Evaluation of a fluorescent staining technique as an indicator of pathogenicity of resting spores of Plasmodiophora brassicae

A report that fluorescent staining can be used as an indicator of the pathogenicity of populations of resting spores of Plasmodiophora brassicae was investigated. In a series of three experiments, resting spores of P. brassicae were subjected to stress induced by physical, ionic and disinfectant treatments. A fluorescent staining technique was used to directly assess the pathogenic activity of the inoculum and subsequent expression of disease symptoms was observed on inoculated broccoli and Chinese cabbage seedlings. Physical treatments, dry heat, pressurised heat and UV light induced gross changes in the pathogenic activity of the spores which was correlated (r = 0.80) with reductions in the severity of symptoms of disease. Ionic treatments resulted in only relatively small reductions in pathogenic activity and there was no correlation between pathogenic activity and eventual symptom expression. None of the nine commercially available disinfectants completely eliminated clubroot. Most were ineffective. There was no correlation between pathogenic activity and the development of disease symptoms in host plants following treatment of the resting spores with the disinfectants. These results indicate that fluorescent staining may be used to predict gross changes in symptom expression, but further work is required to determine whether direct (fluorescent stain) or indirect (plant bait) methods provide a better estimate of resting spore pathogenicity for routine assessment of laboratory trials. Both methods had inherent errors, and where differences between treatment effects were small, no relationship was found between the two measures.