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 sugarbeet root maggot (SBRM) feeding injury. Plots were planted on 9 May, 2016, and no planting-time or postemergence insecticide protection was applied to any plots. Individual treatment plots were single, 14 ft (4.3 m) long rows that were spaced 22 inches (55.9 cm) apart. Experimental design was a randomized complete block with four replications of 38 treatments. Treatments included 29 accessions of Beta vulgaris subsp. vulgaris and one accession 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). Eight additional entries (i.e., PI-605413, PI-608437, PI-658654, PI-676971, PI-535818, PI-179180, PI-181718, and ACH 817 [Crystal 817]) were included for comparative purposes. Larval feeding injury was assessed on up to ten 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 18 August. Sugarbeet root maggot fly activity was monitored in the plot area throughout the growing season by using sticky-stake traps. Activity peaked on 9 June at high rate of about 160 flies per trap per day. Root maggot larval feeding injury levels observed in the trial were also relatively high, as was observed for PI-175596, which incurred an average root injury rating (RI) of 6.75 on the 0 to 9 scale. Unusually high amounts of rainfall (i.e., post-plant total of over 20 inches [51 cm]) occurred in the plot area, which led to long periods of standing water and the loss of several plants (e.g., two entries, for which there were only six and eight observations, were excluded from the analysis). Standing water also could have impacted root maggot egg deposition. As such, these results should be interpreted with some degree of discretion, as they may not completely reflect outcomes that would have occurred under more typical growing conditions. The lowest average SBRM feeding injury in the test (RI = 2.09) was recorded for PI-181718. Other entries that incurred moderately low levels (i.e., RI < 4.0) of feeding injury that were not significantly different from that recorded for PI181718 included the following (in ascending order of injury): PI-179180 (RI = 2.47), PI-535818 (F1010; RI = 2.74), PI-658654 (F1024; RI = 2.84), PI-174063 (RI = 2.93), PI-676971 (F1043; RI = 3.07), PI-560130 (RI = 3.12), PI-515964 (RI = 3.35), PI-518325 (RI = 3.44), PI-515965 (RI = 3.60), PI-608437 (F1016; RI = 3.62), PI-560340 (RI = 3.78), PI-470091 (RI = 3.79), PI-175601 (RI = 3.80), PI-120282 (RI = 3.87), and PI-109040 (RI = 3.88). The relatively low feeding injury incurred by these entries, despite relatively high SBRM infestation levels, suggests that they have moderate to strong tolerance to root maggot feeding injury.