MECHANISMS OF ALTERATION IN BEAN RUST EPIDEMIOLOGY DUE TO INTERCROPPING WITH MAIZE

We performed experiments to identify how maize influences bean rust (caused by Uromyces appendiculatus) in maize-bean intercrops. The effects of competition with maize and interference by maize on dispersal of rust urediniospores were evaluated in trials conducted three times during 1989 and 1990. Alterations in the nondispersal (infection) phase of the pathogen life cycle due to intercropping and competition with maize also were assessed. Overall effects of maize on rust severity were evaluated in another experiment. Competition consistently steepened dispersal gradients (P < 0.10) in trials conducted more than 50 days after planting alone or in combination with interference (intercrop). Interference had no clear effect on dispersal gradients. Estimated total spore deposition per plot was increased (second trial) and decreased (third trial) by competition in both years (P < 0.05). Intercropping only affected infection once, in late 1989, when rust severity was reduced by 96% (P < 0.05). Overall disease was reduced by intercropping at two plot locations in both years (P = 0.07), but not at a third location. Bean leaf area declined because of competition in 1989 but not in 1990. Steep gradients may be due to increased spore escape, and microclimatic changes created by maize are probably responsible for the nondispersal effect.