Modelling Reinfection of Replanted Cocoa by Swollen Shoot Virus in Pandemically Diseased Areas

Reinfection of healthy replanted cocoa (Theobroma cacao) by swollen shoot disease, caused by a mealybug-transmitted virus, has been modelled. Muck of the spread of disease in pandemically diseased areas such as the Eastern Region of Ghana does not follow a pattern of focus expansion; and disease gradient models are not directly applicable. Equations are derived for cases in which reinfection occurs by «radial spread» of vectors, moving short distances from around the perimeter of the replanted area; and by «jump spread» of windborne vectors into the replanted area. The effect of a cordon sanitaire separating the replanting from the surrounding diseased area is analysed. The impact of jump spread appears to be small by contrast to radial spread, but the absence of estimates of the frequency of its occurrence precludes definitive conclusion (...)