Population dynamics of plant-parasite interactions: Thresholds for invasion

Thresholds are derived for the invasion of plant populations by parasites. The theory is developed for a generic model that takes into account two fea tures characteristic of plant-parasite interactions: a dual source of inocu lum (infection from primary or externally introduced inoculum and secondary infection from contact between susceptible and infected host tissue) and a host response to infection load. Each of the threshold criteria is shown t o be the sum of the individual components for primary and secondary infecti on. This indicates that if parasite invasion is not possible through primar y or secondary infection alone, when the two modes of transmission are comb ined, the parasite may be able to invade. The invasion criteria demonstrate that there is a threshold population of susceptible hosts below which the parasite is unable to invade. If there are nonlinearities in the population dynamics (arising through either the transmission process or the host resp onse), there are also threshold densities for the infected hosts and parasi te populations below which invasion does not occur. The implications of the results for the control of plant disease are discussed. (C) 2000 Academic Press.