Evaluation location: Colorado, United States
Thirty beet (Beta vulgaris subsp. maritima (L.) Arcang and Beta vulgaris subsp. vulgaris L.) plant introduction (PI) 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 2011 was a randomized complete-block design with five replications in one-row plots (76 cm row spacing) 4 m long. The field had been planted to sugar beet in 2007 and summer fallowed since then. The soil (Garrett loam, 0 to 1 % slope, pH 7.8) was fumigated with Telone® II in late Oct 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. Field was land leveled in Mar 2011, and bedded a month before planting. Seed was planted on 17 May to moisture and furrow irrigated as needed. No herbicides were used this year. There was a heavy rain right after planting and the field crusted badly. On 24 Jun the decision to replant this experiment was made. The field was rotary hoed on 3 Jul, roto-bedded on 5 Jul, and replanted on 7 Jul. The field was hand weeded on 30 Jul and thinned on 6 Aug. Inoculation with dry, ground, barley grain inoculum of Rhizoctonia solani isolate R-9 (AG-2-2 IIIB) was applied to the crown of the plants on 17 Aug at a rate of 5.3 g m-1 row. A Gandy® electrically driven applicator was used to apply the inoculum and the field was cultivated afterwards to place soil onto the plant crowns. Beets were harvested 28 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. Analysis of variance was performed in SAS (Ver. 9.2) using Proc GLIMMIX on disease index and mean DI. 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 was used to compare entries to the resistant check (FC703) and the highly resistant check (FC709-2).