<b> Variation for virulence on <i>Brassica napus</i> L. amongst <i>Plasmodiophora brassicae</i> collections from France and derived single‐spore isolates </b>

Abstract

Variation for virulence was examined amongst 20 field collections of Plasmodiophora brassicae from France. Out 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 be different from those reported elsewhere. Good pathotype discrimination was obtained using a set of three cultivars of Brassica napus . Five pathotypes, P 1 , P 2 , P 3 , P 4 and P 5 , were detected and their occurrence was unrelated to host type. Pathogenic variation amongst 17 single‐spore isolates derived from three field collections was studied, and 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. The fractionation of different individual pathotypes from one original spore suspension confirmed the genetic heterogeneity of field populations of P. brassicae.