Summary• It has frequently been hypothesized that quantitative resistance increases the durability of qualitative (R-gene mediated) resistance but supporting experimental evidence is rare. To test this hypothesis, near-isogenic lines with ⁄ without the Rgene Rlm6 introduced into two Brassica napus cultivars differing in quantitative resistance to Leptosphaeria maculans were used in a 5-yr field experiment.• Recurrent selection of natural fungal populations was done annually on each of the four plant genotypes, using crop residues from each genotype to inoculate separately the four series of field trials for five consecutive cropping seasons. Severity of phoma stem canker was measured on each genotype and frequencies of avirulence alleles in L. maculans populations were estimated.• Recurrent selection of virulent isolates by Rlm6 in a susceptible background rendered the resistance ineffective by the third cropping season. By contrast, the resistance was still effective after 5 yr of selection by the genotype combining this gene with quantitative resistance. No significant variation in the performance of quantitative resistance alone was noted over the course of the experiment.• We conclude that quantitative resistance can increase the durability of Rlm6. We recommend combining quantitative resistance with R-gene mediated resistance to enhance disease control and crop production.

Environmental factors such as temperature strongly impact microbial communities. In the current context of global warming, it is therefore crucial to understand the effects of these factors on human, animal, or plant pathogens. Here, we used a common‐garden experiment to analyze the thermal responses of three life‐history traits (latent period, lesion growth, spore number) in isolates of the potato late blight pathogen Phytophthora infestans from different climatic zones. We also used a fitness index (FI) aggregating these traits into a single parameter. The experiments revealed patterns of local adaptation to temperature for several traits and for the FI, both between populations and within clonal lineages. Local adaptation to temperature could result from selection for increased survival between epidemics, when isolates are exposed to more extreme climatic conditions than during epidemics. We also showed different thermal responses among two clonal lineages sympatric in western Europe, with lower performances of lineage 13_A2 compared to 6_A1, especially at low temperatures. These data therefore stress the importance of thermal adaptation in a widespread, invasive pathogen, where adaptation is usually considered almost exclusively with respect to host plants. This must now be taken into account to explain, and possibly predict, the global distribution of specific lineages and their epidemic potential.

The main objective of this investigation was to test the 'always more aggressive' hypothesis, often advocated to explain lineage replacements in clonal populations of the potato late blight oomycete Phytophthora infestans. To this end, genotypic and pathogenicity data on 1274 French isolates of P. infestans, collected over the period 2001-2008, were analysed. Overall, the populations sampled showed limited genetic diversity, with four multilocus lineages (1_A1, 2_A1, 8_A1 and 13_A2) accounting for over 80% of the isolates collected. As in other West European countries, drastic changes in these dominant clonal lineages were observed over the course of the 8 years, particularly in the appearance and propagation of the clone 13_A2. However, invasiveness of clones was not associated with higher aggressiveness; on the contrary, dominant clones had generally low or moderate aggressiveness relative to others present at the same time within the same populations. This finding challenges the link between invasive behaviour and increased aggressiveness often assumed for this biotrophic pathogen, and could reflect the existence of a trade-off between intra-season and inter-season transmission. This would be consistent with the concept of inclusive fitness, which involves the abilities to both reproduce and survive.

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