Evaluation location: North Dakota, United States
A commercial field site near St. Thomas (Pembina County) in northeastern North Dakota was selected to evaluate Beta germplasm accessions for potential resistance to SBRM feeding injury. The experiment was planted on 10 May, 2000. No planting-time or postemergence insecticide protection was applied to any plots. Individual treatment plots were single rows that were 35 ft (10.7 m) long and spaced 22 inches (55.9 cm) apart. Experimental design was a randomized complete block with four replications of 30 treatments. Treatments included nine accessions of Beta vulgaris, seven accessions of B. v. subsp. vulgaris, and eight accessions of B. v. subsp. maritima that were obtained from the NPGS Beta collection, courtesy of the USDA-ARS Western Regional Plant Introduction Station (Pullman, WA). Six additional entries (i.e., PI-605413, PI-608437, Beta 3712, Maribo 9363, PI-590659, and Yellow) were included for comparative purposes. One of the commercial hybrid entries, Maribo 9363, was used as the susceptible control. Larval feeding injury was assessed on up to ten sugarbeet roots per plot and rated in accordance with a 0 to 9 scale (0 = no damage and 9 = 75% or more of root surface blackened with feeding scars or a dead plant) on 2 August. Sugarbeet root maggot fly activity was relatively high in the plot area, and peaked between 7 and 8 June at about 230 flies per stake per day. Correspondingly, root maggot larval feeding injury levels observed in the trial were also high, as was observed for PI-142812, which incurred an average root injury rating (RI) of 7.0 on the 0 to 9 scale. The lowest average SBRM feeding injury in the test (RI = 2.28) was recorded for PI-608437 (F1016), which was significantly less injury than that incurred by any other entry in the trial. Other entries that incurred moderately low levels (i.e., RI < 5.0) of feeding injury that were significantly lower than that recorded for the susceptible control (i.e., Maribo 9363) included PI-546378 (RI = 4.85), PI-590659 (FC704; RI = 4.08), and PI-605413 (F1015; RI = 3.45). The relatively low incidence of injury incurred by these entries, despite the high infestation levels that developed during the 2000 growing season, suggests that they have moderate to strong tolerance to sugarbeet root maggot larval feeding injury.