Clubroot is a serious disease affecting many Brassica vegetables, including Oriental types. In the research discussed here, seeds of Chinese flowering cabbage and Shanghai pak choi were sown in a field that was naturally infested with clubroot. Sowings were made in Ontario, Canada, in different years between May and August.
The crops were harvested about six weeks after sowing - and the incidence and severity of clubroot was then assessed.
Records of the weather and of soil temperature at various depths were made throughout each experiment.
The incidence and severity of disease increased with air temperature averaged over the whole growth period. However, the factor that apparently had the most impact was the air temperature from six to 10 days before harvest.
Soil temperature at a depth of 5cm also had an impact, especially in the five days before harvest. Disease incidence was not correlated with rainfall.
The results suggest that warm air temperatures shortly before harvest and warm soil temperatures immediately before harvest increase the likelihood of clubroot.
The effect of warm soil temperatures may be lessened by applying organic or reflective mulches, and high air temperature may be avoided by advancing or delaying the sowing dates for susceptible varieties.
Temperature Prior to Harvest Influences the Incidence and Severity of Clubroot on Two Asian Brassica Vegetables by McDonald and Westerveld (2008). HortScience 43 (5): 1509-1513.Members of ISHS can view HortScience from the website www.ishs.org.