Presence in Leptosphaeria maculans populations of isolates virulent on resistance introgressed into Brassica napus from the B-nigra B genome

The resistance carried by chromosome 4 of the Brassica B genome confers on the Brassica napus-B. nigra addition line 'LA4+' a high level of resistance to Leptosphaeria maculans isolates at the cotyledon stage under controlled conditions and at all growth stages under field conditions. This resistance has never been used commercially in B. napus varieties. To determine the potential durability of this new resistance, it is necessary to investigate whether it exerts selective pressure on fungus populations. A study of the pathogenicity of 57 single ascospore isolates of L. maculans on this line at the cotyledon stage was carried out to examine possible race specificity of the resistance. Two isolates were clearly virulent and three had intermediate aggressiveness on the 'LA4+' line. Two B-group isolates and 53 A-group isolates, including isolates from darkening tissue of resistant plants of the 'LA4+' line and cv. Junius (B. nigra), were virulent on the susceptible isogenic 'LA-' line. These results demonstrate that: (i) the resistance of the 'LA4+' oilseed rape line, conferred on B. napus by B. nigra chromosome 4, is race specific at the cotyledon stage; and (ii) the resistant material can act as a reservoir of virulent isolates for susceptible oilseed rape lines.