Small-scale fungicide spray heterogeneity and the coexistence of resistant and sensitive pathogen trains

Empirical evidence indicates that fungicide-resistant and sensitive strains can coexist for prolonged periods. Coexistence has important practical implications, for example, for the posttreatment recovery of sensitivity and consequently the life expectancy of fungicide products. Despite this, the factors influencing coexistence remain relatively unexplored. Ecological studies have shown that environmental heterogeneity can facilitate the coexistence of different species and subspecific groups. Here we use a simple differential equation model and show that fungicide spray heterogeneity per se is not sufficient for coexistence but that the outcome depends crucially on the competitive relationship between resistant and sensitive strains. The model incorporates the competition between resistant and sensitive pathogen strains for a limited supply of susceptible host tissue on a crop which has received an incomplete coverage of fungicide. We use a combination of invasibility analysis and model simulations to explore the conditions under which coexistence can occur. We further show that the maximum density of healthy host tissue is realized when resistant and sensitive pathogen strains coexist. A set of key influencing parameters are identified and analyzed, and the consequences of the results for disease and resistance management are discussed.