Résumé
Madagascar is one of the biggest rice producers in Africa. About 20% of the production is rainfed rice. Since the 1990's, cold tolerant cultivars adapted to high altitude have been successfully proposed to farmers and adopted in the central highlands of the country. But rice blast pressure on the hillsides is important and is the second constraint (after cold) breeders have to take into account. In the same time, hillsides were more and more put under cultivation, conservation agriculture cropping systems have been proposed to farmers (Scopel et al., 2013), to limit erosion and to ensure the sustainability of rainfed crops. In experimental studies, these conservation agriculture cropping systems were shown to limit blast epidemics (Sester et al., in press). We hypothesized that the specificities of the conservation agriculture cropping systems (no tillage, permanent soil cover and rotations) induced changes in plant nutrition and plant nutritional balance, especially nitrogen assimilation, and that it was the reason of a differential expression of tolerance to blast. To test this hypothesis, pluriannual factorial experiments were conducted in two sites to measure blast disease severity on leaves and on panicles in two cropping systems (conservation agriculture vs conventional) and three nitrogen fertilization levels. Crop development and nitrogen assimilation were measured at four stages and yield components were assessed at harvest. A mineral diagnosis was performed at booting stage on the flag leaf. A relational database has been created to manage agronomic and disease severity rating data from the project. For statistical analysis (performed using Sas) the proportion of damaged leaf area and damaged panicles were fitted to a generalized linear model. We will present the results of the respective impacts of the cropping system environment, the nitrogen fertilization and the crop density on the leaf and panicle blast level. In some particular conditions, a significant impact of the cropping system appeared in addition to the effects explained by crop density, confirming the hypothesis of a plant susceptibility modification. We present then the way these results could be used to adapt cropping systems and to improve upland rice tolerance to blast in a sustainable way. (Résumé d'auteur)