Differences in Resistance Breaking Isolates of Bnyvv from Minnesota and California

Although most resistance genes (R-genes) deployed against virus infections have lasted more than 25 years, the effectiveness of Rz1 that confers partial resistance against Beet necrotic yellow vein virus (BNYVV) was compromised in approximately 10 years in MN/ND by the emergence of resistance breaking (RB) strains of BNYVV (5). Using reverse genetics, Koenig et al. (3) demonstrated that for European A type isolates of BNYVV, valine at position 67 of the BNYVV p25 protein was required to overcome Rz1-mediated resistance and allow normal virus replication. This amino acid substitution was previously associated with breakdown of Rz1 mediated resistance in field infected plants from the California Imperial Valley (CIV, 1, 2). However, Liu and Lewellen (4) did not find a correlation between p25 sequences of numerous North American isolates and virus titer in Rz1-plants in greenhouse assays. This suggested that BNYVV genetic background may affect the resistance breaking requirements of the p25 gene. Therefore, the objective of this work was to look for correlations between BNYVV p25 gene sequences and severity of rhizomania in Rz1-plants in the field.