SPATIAL DYNAMICS OF A MONOCYCLIC DISEASE IN A PERENNIAL CROP

A simple model was developed which represented a rectangular, regularl y-spaced planting of host trees divided initially into a diseased area and a healthy area. The purpose of the model was to investigate disea se dynamics in a plantation over several years, specifically to evalua te the relative importance of dispersal and infection rates, the effec t of inoculum source ageing, the impact of different source sizes and the effect of sanitation. Versions of the model were used to simulate gradients of incidence and severity in the horizontal plane as disease developed with time. Only the case of a within-year monocyclic diseas e was considered, i.e. infections initiated in one year did not produc e inoculum until the next year. One such disease is witches' broom of cocoa (an airborne disease caused by the basidiomycete fungus Crinipel lis perniciosa (Stahel) Singer). The probability of inoculum arriving at an infection court from a source was assumed to decline exponential ly with distance from that source. Infection was modelled stochastical ly: an infection occurred if a calculated infection probability exceed ed a uniformly-distributed random variable, generated for each possibl e infection event. The shapes of the gradients in both the incidence a nd severity models were defined by two parameters. An infection parame ter determined the position of the disease front and a inoculum disper sal parameter, the slope of the front. The effects of reduced inoculum production as sources age was incorporated into the incidence model b ut had only a minor impact on disease spread. The size of the source a rea initially infected was found to be important only when numbers of diseased trees were few. A threshold, above which further increases in source area had little impact on subsequent dynamics, was reached rap idly. Using the severity model, phytosanitation was found to be effect ive in retarding disease spread only at very high efficiencies, often in excess of 90%.