Evaluation location: Colorado, United States
Thirty sea beet (Beta vulgaris subsp. maritima (L.) Arcang accessions from the Beta collection of the USDA-ARS National Plant Germplasm System were screened for resistance to Rhizoctonia root and crown rot, at the USDA-ARS Fort Collins, CO Research Farm. The Rhizoctonia screening nursery in 2012 was a randomized complete-block design with five replications in one-row plots (76 cm row spacing) 4 m long. The soil (Garrett loam, 0 to 1 % slope, pH 7.8) was fumigated with Telone® II in late October 2008, for control of soil borne diseases (esp. rhizomania) and pests. Manure was applied 4 days later and the field was roller harrowed in Nov 2008. The field had been planted to sugarbeet in 2009 and summer fallowed until 2011, when it was planted to Grazex BMR 737 (a sorghum/sudangrass hybrid). The field was roller harrowed, leveled, and bedded 14-16 May of 2012. Seed was planted to moisture on 18 May and furrow irrigated as needed. No herbicides were used. Heavy rain right after planting caused the field to crust badly. On 7 Jun the decision to replant the field was made. The field was rotary bedded to remove plants on 18 Jun, and replanted on 20 Jun. The field was hand weeded and thinned on 15 and 28 Jul. Inoculation with dry, ground, barley grain inoculum of Rhizoctonia solani isolate R-9 (AG-2-2) was applied to the crown of the plants on 2 Aug (8 to 10 leaf stage) at a rate of 5.98 g m-1 of row. A Gandy® electrically driven applicator was used to apply the inoculum and the field was cultivated to place soil into the plant crowns. Beets were harvested 20 Sep with a single row lifter (pulled and cleaned by hand), and each root was rated for rot on a scale of 0 (no damage) to 7 (dead plant with root completely rotted). Average disease severity per plot was determined to create a disease index (DI) for each entry and DI was treated as a continuous variable. Analysis of variance was performed in SAS (Ver. 9.2) using Proc GLM (Data not shown). Data also are represented as the percentage of sugar beet roots in classes 0 through 1, considered as healthy and in classes 0 through 3, considered harvestable. Because the analysis of variance does not group the entries into discrete classes, Dunnett’s one-tailed t-test (p = 0.05) was used to compare entries to the resistant control (FC703), the highly resistant control (FC709-2) and the most susceptible Plant Introduction (PI 518414).
Spring of 2012 in Fort Collins, CO was hot and dry. There was a heavy rainfall right after the first planting. Because of heavy crusting, a western beet roller was used on 30 May to break up the crust. Moisture was good at replanting. With the warm soil temperatures, the replanted beets developed quickly and were ready for inoculation (8 to 10 leaf stage) about 40 days after planting. Temperatures remained high throughout September and there was moderate, uniform disease pressure. There were highly significant differences among entries for DI in this test and good separation among entries, although the susceptible control scored more resistant than expected. Nonetheless, 24 PIs (all with a DI > 3.7 – below the line in the table) were not significantly different (Dunnett’s one-tailed t-test; p = 0.05) than the worst performing Plant Introduction – PI 518414, which had a DI of 5.7. The 4 most resistant PIs (PI 604511, PI 604533, PI 604510, PI 518367) were not significantly different (Dunnett’s one-tailed t-test; p = 0.05) from the highly resistant control (FC709/2) and represent potential sources of resistance to Rhizoctonia root and crown rot. There were 2 PIs that were significantly more resistant than the worst performing line but not significantly different in performance from the resistant control (FC703) – PI 604519 and PI 518360. These germplasms are above the line in the table under PI 518360. These accessions will be retested and, if the resistance is confirmed, entered into the USDA-ARS Rhizoctonia crown and root rot -resistance breeding program at Fort Collins, CO to enhance sugar beet germplasm with increased resistance to rhizoctonia root rot. These results will be accessible to interested parties through the USDA-ARS, NPGS GRIN database (http://www.ars-grin.gov/npgs/index.html).