Analysis of Factors Affecting Disease Increase and Spread in Mixtures of Immune and Susceptible Plants in Computer-Simulated Epidemics

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Abstract
A modification of the simulation model EPIMUL was used to study the effects of host genotype unit area (ground area occupied by an independent, genetically homogeneous unit of a host population), spatial distribution of initial disease, percentage of susceptible plants, rate of disease increase, and steepness of pathogen dispersal gradients on epidemic development of polycyclic, foliar disease in mixtures of immune and susceptible plants. The effectiveness of the mixtures for disease control decreased with increasing genotype unit area and with increasing steepness of the dispersal gradient. The effects of genotype unit area and dispersal gradient steepness on mixture efficacy for disease control were greater when initial disease was distributed uniformly than in epidemics with a single focus of initial disease. Genotype unit area had the greatest effect on mixture efficacy for epidemics with intermediate gradient steepnesses. There was usually a smaller effect of genotype unit area on mixture efficacy when the dispersal gradient was steep and there was very little effect of genotype unit area on mixture efficacy when the gradient was very shallow. The effectiveness of the mixtures in controlling disease declined with increasing percentage of susceptible plants in the mixtures. However, the loss of mixture efficacy with increasing genotype unit area was less in mixtures with a high proportion of susceptible plants than in mixtures with a lower proportion. The mixtures were usually less effective in controlling epidemics with high rates of multiplication per lesion than in controlling epidemics with low rates. For epidemics with a very low rate of multiplication per lesion, the mixtures were less effective in controlling disease when the dispersal gradient was very shallow than when the gradients were of intermediate steepness.

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