Reaction of Lines of the Rapid Cycling Brassica Collection and Arabidopsis thaliana to Four Pathotypes of Plasmodiophora brassicae.

The clubroot reaction of five Rapid Cycling Brassica Collection (RCBC) lines (Brassica carinata, B. juncea, B. napus, B. oleracea, and B. rapa) and 84 lines of Arabidopsis thaliana to pathotypes 2, 3, 5, and 6 of Plasmodiophora brassicae (as classified on William's system) was assessed. Also, the reaction of the Arabidopsis lines to a single-spore isolate of each of pathotypes 3 and 6 was compared with that of a field isolate. Seedlings were inoculated with resting spores of P. brassicae, maintained at 25 and 20°C (day and night, respectively), and assessed for clubroot incidence and severity at 6 weeks after inoculation. Several lines of A. thaliana and RCBC exhibited a differential response to pathotype but none of the lines were immune. Among the RCBC lines, B. napus was resistant to all of the pathotypes; B. oleracea was resistant to pathotypes 2, 3, and 5; B. carinata and B. rapa were resistant to pathotypes 2 and 5; and B. juncea was susceptible to pathotypes 5 and 6 and had an intermediate response to pathotypes 2 and 3. Line Ct-1 of A. thaliana was highly resistant to pathotype 2, Pu2-23 was highly resistant to pathotype 5, and Ws-2 and Sorbo were highly resistant to pathotype 6. These results indicate that the lines of RCBC and A. thaliana have potential for use as model crops for a wide range of studies on clubroot, and could be used to differentiate these four pathotypes of P. brassicae. The reaction of the RCBC lines to pathotype 6 was highly correlated with response under field conditions but the reaction to the single-spore isolates of pathotypes 3 and 6 was not strongly correlated with reaction to the field collections in the Arabidopsis lines.