Pandemics of focal plant disease, a model.

ABSTRACT An analytical model of a pandemic, initiated by a single focus and spreading over a continent, is developed using foci as the smallest units of disease and fields as the smallest units of host. A few generalizing assumptions lead to a parameter-sparse model that may answer general questions on pandemics in a qualitative manner. For pandemic spread of disease during one season, a 'within-season velocity of pandemic spread,' C, is expressed as a set of integral equations. Reduction of inoculum during the off-season is expressed by a 'survival ratio' of inoculum, epsilon. The effect of the off-season is a 'push-back' of the pandemic front over a distance Deltah. It will be shown how Deltah is related to C and epsilon. The mean pandemic spread over successive years is calculated as the 'polyetic velocity of pandemic spread,' V, which depends on C and the push-back distance. The concept of 'pandemic effectiveness' is parameterized. Relations between the two velocities of pandemic spread and several model parameters are studied. Somewhat unexpectedly, velocities of pandemic spread depend only in a very limited way on field density represented by the 'cropping ratio' zeta. This implies that our model and methods will also apply to situations with inhomogeneous field distributions. The effect of parameter values on rates of severity increase are analyzed. Finally, generalizations of the model are developed and their applications discussed.