Occurrence of Plasmodiophora brassicae in Finnish turnip rape and oilseed rape fields

Clubroot, caused by Plasmodiophora brassicae Woronin, is a serious plant disease of cruciferous plants. A field survey of occurrence of clubroot in oilseed fields was conducted in Finland in 2007-2009 and in 1984-1989. At present, the disease is distributed throughout the oilseed cultivation area. Clubroot was found on average from 30% of fields, but its severity was low; fields with high numbers of infected plants and plants with severe symptoms were rare. According to the survey, cultivation frequency of cruciferous plants is the most important factor affecting clubroot occurrence and severity. Clubroot was found in soils with a wide range of pH-values (pH 5-7.6), but symptoms were most severe at low pH. According to the survey, and greenhouse and field trials, high temperature and moisture during the early growth period seem to favour disease development and can cause significant yield losses. In a survival trial, clubroot declined to close to zero after four years in the absence of host plants, but traces of the pathogen were still detectable after a 19-year trial period, making eradication of the pathogen very difficult.