Computerized Simulation of Crown Rust Epidemics in Mixtures of Immune and Susceptible Oat Plants with Different Genotype Unit Areas and Spatial Distributions of Initial Disease

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Abstract
A modified version of the computerized model EP1MUL was used to study effects of host genotype unit area (the ground area occupied by an independent, genetically homogeneous unit of a host population) and spatial distribution of the initial disease on crown rust epidemics in oat populations consisting of 25% susceptible and 75% immune plants. Mixtures with different genotype unit areas were represented by considering each compartment in an N .times. N matrix to be a 0.0025 m2 plant and aggregating plants into units of plants of the same genotype. The effects of genotype unit area and the spatial distribution of initial disease on the effectiveness of the mixtures for oat crown rust control were similar to those found in previous studies conducted in the field. The proportion of rust infection in the mixtures usually increased as genotype unit area was increased from 0.0025 to 0.56 m2. The magnitude of this increase, however, was much larger when initial disease was distributed uniformly than when initial disease was concentrated in a central focus. The position and size of the initial disease focus relative to the position and size of host genotype units were found to influence the effect of genotype unit area on the effectiveness of mixtures for disease control. Spores produced on the first day of spore production were categorized into five groups: those resulting in autoinfections, those resulting in alloinfections, spores dispersed to immune plants in the mixtures, spores deposited on previously infected tissue, and spores dispersed outside of plots. The ratio of autoinfection to alloinfection and focus saturation were found to contribute to the interaction between genotype unit area and the spatial distribution of initial disease in host mixtures.