Modelling reinfection of replanted cocoa by swollen shoot virus in pandemically diseased areas.
Reinfection of healthy replanted cocoa by cacao swollen shoot badnavirus, was modelled. Much of the spread of disease in pandemically diseased areas such as the Eastern Region of Ghana did not follow a pattern of focus expansion and disease gradient models were not directly applicable. Equations were derived for cases in which reinfection occurred by 'radial spread' of vectors (pseudococcids, mainly species of Planococcus, Planococcoides and Phenacoccus), 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 was analysed. The impact of jump spread appeared to be small compared with radial spread, but the absence of estimates of the frequency of its occurrence precluded definitive conclusion. A cordon sanitaire markedly delayed reinfection and this, together with eradication of diseased plants, may provide effective control of the disease. Some problems raised by the simplistic assumptions made in the model are discussed in relation to the epidemiology of swollen shoot disease.