Evaluation of NPGS Germplasm for Resistance to Sugar Beet Root Maggot, 2022

Twenty-nine sugar beet germplasm 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), an insect pest of sugar beet causing significant yield loss in growing areas of the upper Midwest and western states of the United States and adjacent Canadian provinces. The SBRM evaluation was conducted in an SBRM insect nursery at St. Thomas, ND. The germplasm lines, along with the resistance and susceptible checks, were planted (density of 68,062 seeds/A) on 4 Jun 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 three 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 Aug. 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 0–9 (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 the root surface, and 7–9 = severe damage from numerous feeding scars covering over 25% of the root surface), and lines with ratings of 3 or less were considered resistant. The categorical damage rating data were rank transformed and were then analyzed in SAS (Ver. 9.4) using the mixed linear models (Proc MIXED) 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 with nontransformed means are presented in Table 1. Means root damage rating within column followed by a common letter are not significantly different (P < 0.05). aPlant registration number is according to records in NPGS database (https://www.ars-grin.gov/npgs/). bTen roots per plot were evaluated for root damage caused by SBRM using a scale of 0–9 (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). Within a column, means followed by the same letter did not differ significantly based on Fisher’s protected LSD. cP > F was the probability associated with the F-value. LSD = Fisher’s protected least significant difference value (α = 0.05). The SBRM pressure was uniform according to damage on roots of the susceptible check and susceptible commercial varieties on the border. No severe 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 eight sea beet accessions, 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 six sea beet lines were all susceptible with the rating scores ranged from 5.2 to 7.1, which suggested the two sea beet accessions may harbor valuable sources of resistance to SBRM. The two sea beet lines will be evaluated again to confirm SBRM resistance and will be used to develop new SBRM-resistant sugar beet lines in the USDA-ARS SBRM pre-breeding program.1 The authors declare that they have no conflicts of interest. C.C.: Project administration; investigation; methodology; data curation; formal analysis; writing—original draft; funding acquisition. B.C.H.: Data curation; resources; writing—review & editing. K.M.D.: Data curation; formal analysis; writing—review & editing; funding acquisition.

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