Management of clubroot: An overview of the challenges

Management of clubroot (Plasmodiophora brassicae Wor.) is a challenge worldwide, wherever brassica crops are grown. Major gene resistance has been an effective short-term strategy, but is not sustainable because of the high risk of erosion of resistance. To reduce this risk, populations of resting spores in soil must be monitored and kept low. However, spore populations in soil are difficult to quantify. Molecular approaches have been developed to quantify total or viable resting spores in soil, but these cannot reduce the high levels of variability that are present both horizontally and vertically within the soil profile. For example, resting spore concentration ranged from 103 to 106 within a 0.4 ha spot in a heavily infested field. Also, spore levels in heavily infested fields can be so high (> 107) that a reduction of 99% would leave enough spores to cause severe clubroot, which makes assessment of management strategies more difficult. Management strategies need be used in combination to be effective. These include rotation and / or stacking of major resistance genes and quantitative resistance genes, in combination with crop rotation, seeding into cool soil (5-14 °C), soil amendments with lime, boron or calcium cyanamide, solarization, fumigation, and use of non-host crops to stimulate the germination of resting spores. None of these approaches are effective on their own, so effective combinations must be identified that are economical and sustainable alternatives for producers.