Variation for virulence on Brassica napus L amongst Plasmodiophora brassicae collections from France and derived single-spore isolates

Variation for virulence was examined amongst 20 field collections of Plasmodiophora brassicae from France. Our of the 10 brassica lines tested, seven reacted differentially to inoculation; of these. two oilseed rape cultivars exhibited previously unreported differential responses, Some of the differential lines used previously to classify pathotypes of P. brassicae were susceptible to all collections, suggesting that pathogen populations in France may bs different from those reported elsewhere. Good pathotype discrimination Tt as obtained using a set of three cultivars of Brassica napus. Fire pathotypes. P-1, P-2, P-3, P-4 and Pg, were detected and their occurrence was unrelated to host type. Pathogenic variation amongst 17 single-spore isolates derived from three field collections was studied,:ind five pathotypes were identified. Four isolates were classified as pathotype P-1, pathogenic on all three differential hosts, and eight as pathotype P-4, pathogenic on none of the three differentials. The five other isolates were classified as pathotypes P-3, P-6, and P-7, the latter two expressing patterns of reaction not observed for field collections. Tile fractionation of different individual pathotypes from one original spore suspension confirmed the genetic heterogeneity of held populations of P. brassicae.