Résumé
In the densely populated highlands of Madagascar, growing upland rice offers the opportunity to increase the total rice cropping area and to improve food security. However, rice blast was a major constraint for the first cultivars released in the 1990s and consequently limited the extension of upland rice. However, blast epidemics are much less intense in the region of Betafo, where the composition of the soil, an Andosol developed from volcanic rock, is different from the classical Ferralsol of the highlands. A 3-year field experiment (2009-2011) was conducted near Antsirabe to compare blast epidemics on rice grown in Ferralsol vs. an Andosol. Leaf and panicle blast development were monitored and the yield components of upland rice plants growing on the two different soil orders were measured. In 2009 and 2011, leaf and panicle blast development were significantly lower for plants grown on the Andosol compared to those grown on the Ferralsol (final panicle blast reduced by 40% in 2009 and 20% in 2011). The severity of blast was shown to be related to the concentrations of mineral elements in the plant, and the Si content was significantly higher in plants growing on the Andosol. In 2010, the differences of blast incidence between the two soils were less marked (14% reduction of panicle blast at the last scoring date). AUDPC were lower in the Andosol compared to the Ferralsol each year, for leaf and panicle blast. The yield components 1000 full grain weight, dry straw weight and the yield were higher in the Andosol compared to the Ferralsol in 2009 and 2010 but were not significantly different in 2011. These results clearly document that blast development may be impacted by the soil order in which rice is grown, and future agronomic management of blast should focus on improved soil mineral composition such as silicon.