PERSISTENCE AND ENDEMICITY OF PATHOGENS IN PLANT-POPULATIONS OVER TIME AND SPACE

Endemicity is the persistence or constant presence of a pathogen in an ecologically proper spatial unit over many generations. We simulated a simple model over a grid of 8,192 sites to study persistence and the temporal and spatial dynamics of a hypothetical pathosystem of one pathogen and one host. Except for possible increase in the number of leaflets per host, the host population and environment were constant. Two characteristics of the leaf-infecting pathogen influenced persistence. As the potential reproduction per pathogen, iR, decreased, the pathogen was less likely to persist for a given number of generations. The pathogen was more likely to persist over time measured in days when it had a longer infection cycle, but persistence time was essentially constant for all values of latent and infectious periods when time was measured in generations. Several conditions of the host also influenced endemicity. Heterogeneity of iR across hosts increased persistence. Higher host densities and growth of susceptible host tissue increased persistence. Results do not support the theorem that pathogens are endemic when iR = 1 . Theorems must include spatial scale so they can be tested. Removed diseased tissue cannot logically be used in theorems as a predictor of endemicity. Other formulas for predicting the asymptotic disease level did not predict the results of our simulations, presumably because these formulas assumed that susceptible host density would be uniformly distributed and constant.