Pest and disease management
Climate change impact
Climate change increases pest and disease incidence uncertainty. Methods of prevention and elimination will have to change in response to changing pest and disease dynamics. A major factor contributing to pest and disease expansion due to climate change is the increased susceptibility of cocoa when experiencing high ecological stress. Natural enemies (including endophytes) to pests may also be affected negatively by climate change further increasing the capacity of pests to cause damage. Nonetheless, the impact of climate change on pathogen incidence remains unclear.
Description of practices
Farmers should keep a log of their management practices and observe which methods worked the best in which contexts against the spread of pests and diseases. Pruning could be done to increase aeration of the plot and reduce humidity to limit the spread of fungal diseases, for example. In agroforestry production systems, the tree species planted alongside cocoa should not be hosts to any pests or diseases that affect cocoa, and species that are hosts to natural enemies of cocoa pests should be favored. Tree spacing can have a large impact on the incidence and intensity of diseases in non-resistant cocoa varieties (Bieng et al 2017). Sanitary pruning and pesticide use in accordance with Integrated Pest Management principles of minimizing risks to human and environmental health can help limit pest and disease spread in the short-term. Regular monitoring should be carried out in the period after new varieties of cocoa or intercropping species are introduced to a region, because new pathogens may emerge (Keane and Kerr, 1997).
State of the art
Between 30% and 44% of production losses in cocoa are due to five diseases. This is a trend that has remained constant in the past decade as articles from 2004, 2009 and 2015 report similar loss percentages (Okoffo, et al. 2016). Additional production losses are caused by insects and other pests such as mirids and the cocoa pod borer. Some farmers in have significant knowledge gaps as to which tree species are hosts to cocoa pests (Smith-dumont, et al. 2014). Expected climatic changes in West Africa (increased rainfall, increased dry season evapotranspiration) will impact the incidence of black pod and mirids (Flood, 2017). Seasonal disease and pest incidence are also expected to vary (Flood, 2017). Maximum entropy (MAXENT), is considered the best methodology for assessing potential pest and disease impacts (Phillips, et al. 2006). Ideally, Maxent analysis is combined GPS data of the distribution of cocoa farms to limit the incidence of pests and diseases on a larger scale (Flood, 2017).
CLIMATE CHANGE IMPACT
Reduction of number of beneficial insects
Increasing pest and disease presence with increased humidity after periods of heavy rainfall
Cocoa plant stress and susceptibility
Spread of pest and disease varieties (unfamiliar challenges for farmers)
Changing effectiveness of traditional management methods
“Cocoa pests and diseases can lead to large-scale abandonment of cocoa (Long-term boom-and-bust), as has happened in Malaysia, regions of Brazil, the island of Fernando Po etc. (Tscharntke, et al 2011)”
Keane and Kerr (1997)
Diagram sources: Bieng, et al. 2017, and WCF (2017).
Outlook
The literature on the effects of climate change on pests and diseases is very limited (Moraes, et al. 2012). Much of the research is based on qualitative studies of farmer perceptions on climate and disease spread. Some authors argue that conventional methods of eliminating diseases and pests are insufficient. Genetic resistance efforts have yet to be successful on a large-scale, but they have a large potential (Nair, 2010). However, more studies on IPM in cocoa agroforests are required, especially in diverse climatic settings. In Côte d’Ivoire lists of trees that are potential hosts to cocoa pests and disease are sometimes disseminated, even without the backing of scientific evidence to support these claims (Smith-Dumont, et al. 2014). There is a strong need for research-backed shade tree recommendations to reduce the need for pesticides and limit the spread of disease.
Importance in terms of CSA
Productivity: Pests and diseases are the leading cause of production loss in cocoa. With IPM, more value added of cocoa is kept at the farm and community level rather than going to input manufacturers (e.g. manual weeding instead of the application of herbicide).
Adaptation: Climate change will be a defining factor in the interactions between cocoa, pathogens, and other hosts. Developments in IPM and resistant cocoa varieties increase adaptation capacity during periods of changing pest and disease dynamics.
Mitigation: Through the adequate and successful application of IPM cocoa production could be increased substantially, this entails that less land is needed for the same amount of cocoa production, possibly reducing the need for deforestation to increase production.
Complexity and link to other practices
The role of shade trees in pest and disease prevention is complicated, as is the role of other environmental factors associated with pest and disease incidence because these factors are often interdependent (Flood, 2017). Monoculture cultivation and the elimination of tree species in cocoa plots are sometimes associated with higher yields, however, this ignores the effect of the increased environmental stress experienced by cocoa trees which makes them more susceptible to pests and decreasing yields in the long-run (Asase, et al. 2010). Microclimate regulation through pruning and Sanitary pruning are useful mechanisms to reduce the spread of disease along with water management to regulate on-farm humidity. Pest and disease resistant cocoa varieties may become increasingly important in the future, however, the interdependency of factors considered in the disease triangle (host, pathogen, and environment) also makes selection and breeding of disease-resistant varieties more time consuming and complex (Nair, 2010).
Key studies
-Flood, J., 2017 Review of the effects of weather on cocoa pests and diseases [Unpublished]
Study cases
Case study 1: Costa Rica
Tree spacing impacts the individual incidence of Moniliophtora roreri disease in cacao agroforests. – Bieng, et al. 2017
Background
The authors analyze Frosty Pod Rot (FPR) incidence in 20 cacao agroforestry systems in the Upala region of Costa Rica. Trees were spaced at various distances to find whether the barrier effect of cocoa against FPR (i.e. the greater the cocoa tree density the lower the incidence of FPR) is a viable alternative to the use of conventional pesticides which are damaging to the environment.
Relation to CSA
Pests and diseases are a major contributor to yield loss in cocoa. Climate change will affect pest and disease incidence, but it is unclear in which direction. The early implementation of IPM could prevent crop loss in the future (adaptation pillar). Reducing the spacing between trees not only reduces the incidence of FPR but by increasing the number and variety of trees it also increases plot carbon stocks. Findings suggest that the number of cacao trees in a 3.7m radius and the number of fruit trees in a 4.3m radius is significantly inversely related to the incidence of FPR. Spatial neighborhood optimization appears to be a valuable IPM tool for reducing the need for pesticides. The effect of forest tree proximity was not measured because their position in a plot is often not managed by the farmer, unlike the position of fruit and cacao trees which is his or her decision during the establishment of the agroforest. Yield effects of tree spacing were not considered in the study, it is possible that due to nutrient competition cocoa yields would decrease, however, lower incidence of FPR and additional domestic value or income from fruit trees may offset the possible negative effects of nutrient competition.
Case study 2: Nigeria
Crop protection strategies for major diseases of cocoa, coffee and cashew in Nigeria – Adejumo, 2005
Background
This article addresses the need for sustainable cheap methods that farmers can adopt to reduce the incidence of disease in cocoa, coffee, and cashew in Nigeria. These three crops are important exports for foreign exchange earnings although their yields are comparatively lower than in other countries, pests and diseases are partially to blame for this situation. Ideally, crop protection measures have three characteristics: they are cheap, simple and cost-effective, and sustainable. In Nigeria, pod rot and swollen shoot virus are major contributors to cocoa yield losses.
Relation to CSA
The incidence and extension of cocoa pests and diseases are dependent on the variety of cocoa and the amount of rainfall. Farmers often have low levels of education; therefore simplicity of measures will make the correct application more likely and successful. Financial resources are often constrained for farming households. If measures are cheap and cost-effective the likelihood that farmers will adopt those increases. Finally, interventions must be sustainable in terms of the environment impact and long-term protection of crops.
The author reviews, for each of the crops, a series of proposed solutions from the literature. Promising are approaches that combine biological and chemical control, due to their present availability and the combination of on-the-spot results with longer-term protection. Microclimatic conditions on the plot have to be considered for optimal application. Integrated Pest Management requires a cost-effective extension. Finally, future research should evaluate resistance-inducing microorganisms. Microorganisms can provide long-term protection, low-cost and environmental sustainability. The author concludes, therefore that a biological control of diseases is likely to have greater adoption rates while providing “ecologically sound control” of diseases.
Case study 3: Ghana
Agroforestry systems can mitigate the severity of cocoa swollen shoot virus – Andres, et al. 2018
Background
This study relates to the Ghanaian cocoa sector, which suffers large production losses every year due to Cocoa Swollen Shoot Virus (CSSV). To cure and prevent the spread of CSSV cocoa smallholders were advised to identify and cut down infected trees. Estimates suggest that over 200 million cocoa trees have been cut done since 1946 because of CSSV infection. The authors indicate that resistance breeding and mild-strain cross protection have been the focus of the research in the past, while alternative methods of disease management such as agroforestry/diversification have been investigated less. To identify the magnitude of the effect derived from agroforestry, the authors recorded CSSV symptom severity, capsid damage and cocoa yield across fields with different numbers of shade trees.
Relation to CSA
Agroforestry has already been presented as a key CSC method of improving farmer incomes, increase climate change resilience and mitigate some of the effects of deforestation. This study highlights another way in which this production system serves CSC objectives through the reduction in the effects of CSSV disease. Andres, et al. (2018) find that CSSV symptom severity is lowest at 54% shade cover, while yields are highest at 39% shade. These effects were associated with high-light and moisture stress variations. They also make statements on pruning, indicating that shade tree pruning may be required to observe improvements in growth of cocoa trees in agroforests. The results of this study favor the argument of agroforestry over full-sun in yields and disease incidence and validate it as an effective alternative to the post-hoc practice of eliminating and replanting CSSV infected cocoa trees.