During 1986–87 the presence of clubroot in soils sampled from 190 fields was assessed using a bioassay method, based on baiting the soils with Brassica campestris spp. pekinesis cv. Granaat. Clubroot was detected in 148 (72%) of the fields investigated and, on average, 49.2% of the plants were infected according to the bioassay. Subsequent testing of fields in 1990 and 1992 (54 and 81 fields, respectively) where no further Brassica crops had been grown indicated a significant decrease in the degree of infestation to 7.1% in 1992. Clay soils showed, on average, the highest degree of infestation, and high infestation was recorded for a wide range of pH values (5.2–6.6). The highest degree of infestation was recorded on fields where oilseeds were grown five times during the period 1965–85. The results presented show that, in a field with 100% infestation, the level of infestation declined to below the detection level after a period of 17.3 years. The half‐life of the spore inoculum was determined to be 3.6 years. During the sensitivity testing of different Brassica species and cultivars, it was found that Chinese cabbage showed a low percentage of infestation in two soils, whilst some oilseed rape and spring oilseed turnip rape cultivars showed high degrees of infestation in these soils.

A protocol using real-time polymerase chain reaction (PCR) for the direct detection and quantification of Plasmodiophora brassicae in soil samples was developed and used on naturally and artificially infested soil samples containing different concentrations of P. brassicae. Species-specific primers and a TaqMan fluorogenic probe were designed to amplify a small region of P. brassicae ribosomal DNA. Total genomic DNA was extracted and purified from soil samples using commercial kits. The amount of pathogen DNA was quantified using a standard curve generated by including reactions containing different amounts of a plasmid carrying the P. brassicae target sequence. The PCR assay was optimized to give high amplification efficiency and three to four copies of the target DNA sequence were detected. Regression analysis showed that the standard curve was linear over at least six orders of magnitude (R 2 > 0AE99) and that the amplification efficiency was >92%. The detection limit in soil samples corresponded to 500 resting spores g )1 soil. The intersample reproducibility was similar to, or higher than, that of assays for other pathogens quantified in soil samples. Bait plants were used to validate the real-time PCR assay. The protocol developed was used to investigate the spatial distribution of P. brassicae DNA in different fields and a significant difference was found between in-field sampling points. The reproducibility of soil sampling was evaluated and showed no significant differences for samples with low levels of inoculum, whereas at higher levels differences occurred. Indicator kriging was used for mapping the probability of detecting P. brassicae within a 2-ha area of a field. A threshold level of 5 fg plasmid DNA g )1 soil, corresponding to approximately 3 · 10 3 P. brassicae resting spores g )1 soil, is suggested for growing resistant cultivars. The results provide a robust and reliable technique for predicting the risk of disease development and for assessing the distribution of disease within fields.

Cultivation of forage maize is increasing in Sweden. Sole maize is low in protein and supplementation of protein feed is needed. This study investigated whether forage maize and legumes can be intercropped in a system suitable for farmers (simultaneous sowing and harvest) under Swedish climate conditions, and whether intercropping maize and faba bean improves the forage quality compared with sole maize. Two field experiments were performed in which maize intercropped with faba bean (Vicia faba L.) in alternate rows with 0 or 60 kg N/ha was compared with sole maize crop with 120 kg N/ha. Maize dry matter (DM) yield decreased by intercropping (from mean 14,171 kg DM/ha for sole maize to 8,888-10,791 kg DM/ha for intercropped maize). Mean yield of faba bean ranged from 2,907 to 2,966 kg DM/ha. Compared with sole maize, intercropped treatments slightly increased forage protein content (mean increase 10-15 g/kg DM). Furthermore, intercropping increased forage in vitro organic matter digestibility by 4 % units (from 80.8 to 84.7 %), while the starch concentration slightly decreased from 316 to 236-254 g/kg DM. Intercropping increased neutral detergent fibre concentration from 435 to 478-497 g/kg DM and the sugar concentration from 57.3 to 61.5-72.3 g/kg DM. The results showed that intercropping silage maize and faba beans is possible under Swedish climate conditions.

In field experiment studies that tested the effects of three partly resistant cultivars of spring oilseed turnip rape (Brassica campestris), multiplication of clubroot was moderate. When initial soil inoculum levels were 47–72% of infected test plants in the bioassay, only 2.1–10% of the plants were infected after harvest. The yield of partly resistant lines was 5–10% higher compared with that of the susceptible cultivar SW Kulta, which on average yielded 1.73 t/ha. The average disease severity index (DSI) after harvest in these trials was 12.7 for the nonresistant cultivar SW Kulta and ranged between 2.2 and 6.2 for the partly resistant lines. The study demonstrated that in fields where the soil infestation level gives a DSI of less than 10, or when less than 20% of the bait plants are infected in the bioassay, it is possible to avoid the risk of severe yield losses from clubroot infections by integrating a partly resistant cultivar into the crop rotation.

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