Mechanisms of Alteration in Bean Rust Development Due to Intercropping, in Computer‐Simulated Epidemics

Abstract

Computer simulations were performed to study the dynamics of disease in species mixtures. Input variables for the simulator EPIMUL were manipulated to reflect factors evaluated in 1989—1990 field experiments using a bean—maize intercrop and the bean rust pathogen. The variables were maize interference effects on spore dispersal, maize—bean competition as it affected spore dispersal, and effects of maize on infection efficiency of the rust pathogen after dispersal. When simulations were run at a low rate of disease progress (conditions less than optimal for disease development), intercrops reduced disease to 32% of the monocrop level using 1989 data, but had no effect using 1990 data. Reductions of infection efficiency were largely responsible for lowered disease in 1989—based simulations. Interference and competition effects on dispersal when acting together reduced disease to 81% of the monocrop level using 1989 data, but only affected epidemics in 1990—based simulations when acting independently. Disease alteration due to dispersal effects was caused by changes in spore escape from plots and not changes in the spore dispersal gradient slope. All treatment effects diminished as the rate of disease progress increased. We have thus identified potential sources of the variability seen in field studies: effects of intercropping may depend highly on the rate of disease progress; infection efficiency effects, probably microclimatic, can have a strong impact on disease; and dispersal effects, which may be qualitatively consistent among years, can produce very different epidemics in different years.