Suppression of clubroot by dazomet fumigant

Clubroot, caused by Plasmodiophora brassicae, is an important disease in canola (Brassica napus) and other crucifers. Experiments were conducted to examine the effectiveness of the prill formulation of the soil fumigant dazomet (tetrahydro-3,5-dimethyl-2H-1,3,5-thiadiazine-2-thione, trade name Basamid) on infection by P. brassicae, clubroot severity, and the growth and yield of canola. In greenhouse studies, seedling emergence and plant height increased while infection (both primary and secondary) and clubroot severity decreased, with increasing rates of a dazomet pretreatment. Under field conditions, seedling emergence, gall mass, and seed yield were reduced, especially at high rates of dazomet application (e.g., 0.4-0.8 t a.i. ha-1). A study conducted under controlled conditions indicated that an inadequate interval between the dazomet treatment and seeding was the underlying cause of the phytotoxic effect on canola in the field experiments. Further field studies showed that covering the soil with construction-grade plastic after dazomet application increased its efficacy, reducing gall weight and clubroot severity and increasing seed yield. Fumigation with dazomet also increased emergence, plant survival, and plant biomass in P. brassicae-infested soils that were inoculated with the soil-borne fungal pathogens Fusarium avenaceum, Pythium ultimum, and Rhizoctonia solani. These results suggest that dazomet is effective against both clubroot and seedling blight in canola.