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Modelling reinfection of replanted cocoa by swollen shoot virus in pandemically diseased areas

Published by:
Publication date
Type of Publication:
Articles & Journals
Focus Region:
Sub-Saharan Africa
Focus Topic:
Health & Diseases
Type of Risk:
Biological & environmental
Jeger, M.J.; Thresh, J.M.

Reinfection of healthy replanted cocoa (Theobroma cacao) by swollen shoot disease, caused by a mealybug-transmitted virus, has been modelled. Much 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. 2. 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. 3. 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. A cordon sanitaire markedly delays reinfection and this, together with eradication of diseased plants, may provide effective control of the disease. 4. Some problems raised by the simplistic assumptions made in the model are discussed in relation to the epidemiology of swollen shoot disease.