Development of Plasmodiophora brassicae in the root cortex of cabbage over time

Pathogen development in the root cortex of three cabbage (Brassica oleracea var. capitata) cultivars that differed in clubroot reaction (susceptible Bronco', partially resistant B-2819', resistant Tekila') was assessed over time in plants inoculated with 1x10(6), 1x10(7) and 2x10(8) spores mL(-1) of Plasmodiophora brassica in a controlled environment study. Cortical colonization and symptom development in Bronco' and B-2819' were affected by inoculum concentration. When inoculated at 1x10(6) and 1x10(7) sporesmL(-1), clubroot levels were generally higher on Bronco' relative to B-2819'. However, there were no differences at 2x10(8) spores because incidence and severity in both Bronco' and B-2819' were near the maximum. Similarly, the area colonized by P. brassicae (%) on root cross-sections was much higher in Bronco' than B-2819' at 1x10(6) spores, but differences in cortical colonization, if any, were small at higher inoculum concentrations. The concentration of P. brassicae DNA in inoculated roots (assessed using qPCR) was higher in Bronco' relative to B-2819' at 1x10(6) spores, and there were no differences between the two cultivars at the highest inoculum concentration, which matched with the pattern of symptom development. In contrast, symptom development in B-2819' lagged behind increases in DNA level at 1x10(7) spores. No cortical infection was observed in Tekila' at the two lower inoculum concentrations, and only a few spindle clubs developed at the highest concentration. These results demonstrated that this source of partial resistance was strongly affected by inoculum concentration, while strong resistance was much less affected.