Résumé
Rice yellow mottle virus (RYMV) is the most important rice-infecting virus in Africa. Highly resistant rice (Oryza spp.) cultivars Gigante and Tog5681 were challenged with virus isolates from five countries of the west and central African Sudano-savannah zone in order to investigate the occurrence and prevalence of resistance-breaking (RB) isolates. High resistance was overcome by 38.6% of the isolates. RB isolates could be divided into three main pathogenic groups. Isolates of the first group (17.5%) and of the second group (16.4%) were able to break down the resistance of Gigante only and of Tog5681 only, respectively. Resistance in both cultivars was overcome simultaneously by isolates of the third group (4.7%). In each group, some isolates induced symptoms, whereas plant infection by others was evidenced only by serological tests. RB isolates occurred in all five countries with varying frequencies (19 to 57%). The wide geographical distribution and high frequencies of RB isolates represent a high risk for the durability of resistance to RYMV in the Sudano-savannah zone. Rice yellow mottle, caused by Rice yellow mottle virus (RYMV), is a major threat to the cultivation of rice (Oryza sativa) in Africa. The disease is widespread in Africa, with estimated yield losses between 25 and 100%, but has not been reported elsewhere. Control measures against rice yellow mottle are mainly directed to breeding for resistance. Most rice cultivars are susceptible to the virus, but the cultivar Gigante and some African rice (O. glaberrima) cultivars like Tog5681, Tog5672, and Tog5675 are highly resistant to RYMV. To develop varieties with better resistance, the high resistance sources are being crossed with high yield susceptible rice lines. However, it has been reported recently that some RYMV isolates were capable (naturally or after serial inoculations) of breaking down the high resistance observed in Gigante and Tog5681. If such resistance-breaking (RB) isolates were frequent in field conditions, it would undermine the stability of resistance to RYMV. In this study, highly resistant rice cultivars Gigante and Tog5681 were challenged with virus isolates from five countries of the West and Central African Sudano-savannah zone in order to investigate the occurrence of RB isolates. High resistance in both cultivars was overcome by 38.6% of the isolates, and RB isolates could be divided into three main pathogenic groups. Isolates in the first group (17.5%) and second group (16.4%) were able to break down resistance in Gigante only and Tog5681 only, respectively. Resistance in both cultivars was overcome simultaneously by isolates of the third group (4.7%). In each group, some isolates induced symptoms, whereas plant infection by others was symptomless. RB isolates occurred in all five countries, with frequencies between 19 and 57%. The wide geographical distribution and high frequencies of RB isolates represent a potential high risk for the durability of resistance to RYMV in the Sudano-savannah zone. This suggests that in addition to breeding for resistance, other control measures like phytosanitation practices are needed in an integrated management frame of RYMV.