USDA-ARS Ft. Collins germplasm screened for resistance to Beet curly top, 2015

Fifty sugar beet (Beta vulgaris L.) germplasm lines produced by the USDA-ARS Ft. Collins sugar beet program and two commercial check cultivars [SV2012RR (susceptible) and HM PM90 (resistant)] were screened for resistance to Beet curly top virus (BCTV). The curly top evaluation was conducted at the USDA-ARS North Farm in Kimberly, ID, which had been in barley in 2014. The germplasm was planted (density of 142,560 seeds/A) on May 27. The plots were two rows 10 ft long with 22-in row spacing and arranged in a randomized complete block design with three replications. The fields were sprinkler irrigated, cultivated, and hand weeded as necessary. Plant populations were thinned on June 20 Plants were inoculated at the four- to six-leaf growth stage on June 24 Jun with approximately six viruliferous beet leafhoppers per plant. The beet leafhoppers were redistributed by dragging a tarp through the field. The plants were sprayed with Lorsban 4E (1.5 pints/A) on July 7 to kill the beet leafhoppers. Plots were rated for foliar symptom development on 13 and 20 Jul using a disease index scale (DI) of 0 to 9 (0 = healthy and 9 = dead). Data were analyzed in SAS using the general linear models procedure (Proc GLM), and Fisher’s protected least significant difference (LSD; α = 0.05) was used for mean comparisons. We are reporting the second and most severe disease rating scores. There were significant differences among entries. Additionally, an analysis of variance (PROC MIXED) was performed on DI, and Dunnett’s one-tailed t-test (p = 0.05), adjusted for sample size, was used to compare all entries to the resistant control (HM PM90) and the most susceptible germplasm (20121034) for DI. Curly top symptom development was uniform and no other disease problems were evident in the plot area. The resistant and susceptible checks performed as expected for both visual ratings. The DI was 4.0 in the resistant control and 7.9 in the most susceptible entry. Those entries, for which DI <5.5, were not significantly different than the resistant control (HM PM90) (Dunnett’s one tailed t-test, p = 0.05). Similarly all entries for which DI > 6.2, were not significantly different than the most susceptible entry (20121034) (Dunnett’s one tailed t-test, p = 0.05). Those entries for which DI > 5.4 and DI < 6.4 showed a moderate resistance, significantly more resistant than the most susceptible entry but significantly more susceptible than the resistant control. Based on their performance, entries will be released for resistance to BCTV or re-selected to improve their resistance to BCTV. All germplasms developed by the USDA-ARS pre-breeding program at Fort Collins are screened for BCTV before release; even if they have not been selected primarily for BCTV-resistance, because this is useful information for other plant breeders wishing to incorporate released germplasm into their breeding programs.