Influence of Host Diversity on Development of Epidemics: An Evaluation and Elaboration of Mixture Theory

Abstract

Skelsey, P., Rossing, W. A. H., Kessel, G. J. T., Powell, J., and van der Werf, W. 2005. Influence of host diversity on development of epidemics: An evaluation and elaboration of mixture theory. Phytopathology 95:328-338.A spatiotemporal/integro-difference equation model was developed and utilized to study the progress of epidemics in spatially heterogeneous mixtures of susceptible and resistant host plants. The effects of different scales and patterns of host genotypes on the development of focal and general epidemics were investigated using potato late blight as a case study. Two different radial Laplace kernels and a two-dimensional Gaussian kernel were used for modeling the dispersal of spores. An analytical expression for the apparent infection rate, r, in general epidemics was tested by comparison with dynamic simulations. A genotype connectivity parameter, q, was introduced into the formula for r. This parameter quantifies the probability of pathogen inoculum produced on a certain host genotype unit reaching the same or another unit of the same genotype. The analytical expression for the apparent infection rate provided accurate predictions of realized r in the simulations of general epidemics. The relationship between r and the radial velocity of focus expansion, c, in focal epidemics, was linear in accordance with theory for homogeneous genotype mixtures. The findings suggest that genotype mixtures that are effective in reducing general epidemics of Phytophthora infestans will likewise curtail focal epidemics and vice versa.

Intraspecific functional diversity in hosts and its effect on disease risk across a climatic gradient

“…Mixtures may be most effective if they combine genotypes with different forms of resistance, such that pathogen populations are partitioned between the genotypes and all genotypes may experience reduced disease. While these experimental mixtures were composed of only two genotypes for simplicity, mixtures of larger numbers of genotypes may be more useful for disease reduction (Mundt 2002, Skelsey et al 2005, though good strategies are needed for formulating the most useful complex mixtures (Garrett and Cox 2008). Our field results suggest that the utility of mixtures for reducing disease will vary over the course of the season, as discussed by Wolfe and Barrett (1980), and as a function of the disease-conduciveness of environmental conditions.…”

“…Disease severity is often measured as the percentage of the maximum number of infections possible Mundt 1999, Phillips et al 2005) because many plant pathogens, especially fungal plant pathogens, are not systemic but produce a large number of local lesions per plant. Models of the effects of mixtures on epidemics have generally addressed resistant and immune types (Leonard 1969, Mundt et al 1986, van den Bosch et al 1990, Skelsey et al 2005, though sometimes in practice all types in a mixture may have some form of susceptibility. Our first hypothesis was that greater functional divergence, exemplified by mixtures in which types have greater differences in phenotypic resistance, would experience greater host-diversity effects for reduced late blight.…”

Sensitivity analysis of leaf wetness duration within a potato canopy

“…Moreover, reliable estimates can lead to a better understanding of the epidemiology of many aerially and splash-dispersed diseases. Leaf wetness simulation models, possibly in combination with knowledge of the state and progress rate of the disease, can be employed to schedule chemical applications (Skelsey et al, 2005(Skelsey et al, , 2008.…”

Debate on the Exploitation of Natural Plant Diversity to Create Late Blight Resistance in Potato

“…This issue is currently investigated in the Netherlands under the DuRPh project by combining simulation studies and field experimentation to analyze different options for spatial and temporal diversification of resistance under various agronomic conditions (Haverkort et al 2008). Strategies will be selected and these will subsequently be tested in the field during several years in order to assess whether these strategies are capable of reducing infection risk and slowing down the development of potato late blight epidemics (see also Skelsey et al 2005). Also in the BIOEXPLOIT project a lot of attention is being paid to the proper management of resistance (see also Haverkort et al 2009, this issue).…”