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%.