Towards Integrated Management of the Pests and Pathogens of Cassava in Africa

Studies on pathogenic agents of cultivated plants are generally organized on a binomial basis with examination of a host plant and specific parasites or pests. As cassava in Africa has few important pests and they display a limited range of biological features and relations with the host, it was feasible for ORSTOM (L'Institut française de recherché scientifique pour le development en coopératon) researchers to study each of them over a period of 25 years in Côte d'Ivoire, Congo and Togo and such work is currently in progress in Benin and France. The diseases and pests concerned are African cassava mosaic disease and its whitefly vector ( Bemisia tabaci ), cassava bacterial blight caused by Xanthomonas campestris pv manihotis , cassava mealybug ( Phenacoccus manihoti ), and cassava green mite ( Mononychelus progresivus ). Research initially concerned only specific cassava-pathogen or cassava-pest combinations, without attention being paid to the system as a whole, despite obvious epidemic convergences resulting from a common environment, analogies between effects on the host plant and probable interactions between the various pests of the same crop. A biocenotic approach was developed to integrate the various research activities. It is aimed at analysis of the functioning of plant-pathogen and plant-pest systems and enables the design and evaluation of options for integrated management. ‘Transversal' comparisons of cassava parasite models cover pathogenic agents (pests and diseases) and their variability, their biological cycles and the climatic factors affecting them, relations with the host (genetic variability, physiology and crop management), the effect of entomophagous species and entomopathogens or phytophagous pests and the integration potential of modeling. The ORSTOM studies show the complexity of the ecology of pathogenic agents and pests in the cassava biocenosis. The extent of the cassava cropping area and the variety of production situations, the variability of pests (phytophagous and entomophagous organisms) and diseases and the many interactions between the factors regulating epidemic mechanisms leads to a complex mosaic whose structure is described. The various sequences of analysis of a system of parasite constraint are shown in matrix from (variability of pathogen agents, vectors and phytophagous organisms, host-parasite relation modes, epidemiology and population dynamics, modeling) for the various organisms investigated to date. The analysis reveals the fundamental achievements and gaps in knowledge and also the most appropriate areas for combining approaches. The body of knowledge assembled at different sites, during different periods and concerning a variety of organisms and the analysis of this knowledge show it is not necessary at this stage to collect new information but to make a synthesis (modeling) that will open up original lines of research with combinations of approaches and solutions