makers involved in the control of cassava viral diseases. This includes policy-makers wanting to best identify strategies to reduce yield losses and increase resilience to disease, breeders looking to distribute improved or disease-free material with maximum effect and governmental and non-governmental organisations similarly wanting to distribute material and establish interventions with best impact. Our work will also assist seed system growers wanting to market their material in an efficient manner, and improve the performance of their material through effective management.

It is unclear what an optimal approach to disease control for either of these economically important diseases in West Africa would be, let alone tackling them combined. The outputs of this research will therefore enable the above decision-makers to develop appropriate, resilient approaches to control, which will decrease losses due to disease. Advice on this will be deliverable within 5 years, and will shape future strategies deployed in the longer term to combat these diseases.

Society will directly benefit over the longer term since the outputs of this project will promote food security in West Africa. Cassava is a highly drought resistant crop and is consumed by over 70% of the continent's population. By improving the success of control strategies for the cassava diseases CMD and CBSD, this project will stabilise and increase the production of cassava across the region, helping to deliver and safe and sufficient supply of nutritious food in LMICs.

Finally, through the auspices of our collaborating partners and the network of decision makers described, our research will benefit economic development in LMICs including individual cassava growers, consumers and processors through reduced disease presence, and hence reduced yield losses. This will increase their quality of life through improved economic welfare and the delivery of more sustainable food security, promoting the economy of the entire region.