Report to the Oregon Processed Vegetable Commission

Aaron Heinrich and Alex Stone
OSU Department of Horticulture

Executive summary
This research has demonstrated that liming clubroot infected soils to a pH ≥7.1 is an effective practice for reducing both the incidence and severity of clubroot. Liming does not kill the pathogen but rather prevents disease spores from infecting the plant. This research demonstrated that highly reactive calcitic lime products could be substituted for hydrated lime as they effectively raise the pH of the soil to the target pH of ≥7.1 within a week after application. The project also demonstrated that boron and Serenade drenches did not suppress clubroot under field conditions.

In the greenhouse, liming two heavily infested soils to a pH >7.1 almost completely eliminated clubroot symptoms on cauliflower (cv ‘Artica’). Under field conditions, liming a soil to a pH ≥7.1 reduced clubroot incidence by 44-77% and severity by 74-90% in 3 trials conducted in the same field in the spring and fall of 2014. In the field, liming to pH ≥7.1 does not typically completely eliminate clubroot symptoms. Zones of lower pH occur in some areas of the soil volume due to the incomplete mixing of the lime into the soil volume, and where that occurs (in combination with clubroot spores), clubroot symptoms develop. Nonetheless, clubroot disease incidence and severity are much lower and yields are higher in limed fields.

The profitability of liming Willamette Valley soils to a target pH of ≥7.1 as a clubroot control strategy will depend on several factors: the degree to which clubroot will reduce yield if no lime were applied, the cost of the lime product used and the lime rate (which depends on the pH buffering capacity of the soil as influenced by clay content, organic matter, and pH), and the value of the crop. Liming should always be used as one tool in an integrated clubroot management tool box that also includes rotation (4 or 5+ years), soil and irrigation management (to minimize waterlogging), and sanitation (use of clubroot-free transplants and prevention of clubroot movement from field to field).