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

Abstract

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 1 × 10⁶, 1 × 10⁷ and 2 × 10⁸ 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 1 × 10⁶ and 1 × 10⁷ spores·mL⁻¹, clubroot levels were generally higher on ‘Bronco’ relative to ‘B-2819’. However, there were no differences at 2 × 10⁸ 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 1 × 10⁶ 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 1 × 10⁶ 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 1 × 10⁷ 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.