Evaluation location: Colorado, United States
The Rhizoctonia screening nursery in 2010 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 sugarbeet in 2006 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 March 2010, and bedded a month before planting. Seed was planted on 25 May to moisture, fertilized with 44.7 kg ha-1 of ESN? (Agrium Advanced Technologies, Sylacauga, AL) and furrow irrigated as needed. Only pre-plant (glyphosate and clopyralid) and pre-emergence herbicides (glyphosate and clopyralid) were used this year. The field was thinned (20 - 25 cm spacing) and hand weeded 18 to 20 Jun, and hand weeded again 3 to 4 Jul and 24 Jul. Inoculation with dry, ground, barley grain colonized with Rhizoctonia solani isolate R-9 (AG-2-2 IIIB) was applied to the crown of the plants on 15 Jul at a rate of 6.9 g m-1 row. A Gandy? electrically driven applicator was used to apply the inoculum and the plots were cultivated afterwards to place soil onto the plant crowns. Beets were harvested from 16 to 18 Aug, 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. . The scores were adjusted to a 1 (healthy) to 9 (dead) scale: [DI*(9/8)]+1. Analyses of variance (PROC ANOVA/GLM) were performed on disease indices DI. 2010 had a cool spring with good early season rainfall in Fort Collins, CO. Between 1 Jan and planting we had about 15 cm of rain allowing us to plant to moisture. The crop grew well and was inoculated 15 Jul. In Jul and Aug, daytime temperatures remained high and there was an excellent infection and very severe epiphytotic. There was good separation of disease severity between the highly resistant and highly susceptible lines. Weed pressure was light and, although we saw very little curly top in the field, we noted heavy leaf hopper pressure and sprayed lorsban and mustang on 30 Jun. Four of the PIs had a DI score of 4.6, which was not significantly different from the highly resistant control. Two more PIs were rated with a DI under 5.0. There was segregation for resistance in these accessions with some of the roots looking very clean on evaluation. There may be resistance genes in these accessions and they will be reevaluated to confirm resistance to Rhizoctonia root and crown rot.
Report not yet published but accepted for publication: Panella, L., T. Vagher, A. L. Fenwick, and K. M. Webb. 2011. Rhizoctonia crown and root rot resistance of Beta PIs from the USDA-ARS National Plant Germplasm System, 2010. Plant Disease Management Reports. Online publication. (Accepted 2/9/11)