Conventional and novel treatments for control of clubroot disease of brassicas
Stewart, Kelly Louise
The aim of this project was to develop treatments that would be able to reduce the survival of clubroot spores in field soil and protect the roots of young transplants against infection. The project focused on using combinations of treatments which integrate novel and existing controls with emphasis placed on the sustainable use of waste materials, plant materials and bioactives. A wide range of treatments were screened individually and in combination under glasshouse and field conditions, e.g. fungicides, nutritional amendments, companion planting, plant saponins and biocontrol agents. Many of these treatments were able to reduce clubroot severity to varying degrees. Glasshouse treatments were more successful at controlling disease than those applied in the field. The most effective treatments - when applied correctly - contained calcium, e.g. lime as calcium oxide or LimeX (a by-product of the sugarbeet processing industry), and crushed scallop and whelk shells (a by-product of the fishing industry). Whilst the effects of calcium and pH on clubroot are not new, growers need to think more in terms of dose of calcium applied in the field rather than just pH, and also, the time of addition of lime to soil before planting needs serious consideration as it may be optimal to apply lime less than two weeks before transplanting. Experimental results have shown that soil microflora plays a major role in the development of clubroot disease and that the membrane potential of growing roots may be one of the most important factors in preventing P. brassicae from entering plant roots and causing disease due to the effect that calcium and pH have on clubroot control. The experiments have also shown that there are interactions between treatments such as fungicides, limes, soil nutritional level and soil microflora at controlling disease and some treatments may reduce the effectiveness of other treatments at controlling disease. Another factor that is known to affect the effectiveness of treatments at controlling clubroot is the initial spore load in the soil. Therefore, an additional aim of the project was to develop a rapid, quantitative PCR based diagnostic test that could measure the level of clubroot spores directly from soil. Plasmodiophora brassicae DNA was successfully extracted and amplified from artificially inoculated soils and from naturally infested field soils using real-time PCR with selected sets of primers and probes. Many different types of soil DNA extraction methods were tested and standard curves relating to different levels of spore inoculum were created. This project has generated useful information as to why there are contradictory results in clubroot research about the effect of various treatments at controlling clubroot. This information may also be the basis of practical advice to brassica growers on best practices to use to achieve optimal clubroot control in the field. Options in relation to new sustainable control treatments are discussed in the light of the results from both glasshouse and field experiments. These involve planting brassicas on raised beds and applying treatments strategically around the root zone. LimeX 70 or powdered calcium oxide were demonstrated to be the most optimal lime treatments for control, and a split application of Perlka (granular calcium cyanamide) may prove to be a consistently effective method for controlling clubroot.