MECHANISMS OF ALTERATION IN BEAN RUST DEVELOPMENT DUE TO INTERCROPPING, IN COMPUTER-SIMULATED EPIDEMICS

Computer simulations were performed to study the dynamics of disease i n species mixtures. Input variables for the simulator EPIMUL were mani pulated to reflect factors evaluated in 1989-1990 field experiments us ing a bean-maize intercrop and the bean rust pathogen. The variables w ere maize interference effects on spore dispersal, maize-bean competit ion 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 f or disease development), intercrops reduced disease to 32% of the mono crop level using 1989 data, but had no effect using 1990 data. Reducti ons of infection efficiency were largely responsible for lowered disea se 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 simul ations when acting independently. Disease alteration due to dispersal effects was caused by changes in spore escape from plots and not chang es in the spore dispersal gradient slope. AH treatment effects diminis hed as the rate of disease progress increased. We have thus identified potential sources of the variability seen in held studies: effects of intercropping may depend highly on rate of disease progress; infectio n efficiency effects, probably microclimatic, can have a strong impact on disease; and dispersal effects, which may be qualitatively consist ent among years, can produce very different epidemics in different yea rs.