Résumé
In Côte d'Ivoire, the S2 strain of Rice yellow mottle virus (RYMV) predominated in the forested zones, including the “rice belt” to the west, in each of the cropping systems where rice was grown. The S1 strain occurred more frequently in the northern Guinean savanna, and only S1 isolates were found further north in the Sahelo-Soudanian zones. In mixed infection, S2 dominated over S1 both in viral capsid and RNA contents under temperature regimes encompassing those observed in savanna and forested zones of Côte d'Ivoire. There was no evidence of interactions in virus accumulation between the West African strains S1 or S2 with the more distantly related East African strain S4. Field trials emphasized the impact of RYMV, which induced yield losses of 40 to 60% in several widely grown cultivars of Oryza sativa indica and O. sativa japonica. We report the high resistance of the O. indica cv. Gigante under field conditions which was apparent with all the S1 and S2 isolates tested. Responses to RYMV infection of several cultivars were isolate dependent. With most differential cultivars, responses were not strain specific, with the exception of the O. japonica cv. Idsa6, in which the S2 isolates always induced higher yield losses than the S1 isolates. Rice cultivation throughout Africa is severely affected by Rice yellow mottle virus (RYMV). In Côte d'Ivoire and in neighboring countries of West Africa, two strains of RYMV occurred, RYMV-S1 being more prevalent in the savanna in the north and RYMV-S2 in the forested zones in the south. When coinoculated, S2 dominated over S1 in virus titer under temperature regimes encompassing those observed in savanna and forested zones of Côte d'Ivoire. There was a significant relationship between symptom intensity and yield loss, which confirmed the importance of making symptom assessments in screening tests. However, yield loss estimates gave more consistent results and greater discrimination, possibly because symptoms are masked in specific growing conditions. Field trials emphasized the impact of RYMV, which induced yield losses of 40 to 60% in several widely grown cultivars of Oryza sativa indica and O. sativa japonica. We report the high resistance of O. indica cv. Gigante under field conditions which was apparent with all of the S1 and S2 isolates tested. This emphasizes the importance of the current plant breeding projects to transfer this type of resistance to the currently susceptible irrigated and lowland cultivars. This work documents a variability of cultivar-isolate interactions which, practically, indicates that breeding projects should include a screening against a range of isolates differing in pathogenicity.