H. Brun et al., Presence in Leptosphaeria maculans populations of isolates virulent on resistance introgressed into Brassica napus from the B-nigra B genome, PLANT PATH, 50(1), 2001, pp. 69-74
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 resistanc
e 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 interm
ediate aggressiveness on the 'LA4+' line. Two B-group isolates and 53 A-gro
up isolates, including isolates from darkening tissue of resistant plants o
f the 'LA4+' line and cv. Junius (B. nigra), were virulent on the susceptib
le isogenic 'LA-' line. These results demonstrate that: (i) the resistance
of the 'LA4+' oilseed rape line, conferred on B. napus by B. nigra chromoso
me 4, is race specific at the cotyledon stage; and (ii) the resistant mater
ial can act as a reservoir of virulent isolates for susceptible oilseed rap
e lines.