ABSTRACT
Rhizomania, a serious disease of sugar beet (Beta vulgaris), is caused by Beet necrotic yellow vein virus (BNYVV). Resistance allele Rz1 has been widely incorporated into commercial cultivars. Recently, resistance-breaking isolates of BNYVV (RB-BNYVV) were identified and characterized. When the occurrence of RB-BNYVV was surveyed throughout the sugar-beet-growing areas in the United States, most soil samples contained Beet oak-leaf virus (BOLV) as well. BNYVV and BOLV often occurred in the same field and sometimes in the same sugar beet plant. The possibility of interactions between these two Polymyxa betae-transmitted sugar beet viruses was tested. Plants grown in soils infested with aviruliferous P. betae or carrying RB-BNYVV and BOLV, alone and in combination, were compared with plants grown in noninfested soil for differences in plant fresh weight and virus content as measured by enzyme-linked immunosorbent assay (ELISA). Rz1 and Rz2 resistance genes that condition resistance to BNYVV did not confer resistance to BOLV. BNYVV ELISA values were significantly higher in single infections than in mixed infections with BOLV in both the rhizomania-resistant and -susceptible cultivars. In contrast, ELISA values of BOLV were not significantly different between single and mixed infections in both the rhizomania-resistant and -susceptible cultivars. Results indicate that BOLV may suppress BNYVV in mixed infections. Soils infested with P. betae significantly reduced fresh weight of sugar beet seedlings regardless of whether they were with or without one or both viruses or resistance genes.
