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 ° C) followed by warm temperature (20 ° 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.

International audienceAssessing plant disease severity and pathogen population size is central to epidemiological studies that help to devise disease control practices for crop protection. Among current methods, there is a trade-off between accuracy, defined as the closeness of the estimated value to the true value, and cost, defined as the consumption of resources that have to be spent in order to achieve the appropriate measurement. On the one hand, accurate methods based on counting lesion numbers per plant are time consuming. On the other hand, quick methods based on evaluations of diseased area, are adequate for varietal evaluation, but not sufficient for a quantitative ranking of numerous observations such as those required for an estimation of spore dispersal. A new method based on counting of leaf spots observed during 1 min in a delimitated one square meter area (lesions counted/m(2)/min, Mac.m2) was tested, using phoma stem canker as a case study in experimental plots and farmers' fields of oilseed rape. We showed that direct sampling of disease symptoms is feasible, reduces observation time and yields a continuous quantitative variable. We confirmed that: (i) lesions counted/m(2)/min (Mac.m2) values were correlated with mean number of leaf spots per plant (lesions/plant, Mac.pl); (ii) repeat phoma leaf spot counts of the same sample area by the two methods (Mac.m2 and Mac.pl) were correlated when assessed either by the same or different observers; and (iii) ranking of field plots with different disease severity was coherent among observers. We conclude that the lesions counted/m(2)/min (Mac.m2) method is equal in performance to the lesions/plant (Mac.pl) method, and was faster. Used with care, this method will facilitate studies requiring disease severity estimates, which were previously hampered by cost and/or tim