A model was developed to evaluate long-term temporal changes in disease severity (DS) of the clubroot disease of Chinese cabbage in the field with various management strategies. The model consists of a dose-response curve (DRC) of resting spore (RS) density of Plasmodiophora brassicae and DS, and the rate of RS density change due to leafy daikon cropping as a decoy plant, plowing the clubbed roots in the soil after harvest, and natural reduction. The DRC was estimated by greenhouse experiments. Three DRCs were estimated by experiments carried out in 1995-1997. The DRC in 1996 predicted the highest DS at all RS densities and hence was considered to represent a conducive condition, and that in 1997 predicted the lowest DS and was considered to represent a suppressive condition, respectively. Field data for three years (1996-1998) fell into an area surrounded by both DRC. This result confirmed the validity of the DRCs. Also the model was validated by field experiments where leafy daikon was cultivated before planting Chinese cabbage. The usefulness of the model for long-term prediction of temporal changes of DS was discussed.