Influence of particle size and application method on the efficacy of calcium cyanamide for control of clubroot of vegetable brassicas

The ability of calcium cyanamide to control clubroot caused by Plasmodiophora brassicae was evaluated in field trials conducted over a 6-year period (1996-2001). During this time a decrease in efficacy was recorded and the cost of the product increased. These factors were addressed in a subsequent glasshouse trial, in which the cause of the observed decrease in efficacy was investigated, and field trials, designed to develop and evaluate improved methods of application to reduce the cost of treatment. Broadcast application of a 'standard' formulation of calcium cyanamide (containing particles 68% w/w >850 micrometer and 31% w/w 300-850 micrometer) at 1 t/ha consistently reduced root galling due to clubroot and increased the marketable yield of broccoli by between 1.1 and 4.7 t/ha in a series of five field trials. The efficacy of calcium cyanamide decreased in subsequent field trials when the commercial formulation changed to a macrogranule from which the fine particle component had been removed. Three formulations, calcium cyanamide 'powder', 'standard' and 'macrogran', were subsequently evaluated in a glasshouse experiment. The 'powder' containing small particles (98% w/w<300 micrometer) was the most effective formulation virtually eliminating root galling due to clubroot when used at the high rate (1000 kg/ha). The 'standard' formulation used by growers until 1998 was more effective than the 'macrogran' product (98% w/w >850 micrometer) currently used, indicating that the small particle fraction is responsible for the reduction in root galling due to clubroot. A new method of application was developed to incorporate the product into two bands approximately 15 cm deep. This method of application reduced the amount of calcium cyanamide required for control of clubroot by approximately two thirds. This has reduced the cost of treatment from A$1400/ha to A$467/ha (US$896 to US$299). Use of this method together with the anticipated reintroduction of the 'standard' formulation of calcium cyanamide will provide an efficacious and cost-effective treatment for Australian growers to manage clubroot in vegetable brassicas.