Interpretive Summary: Thirty-one sugar beet germplasm breeding lines and releases produced by the USDA-ARS sugarbeet program at Salinas, CA, and one line from the USDA-ARS sugarbeet program at East Lansing, MI, were evaluated for resistance to rhizomania under south-central Idaho conditions at Twin Falls, ID. The field had been identified as having Beet necrotic yellow vein virus (BNYVV), which overcomes resistance conditioned by the Rz1 gene. Sugar beet lines containing the rhizomania resistance gene Rz1, Rz2, or Rz1 plus Rz2 were used as controls in each plot. Plants were topped, roots lifted then hand pulled for scoring on 12-13 Oct. Each root was scored based on the traditional 0 to 6 scale which was converted to a 0 to 9 scale. The categories were 0, 1, 3, 5, 6, 7 and 9 with 0-3 being resistant, 5-7 susceptible and 9 is dead. The average disease severity was determined to create a disease index for each entry and control. Analysis of the data showed significant differences among the entries and the controls. There were significant differences among the entries and none of the entries are better than the mean of the Rz2 resistant controls. Both the Rz2 and Rz1 plus Rz2 controls were significantly better than the Rz1 control, thus indicating that the virus in this field does partially defeat Rz1. Selections were made for advancement to next years field trial for evaluation of rhizomania.

Technical Abstract: Thirty-one sugar beet germplasm breeding lines and releases produced by the USDA-ARS sugarbeet program at Salinas, CA, and one line from the USDA-ARS sugarbeet program at East Lansing, MI, were evaluated for resistance to rhizomania under south-central Idaho conditions at Twin Falls, ID. The field had been identified as having Beet necrotic yellow vein virus (BNYVV), which overcomes resistance conditioned by the Rz1 gene. The trial layout was a random complete block with eight replications. Sugar beet lines containing the rhizomania resistance genes Rz1 and Rz2 were used as controls in each plot. Plants were topped, roots lifted then hand pulled for scoring on 12-13 Oct. Each root was scored based on the traditional 0 to 6 scale which was converted to a 0 to 9 scale. The categories were 0, 1, 3, 5, 6, 7 and 9 with 0-3 being resistant, 5-7 susceptible and 9 is dead. The average disease severity was determined to create a disease index (DI) for each entry and control. A second index (DI2) was calculated for each entry-plot to evaluate the entry relative to the mean of the Rz2 and Rz1 plus Rz2 local controls. Differences among the entries and the controls for DI were significant and differences among the entries for DI2 were significant. Separation of means using Least Significant Difference for DI2 showed that there were significant differences among the entries and given that the resistant control value is 100, none of the entries are better than the mean of the Rz2 resistant controls. Dunnett’s test to compare the entries to the three different controls for DI2 indicated that both the Rz2 and Rz1 plus Rz2 controls were significantly better than the Rz1 control, thus indicating that the virus in this field does partially defeat Rz1. Selections were made for advancement to next years field trial for evaluation of rhizomania.