Evaluation location: North Dakota, United States
Twenty-nine sugar beet (Beta vulgaris L.) lines from the National Plant Germplasm System (NPGS) at Pullman WA, and two resistant (F1024 and F1016) and one susceptible (C817) check cultivars were screened for resistance to sugar beet root maggot (SBRM), a severe pest insect of sugar beet causing significant yield loss in growing areas of the upper mid-west and western states of the U.S. and adjacent Canadian provinces. The SBRM evaluation was conducted at the sugar beet root maggot nursery in St. Thomas, ND. The germplasm lines along with the resistance and susceptible checks were planted (density of 68,062 seeds/A) on 4 June 22. Each plot included two rows of 8.5-m long, and between-row space was set at 56 cm. A randomized complete block design was used for resistance evaluation with 3 replications included. The sugar beet nursery field was managed according to standard cultural practices as conducted in northern North Dakota. The evaluation relied on endemic field insect infestation that caused feeding damage on roots by SBRM. The plots were rated for size and area of feeding scars on the roots on 23 August. Ten roots from middle of two rows in each plot were manually dug out and were then hand cleaned in water using a brush to remove all soil on roots to clearly show insect feeding scars. Damage on the roots was rated using a widely accepted scale of 0 to 9 (0 = no damage, 1-3 = minor damage from few feeding scars, 4-6 =moderate to heavy damage from large feeding scars covering less than 25% of root surface, and 7-9 = severe damage from numerous feeding scars covering over 25% of root surface), and lines with ratings of 3 or less were considered resistant. The root rating data were rank transformed and analyzed in SAS (Ver. 9.4) using the mixed linear model (Proc MIXED) procedure with germplasm lines as fixed effect and blocks as random effect, and Fisher’s protected least significant difference (α = 0.05) was used for mean comparisons, which are presented in the table.
The SBRM pressure was uniform according to damage on roots of the susceptible check and susceptible commercial varieties on the border. No sever other disease problems were observed in the plot area. The two resistant checks F1024 and F1016 both have damage ratings around 2, which agree with observations from previous years. Of the total 29 NPGS accessions evaluated and reported in the table, all lines of cultivated beet (sugar beet, fodder beet and table beet) showed moderate to heavy damages caused by SBRM with average ratings ranged from 4.2 to 7.1. Of the 8 accessions of sea beet (B. vulgaris ssp. maritima), lines PI 518344 and PI 518408 were both resistant and showed a mean damage rate of 2.1 and 2.6, respectively, and the remained 6 B. maritima lines were all susceptible with the damage ratings ranged from 5.2 to 7.1. Therefore, two B. maritima lines PI 518344 and PI 518408 will be evaluated again to confirm their resistance. If resistance is confirmed, these germplasm lines will be used for trait introgression in the USDA-ARS root maggot pre-breeding program.