MODELING THE TEMPORAL PRIMARY SPREAD OF AFRICAN CASSAVA MOSAIC-VIRUS INTO PLANTINGS

The rate of temporal primary spread of African cassava mosaic virus in to cassava plantings has been shown to be dependent on the planting da te, P, and on the plant age, t. In this paper, the relationships betwe en the rate of disease progress, P, and t were expressed mathematicall y. The appropriate functions were chosen, and their parameters were de rived by nonlinear regression using a set of experimental data obtaine d at Adiopodoume (Ivory Coast, West Africa). The resulting equations w ere incorporated into a monomolecular model with a variable rate r(P) (the product of the change of rate of disease incidence when P was fix ed), k, a constant, y, the disease incidence, and t, the time: dy/dt = k.r(P)(t)(1 - y). The modeled disease progress curves were obtained b y numerical integration of the differential equation. The close fit be tween the modeled and the experimental curves showed that the main tre nds of the epidemics were represented. The model was tested with a set of data obtained in Tanzania (East Africa), and the structure of the model was validated, as there was also a good fit between the observed and modeled disease progress curves. Finally, assumptions were made o n the remaining variation around the modeled curves.