Evaluation location: Colorado, United States
Twenty-nine beet (Beta vulgaris subsp. vulgaris and Beta vulgaris subsp. maritima (L.) Arcang) accessions from the Beta collection of the USDA-Agricultural Research Service National Plant Germplasm System were screened for resistance to Rhizoctonia crown and root rot (RCRR) at the Colorado State University Agricultural Research, Development, and Education Center in Fort Collins, Colorado. The Beta PI Rhizoctonia nursery used two highly resistant germplasm (FC709-2 and FC705/1), one resistant germplasm (FC703) and one susceptible germplasm (FC901/C817) as controls. The nursery was planted in a completely randomized design with five replications in one-row plots (76 cm row spacing) 3.7 m long. The soil is a Fort Collins loam, (0 to 1 % slope, pH 7.2). In 2015 and 2016, the field was planted to corn, followed by pinto bean in 2017 and hard red winter wheat in 2018. In 2019, the field was not fertilized due to available nitrogen and was bedded on 15 May, and the field was planted on 30 May with an initial irrigation of 1.9cm on 1 Jun. The field was watered weekly to assist with germination and seedling emergence. A hard rain event on 5 June (1.4cm) caused soil crusting issues impacting emergence. The field was weeded by hand and thinned on 26 Jun. The field was inoculated with dry, ground hulless barley grain infested Rhizoctonia solani isolate R-9 (AG2-2 IIIB). The inoculum was applied to the crown of the plants on 31 Jul (8 to 12 leaf growth stage) at a rate of 9 g m-1 of row using a Gandy® electrically driven applicator. The field was cultivated afterwards to place soil onto the plant crowns. A large hail event and rain occurred on 5 Jul. A total of 2.3 cm was recorded over 36 hrs. Most plants were at the 4-6 leaf stage, and approximately 19% of the plants across the test died due to hail damage. A single entry was completely lost to hail damage (PI 198680, not shown in data table), and 7 entries had <30 plants over all 5 replicates. From planting to harvest, there was 10.8 cm of rainfall, and 44.45 cm of water applied via overhead linear irrigation. During the season, the average daytime high was 85F, and the average nighttime low was 54F. Disease progression was excellent and at harvest we had severe and uniform levels of RCRR in the nursery.
Roots were defoliated and harvested 4 Sep with a single row lifter (pulled and cleaned by hand), and each root was rated for RRCR on a scale of 0 (no damage) to 7 (dead plant with root completely rotted) (Plant Dis. Rep. 63:518-522). 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 (PROC GLIMMIX) was performed in SAS (Ver. 9.4) on DI. Data are also represented as the percentage of sugar beet roots in classes 0 through 1, considered as healthy and in classes 0 through 3, considered harvestable. Data in classes 0-1 and 0-3 were transformed using arcsine square root to normalize the data for analyses %0-1 and %0-3. Dunnett’s one-tailed t-test (P = 0.05) was used to compare entries to the highly resistant control FC705-1 (19851032H) and the most susceptible line tested (PI 344063). Of the 2019 entries, 12 were screened in recent years (2012 or 2018) and showed moderate to high levels of resistance. The other 17 were repeats from the 2016 screen which failed due to inadequate disease pressure. There were significant differences among entries for DI in this test. Two entries, PI604510 and NSL 31344, were not significantly different (P = 0.05) from the highly resistant control (FC 705-1) and represents potentially novel sources or combinations of resistance to RCRR. PI 344063 was the most susceptible entry, with a DI of 6.7. There were 20 entries not significantly different from PI 344063, suggesting a lack of resistance in these entries. These accessions will be retested and, if resistance is confirmed, incorporated into the USDA-ARS RRCR resistance breeding program at Fort Collins, CO to enhance sugar beet germplasm. These results will be accessible to interested parties through the USDA-ARS, NPGS GRIN database http://www.ars-grin.gov.