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 P lasmodiophora brassicae from France. Our of the 10 brassica lines test ed, 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 pathotype s of P. brassicae were susceptible to all collections, suggesting that pathogen populations in France may bs different from those reported e lsewhere. 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 t hree field collections was studied,:ind five pathotypes were identifie d. Four isolates were classified as pathotype P-1, pathogenic on all t hree differential hosts, and eight as pathotype P-4, pathogenic on non e 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 di fferent individual pathotypes from one original spore suspension confi rmed the genetic heterogeneity of held populations of P. brassicae.