MONITORING AND CONTROL OF PLASMODIOPHORA BRASSICAE IN SPRING OILSEED BRASSICA CROPS

Abstract

In Swedish oilseed crops severe attacks and subsequent marked yield reductions from clubroot infections were observed during the 1980s. The presence of clubroot in soil samples from 190 fields was assessed using a bioassay based on baiting the soils with Brassica campestris spp. pekinesis (Chinese cabbage) 'Granaat'. Clubroot was detected in 148 (78%) of the fields. Subsequent testing showed that disease incidence was significantly decreased after Brassica crops ceased to be grown. The half-life of spore inoculum was determined to 3.6 years for a field with 100% infestation. The level of infestation declined to below the detection level after a period of 17.3 years. Observations on yield loss from P. brassicae infections in spring oilseed rape (B. napus L.) are reported. In field tests of partly resistant cultivars of spring oilseed turnip (B. rapa L.), multiplication of club root was moderate. The average disease severity index (DSI) after harvest ranged between 3.6 and 4.4 for the partly resistant lines, as compared to 15.3 for the susceptible control cultivar. A partly resistant cultivar could preferably, according to these results, be integrated in the crop rotation where the soil infestation level gives a DSI of less than 10. PCR based methods were developed for detecting DNA in soils naturally infested with P. brassicae at levels of soil inoculum higher than DSI=3.1.