Usefulness of nonhost plants in managing Plasmodiophora brassicae

Germination of resting spores of Plasmodiophora brassicae, causal agent of clubroot in crucifers, may be stimulated by certain nonhost plants. Without a host plant to infect, such germination would lead to a reduced persistence of resting spores in the soil. The effect of four nonhost plants on P. brassicae was investigated in a 3-year field experiment and a 14-month glasshouse experiment. Three of the plant species used, leek (Allium porrum), winter rye (Secale cereale) and perennial ryegrass (Lolium perenne), have been reported to stimulate resting spore germination, while the fourth, red clover (Trifolium pratense), does not. In the field experiment, none of the plant species reduced the concentration of P. brassicae in soils when tested with bioassay plants (Chinese cabbage, Brassica rapa var. pekinensis). In the glasshouse experiment, there was a lower disease level in all plant treatments compared with the plant-free control following incorporation and decomposition of plant roots. At this time, pH in the soils with plant treatments was higher than that in the control soil. There were no indications of a species-specific interaction between any of the nonhost plants investigated and P. brassicae, and it cannot be concluded that any of them would be useful in the sanitation of P. brassicae-infested soils within short time periods.