The influence of competitive effects between two isolates, of the number of infection sites on cotyledons and of host polygenic resistance on the systemic growth of Leptosphaeria maculans, the cause of phoma stem canker in oilseed rape (Brassica napus), were investigated. Controlled-condition experiments were conducted with two oilseed rape doubled haploid lines, one susceptible and the other with a high level of polygenic resistance, inoculated via wounded cotyledons with conidial suspensions obtained from two isolates. Expression of cankers in plants was enhanced by exposing inoculated plants to low temperature (6 degrees C) followed by warm temperature (20 degrees C). The fungus was detected by PCR amplifications of three minisatellite markers in all stems with visible canker symptoms and also in the stems of 14 of the 59 plants without visible cankers on the hypocotyls. Disease severity increased with the number of infection sites on cotyledons: in one of the three replicate experiments, the mean external necrosis length on the hypocotyl ranged from 6.47 to 35.3 mm for one and eight infections sites on cotyledons, respectively. The probability of an isolate reaching the hypocotyl from inoculated cotyledons decreased with increasing competing inoculum load on cotyledons: for instance, for isolate A290v it decreased from 1 when inoculated alone to 0.28 when coinoculated with six drops of competing isolate P27d. Polygenic resistance significantly reduced disease incidence and severity. For instance, in one of the three replicate experiments, disease incidence ranged from more than 74% in susceptible plants to 16% in resistant ones, while mean external necrosis length was up to 35.3 and 6.5 mm on susceptible and on resistant plants, respectively. This study offers new possibilities for assessing levels of polygenic resistance to stem canker in B. napus and studying the aggressiveness of L. maculans isolates