Rhizoctonia root rot is a serious disease problem in several sugarbeet-growing regions, with the result sometimes being dramatic — and expensive — reductions in tonnage and quality. Low levels of infection can easily cause yield losses in excess of a ton per acre, while high infection levels can cut yields by more than 10 tons per acre. The quality of surviving beets can also be impacted, sometimes resulting in significant losses in recoverable sugar.
Management steps typically revolve around the planting of Rhizoctonia-resistant (tolerant) varieties, certain agronomic practices (e.g., rotation and keeping contaminated soil off beet crowns) — and the employment of seed treatment products and/or applied fungicides. While tolerant varieties also will benefit from the use of fungicides, they are most helpful on varieties that are more susceptible to infections by the Rhizoctonia solani fungus.
During 2009 and 2010, the Michigan Sugarbeet Advancement initiative established a study to determine the efficacy and economic impact of various application strategies for the use of Quadris® flowable fungicide to control Rhizoctonia root rot. The different treatments encompassed:
• In-furrow T-band
• In-furrow T-band + a low-rate application at the six- to eight-leaf stage
• Normal rate at the two- to four-leaf stage
• Normal rate at the six- to eight-leaf stage
• A low-rate application twice (two- to four-leaf stage and again at the six- to eight-leaf stage)
We also evaluated in-furrow Quadris for its impact on sugarbeet plant emergence. In addition, the effectiveness of Quadris at the six- to eight-leaf foliar application stage was compared with that of Proline®480 SC. Finally, we looked at the effectiveness of “rescue” treatments of Quadris, both banded and broadcast.
Three trials were conducted in 2009 and two in 2010 (plus two other “rescue” ones), with multiple replications at all sites. All consisted of strip trials in commercial fields with histories of moderate to heavy infections of Rhizoctonia. Also, all were naturally infected sites; none of the trials were inoculated. Rhizoctonia-susceptible varieties were planted at all locations.
Control evaluation was conducted by taking counts of dead or dying beets (per 1,200 feet of row) in mid-August. Infection levels at these trial locations were considered low to moderate, with the check having 150-250 dead beets. The emergence data were taken from the five trials plus 10 additional sites.
The in-furrow T-band applications were performed with the grower’s equipment, spraying a 2.75- to 5.5-inch band between the planter’s disk openers and press wheels. Rates ranged from 4.0 to 7.5 oz/acre, depending upon the band width.
The foliar treatments were applied in a 7.0-inch band. Low and normal rates of these applications were 0.4 and 0.6 oz/1,000 feet of row, respectively. Proline + NIS was applied in a 7.0-inch band at 5.7 oz/acre.
Four of the five trials had similar emergence data, so results from those trials were combined. Since one trial incurred a significant reduction of its plant stand for the in-furrow treatments, its data were compiled separately.
For the four combined trials, the average percent controls (2009 and 2010 together) over the check ended up as follows:
• Six- to eight-leaf, normal rate: 82% better than check
• In-furrow, six- to eight-leaf, low rate: 82% better
• Two- to four-leaf and six- to eight-leaf: 78% better
• In-furrow T-band alone: 72% better
• Two- to four-leaf, normal rate: 48% better
These levels of control were based on the check having 202 dead or dying beets per 1,200 feet of row.
When it came to tonnage and recoverable sugar per acre, results for the four combined trials averaged out as follows (tons/acre and pounds/acre recoverable sugar, respectively):
• In-furrow, six- to eight-leaf, low rate: 29.2 tons and 8,799 lbs.
• In-furrow T-band alone: 28.7 tons and 8,544 lbs.
• Six- to eight-leaf, normal rate: 28.3 tons and 8,414 lbs.
• Two- to four-leaf and six- to eight-leaf: 27.6 tons and 8,184 lbs.
• Two- to four-leaf, normal rate: 26.9 tons and 7,991 lbs.
• Check: 24.7 tons and 7,139 lbs.
So the best treatment beat the check in terms of recoverable sugar per acre, by 1,660 pounds, while the “worst” beat the check by 850 pounds.
The trial that was not combined due to emergence issues showed generally similar results as the combined trials in terms of dead beet counts. Due to the reduced emergence, tons per acre for the in-furrow treatments were about one ton lower compared to the foliar treatments with similar levels of control.
On average (four trials in each of two years), even with low to moderate levels of Rhizoctonia infection, the per-acre net return of Quadris over the check trials ranged from $94 to $209, depending upon the rate, timing and method used. The best treatment in these trials improved recoverable sugar per ton by 14 pounds and percent sugar by 0.7. Even the “worst” treatment increased RST by 8 pounds and sugar content by 0.3%.
Three trials compared Quadris and Proline where foliar applications were made at the six- to eight-leaf stage. Quadris provided significantly better control and tonnage than did Proline; but Proline likewise provided a significant improvement over the check.
In the “rescue” treatment trials, Quadris was applied basically as soon as any dead or dying beets were noticed (typically between the 10- to 14-leaf stage). Two methods were used: (1) banded (forced onto the beet crown) at the recommended rate, and (2) broadcast at a similar rate. Banding did provide moderate control, while the broadcast treatment gave about half the control of banding.
A rescue application will not be as effective as a well-timed earlier treatment; but it could be helpful if a person is monitoring the field often and sees problems developing early on. Applications as row closure approaches would be significantly less effective, for two reasons: (1) Significant infections already will have occurred, and (2) getting the fungicide onto the crown of the plants will be difficult.
* Steve Poindexter is senior extension sugarbeet educator with Michigan State University and director of Michigan Sugarbeet Advancement. Tom Wenzel is research technician with Michigan Sugarbeet Advancement.
Don Lilleboe is editor of 'The Sugarbeet Grower' and 'Gilmore Sugar Manual.'