Various factors influencing the amount of discase on the stem-base of wheat, when grown as a component of a bicrop, with white clover, arc described. The amount of crop debris on the soil surface remained higher in monocrop than bicropped plots. Furthermore, the rate of debris decay was faster in bicrops than in monocrops. Population levels of P. herpotrichoides and Fusarium spp. were higher on debris within bicrops than on debris within monocrop plots. However, because debris decomposition was morc rapid in bicrops, inoculum availability was of shorter duration in bicrops than monocrop plots. In soil, populations of P herpotrichoides were greater in bicropped plots than in monocrops, although no significant differences were observed for Fusarium spp. population levels. Pathogen cross-infection between bicrop components and changes in microclimate within crop canopies were shown to occur, and isolates of Fusarium from clover were shown to induce significant levels of disease on cereal seedlings. The effects of each of these factors on discase development are discussed in relation to previous reports of disease levels in the field.