Genetic resistance is the only way to control mosaic damage in winter barley. At least 18 resistance genes are known, but until recently, rym4 was the most used by breeders to control Barley yellow mosaic virus-1 (BaYMV-1), predominant in France and Barley mild mosaic virus (BaMMV). However, since 2009, new significant damage has been observed in malting barley in a widening production area, indicating possible overcoming of resistance, with the development of BaYMV-2. The objectives of the Mosa-hordeum project were to: identify new viruses or pathotypes; develop new detection tools; confirm cultivar resistance observed in field using MB tools; confirm efficiency of resistance genes cited in literary reviews; quantify impact on yield and malting quality. A complete viral inventory was carried out 2013- 2016 on affected barley crops in 108 sites using: real-time PCR; Sanger sequencing; Next-Generation Sequencing; and serological tests. A new tool, based on derived Cleaved Amplified Polymorphic Sequences (dCAPS), developed to investigate BaYMV-1 and 2 distributions, demonstrated that BaYMV-2 is predominant (> 95%) in diseased samples. BaYMV-1 and BaMMV were also identified in co-infection with BaYMV-2 on susceptible cultivars. Comparison of yield components, malting and beer quality obtained on healthy/contaminated plots with couples of cultivars revealed variation in yield losses, up to 3t/ha, reduction in number of spikes and kernels/m², smaller kernels, slight increase in protein content and decrease of malt extract. Phylogenetic analyses indicated that the rym4 resistance-breaking ability of BaYMV-2 independently evolved on multiple occasions. In limited number of trials, rym5 resistance was overcome by a variant of BaMMV. The implantation of differentials in 21 contaminated trials confirmed the efficiency of 11 resistance genes against the BaYMV-2/BaMMV complex. To monitor BaYMV and BaMMV resistance for registration in the National List real-time PCR appears more efficient than dCAPS. These findings will help breeders achieve sustainable resistance.< Réduire