Modelling pathogen competition and displacement- Phytophthora infestans in Scandinavia

Populations of Phytophthora infestans have displaced less fit populations in a number of regions, but not in Scandinavia. In order to study this process, a simple epidemic model was developed, based on the Lotka-Volterra model for competition. The model contains two epidemics, each with logistic growth of two separate populations that interact with each other in that they compete for the same resource base (the plant). In Scandinavia, epidemics from suspected oospore infections are thought to start earlier than those originating from tubers, so routines were added to allow the epidemics to start at different time points. A numerical solution to the model was developed using the deSolve package of the open-source statistical program 'R'. The resulting model allows examination of the relative importance of different values for the apparent infection rates and starting parameters for the two sub-epidemics. If epidemics from oospore infections start earlier than those from tuber infections, the delay for epidemic initiation via tuber infection would require extremely high values of r in order for this population to dominate at the end of the season. This could be one reason for the lack of persistent clones in the Scandinavian Phytophthora infestans population.