Barley yellow mosaic virus is overcoming RYM4 resistance in Belgium.

Barley yellow mosaic virus (BaYMV) is the causal agent of a soil-borne systemic mosaic disease on barley. It has been reported in Belgium since the 1980s. The control of this disease is managed almost exclusively through the use of resistant varieties. The resistance of most commercial barley cultivars grown in Europe is conferred mainly by a single recessive gene, rym4. This monogenic resistance provides immunity against BaYMV pathotype 1 and has been mapped on barley chromosome 3HL and shown to be caused by mutations in the translation initiation factor eIF4E. Another pathotype, BaYMV pathotype 2, which appeared in the late 1980s (in Belgium, in the early 1990s), is able to overcome the rym4-controlled resistance. Until recently, this pathotype remained confined to specific locations. During a systematic survey in 2003, mosaic symptoms were observed only on susceptible barley cultivars collected in Belgian fields. BaYMV was detected by ELISA and RT-PCR on the susceptible cultivars and only by RT-PCR on the resistant cultivars. In 2004, mosaic symptoms were observed on susceptible and resistant cultivars. BaYMV was detected by ELISA and RT-PCR on both cultivars. In addition to developing RT-PCR methods for detecting and identifying BaYMV and Barley mild mosaic virus (BaMMV), an RT-PCR targeting the VPg/NIa viral protein part of the genome, known to discriminate the two BaYMV pathotypes, was set up to accurately identify the pathotype(s) now present in Belgium. The sequences from the generated amplicons revealed the single nucleotide substitution resulting in an amino acid change from lysine to asparagine specific to BaYMV pathotype 2. The possible reasons for the change in the BaYMV pathotype situation in Belgium, such as climatic change or a progressive build-up of soil inoculum potential, will be discussed, as well as the use of eIF4E-based resistance.