Modélisation de la dynamique d’évolution spatio-temporelle de la pourriture brune causée par phytophthora megakarya parasite du cacaoyer (théobroma cacao l.).

Abstract

Cocoa black pod rot, caused by several Phytophthora spp., like Phytophthora megakarya, is the most important cocoa disease in the world and the main disease threatening cocoa production in Africa. Black pod epidemics are responsible for yield losses of up to 80-90% in the absence of control measures. In this study, we demonstrate how the use of mathematical modeling can be useful in understanding the mechanisms behind black pod rot epidemics and also in identifying action levers on which we could act to control the disease. Although cocoa black pod rot is a major concern and much attention has been dedicated to it in recent years, many questions still remain regarding the factors that govern the dynamics of the disease. For example, the relative importance of the two different sources of inoculum (primary and secondary) and the spore dispersal dynamics are not well understood. In an attempt to provide answers to these questions, a temporal compartmental model and a spatio-temporal model have been developed and studied. The built models made it possible to study the impact of sanitary harvest on black pod epidemics and to assess the impact of shade on system dynamics. Analysis of the temporal model led to highlight two epidemiological thresholds (a specific threshold for the studied system R0 and a global threshold R0;g) linked to model parameters and disease transmission pathways. which reveals the relative importance of certain compartments in disease dynamics. Black pod epidemics persistence and extinction conditions in the system were discussed according to the fact that values of the two mentionned thresholds (R0 et R0;g) were respectively greater or less than 1. Simulations of the temporal model confirmed that intense (40 to 60 % of infected pods removed from the system every day) and frequent pod stripping every 4 days could be determinant for disease control in the field. Shading data collected in the cocoa plot led to a better goodness of fit of the spatio-temporal model and also made it possible to identify a greater number of infected pods located in areas of the plot with higher shading values. The numerical simulations provided made it possible to illustrate the temporal and spatio-temporal evolution of the disease caused by Phytophthora megakarya in a cocoa plot. To refine numerical simulations of the pathosystem, a mechanical-statistical approach was used to estimate model prameters from real observations of a specific cocoa plot. Following the investigation on the effect of environmental factors on system dynamics, the in vitro light (linked to shading) effect on P. megakarya evolution (growth and sporulation) was also evaluated in the laboratory during this study. Optimal lighting and inhibition conditions have been identified for the growth and sporulation of P. megakarya. The results obtained are in general agreement with the literature and previous results concerning shading effects on the pathosystem. Recommendations in terms of promoting cocoa cultivation in less shaded systems may be evident if these results are confirmed.