Dynamics of the rice yellow mottle disease in western Burkina Faso: epidemic monitoring, spatio-temporal variation of viral diversity and pathogenicity in a disease hotspot
The rice yellow mottle virus (RYMV) is a model in plant virus molecular epidemiology and phylogeography, with the reconstruction of historical introduction routes at the scale of the African continent. However, information on patterns of viral prevalence and viral diversity over multiple years at local scale remain scarce, in spite of potential implications for crop protection. Here we describe a five-years monitoring of RYMV prevalence in six sites from western Burkina Faso. This study confirmed one irrigated site as a disease hotspot, and found two rainfed lowland sites with occasional high prevalence levels. Within studied field, a pattern of disease aggregation was evidenced at a five-meter distance, as expected for a mechanically transmitted virus. Next, we monitored RYMV genetic diversity in the irrigated disease hotspot site, revealing a high viral diversity, with the current coexistence of various distinct genetic groups at the site-scale (irrigated perimeter of ca. 520 ha), and also within various specific fields (25 meters side). One genetic lineage, named S1bzn, is the most recently introduced group and increased in frequency over the studied period. Its genome results from a recombination between two other lineages. Finally, experimental work evidenced no differences between three rice varieties cultivated in Burkina Faso in terms of resistance level, and no statistical effect of RYMV genetic group on symptom expression and viral load. We found however, that infection outcome depended on the specific RYMV isolate, with various isolates from the lineage S1bzn found to be particularly aggressive, including one accumulating at highest level. Overall, this study documents a case of high viral prevalence and high viral diversity, with the co-occurrence of divergent genetic lineages at small geographic scale. A recently introduced lineage, that includes viral isolates with high symptoms and accumulation in controlled conditions, could be recently rising though natural selection. Following up the monitoring of RYMV genetic and pathogenic diversity in the area is required to confirm this trend and further understand the factors driving the maintenance of viral diversity at local scale.