QUANTITATIVE EPIDEMIOLOGY OF BANANA BUNCHY TOP VIRUS-DISEASE AND ITS CONTROL

Models of a banana bunchy top virus disease epidemic were developed to incorporate the two key features of an epidemic in a plantation in th e Philippines: an exponential increase in disease incidence over 10 ye ars, and a declining gradient of incidence from the outside edge of th e plantation to the centre. A non-spatial model consisted of three dif ference equations to describe the numbers of latently infected and of infectious plants in the plantation and the size of the inoculum sourc e outside the plantation. In a spatial model the outside portion of th e plantation was divided into eight blocks running parallel to the out side edge. The dispersal gradient of the inoculum was assumed to be ne gative exponential. Analysis of the two models showed that for disease incidence to increase exponentially over time, the rate of disease pr ogress could be dependent either on internal spread and roguing rate ( proportion of diseased plants removed and replaced per unit time) or o n the rate of increase of external inoculum pressure. The observed inc idence gradient from the edge to the centre of the plot could be expla ined only if external inoculum dominated the parameters in the spatial model. This model was also used to explore a variable roguing rate ac ross blocks. Simulations indicated that this may produce small gains o ver the adoption of a constant roguing rate over all blocks, but was r isky because a shift of roguing emphasis only slightly too far towards the outside blocks can result in a dramatic increase in disease.