Cultivar and species mixture effect on wheat septoria tritici blotch spreading
Résumé
Within the framework of pesticide reduction policy, it is necessary to find alternative practices to control wheat diseases. For air-borne diseases such as rusts and powdery mildew, cultivar and species mixtures can reduce severity by 40 to 80% compared to the mean of the pure stands. However, for rain-borne diseases such as septoria tritici blotch (STB), caused by the pathogenic fungus Mycosphaerella graminicola, severity may be reduced by only 6 to 27% (de Vallavieille-Pope, 2004). The benefits of cultivar and species mixtures (such as disease management and agronomic performance) rely on combinations of disease pressure, cultural practices (e.g. fertilization), and mixture proportions. The main factors involved in the reduction of disease severity in mixtures are the proportion of susceptible/resistant host leading to the genetic barrier effect, and the induced resistance (Finckh et al., 2000). In the case of foliar fungal disease dispersed by rain splash, a physical barrier to the transfer of splash droplets carrying the spores also contributes to the reduction of the disease severity (Saint-Jean et al., 2008). Thus, in order to control STB epidemics, it is necessary to study the fungus dispersal within mixtures. For the particular case of species mixtures such as cereals/legumes there are nitrogen exchanges between the two components, which leads us to investigate the effects of the nitrogen quantity and origin on the STB development. An experiment was set up at Grignon for several years. Different proportions of a susceptible and a resistant wheat cultivars were tested. Mixtures with the susceptible wheat cultivar and pea were also tested. Two levels of nitrogen supply were applied on each mixture. Diseased surfaces, emergence of necrotic surface, were assessed weekly. Meteorological and spore fluxes were also assessed during the epidemic seasons. Depending on the disease pressure and the direct and indirect nitrogen supply, our findings show that the emergence of leaf necrotic (diseased) surfaces can be reduced by 60% in cultivar mixtures compared to the pure stands.
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