Rhizoctonia in Michigan ~ Update on Research & Recommendations By Steve Poindexter

   Michigan Sugar Company has a goal to improve beet quality by increasing the co-op’s average sugar content to 19%. This goal is achievable, but it will take increased management and use of higher sugar varieties.
      Many of our new varieties have a high tonnage and sugar potential; but several likewise are very susceptible to Cercospora leafspot and Rhizoctonia. Left unchecked, both diseases can greatly affect yield and quality. By using the BeetCast leafspot prediction model and appropriate fungicides, growers are doing an excellent job of minimizing the impact of Cercospora leafspot. While Rhizoctonia is more difficult to manage, we also have made great strides in reducing its effect on yield and quality

     Rhizoctonia management begins with matching varieties with specific fields. Sugarbeet fields with a history of high Rhizoctonia levels are not good candidates for planting of any highly susceptible varieties. In these fields, a Rhizoctonia-resistant variety is the best choice. Several varieties carry varying levels of resistance. Growers always need to remember, though, that “resistance” does not mean “immunity.”
     Michigan Sugarbeet Advancement research has shown that a resistant variety placed in a heavy infestation of Rhizoctonia may reduce infections by about 75% as compared to a susceptible check. Couple this resistance with an in-furrow or properly timed foliar application of Quadris®, and infections likely will be reduced by more than 90% while maintaining a good return on investment for Quadris.
     Fields that have not shown a history of significant Rhizoctonia are excellent candidates for some of the new high tonnage and sugar varieties. Many of these varieties are very susceptible to Rhizoctonia and almost always give an economical response to a Quadris application. For this reason, every Michigan grower is encouraged to equip their operation with the ability to apply Quadris either in-furrow or with a foliar band application.
     Research conducted in 2009 showed economic responses to a Quadris application ranging from $85 to $212 per acre in fields with moderate infection levels. Sugarbeet quality increased from 291 to 305 pounds of sugar per ton on the best treatments. This reflects an average improvement of 0.6% in sugar along with an improvement in clear juice purity.
     The 2009 research results on susceptible varieties were very similar to those of previously conducted Rhizoctonia trials. In-furrow T-band applications at planting generally provided the most consistent control. The label recommendation is 10.5 or 14.3 ounces per acre in a 7-inch band for 30- or 22-inch rows, respectively. Sugarbeet Advancement research in 2009 suggests that in-furrow width and Quadris rates may be reduced by half and still provide results comparable to a well-timed banded foliar application.
      Growers who choose to reduce the in-furrow width should maintain a rate of 1.5 ounces per inch of band in 30- inch rows and 2.0 ounces per inch of band in 22-inch rows. Do not dribble in-furrow or apply with in-furrow fertilizer, as emergence reduction and lack of efficacy may result. More research needs to be conducted comparing efficacy of narrow in-furrow band widths and rates to the standard 7-inch band.
   In the last few years, the standard recommendation for foliar application has been to apply 10.5 or 14.3 ounces per acre in a 7-inch band for 30- or 22-inch rows at the four- to six-leaf stage. In 2009, foliar applications showed best efficacy in full-rate applications at the six- to eight-leaf stage. Rhizoctonia is a warm-season disease, and research in other states indicates that the infection period generally occurs when the mean soil temperature reaches 70 degrees at the 4.0-inch depth with moist soil conditions.
     Because 2009 saw a prolonged cool spring in Michigan, soils were slow to warm. This favored the later application of Quadris applied just before optimum temperature for infection. Research indicated that the two- to four-leaf application still gave a significant 51% control over the check, compared to 83% control at the six- to eight-leaf stage. Research also showed a reduction of Quadris efficacy when the foliar rate was reduced to the lowest labeled rate of 7.0 ounces, as compared to 10.5 ounces in 30-inch rows.
   So an in-furrow and /or a well- timed foliar application of Quadris is important to effective management of Rhizoctonia. Quadris needs to be used in conjunction with matching field disease history with variety tolerance.
      Reducing in-furrow band width and rate by half appears to have efficacy similar to a well-timed foliar band application. In a 7-inch foliar band, reducing the Quadris rate from 10.5 to 7.0 ounces per acre in 30-inch rows reduced control. In-furrow applications in 2009 did not significantly affect beet emergence.
     Improved Rhizoctonia control will increase sugarbeet quality, yield and grower profitability.

     Steve Poindexter is senior sugarbeet extension educator and Michigan Sugarbeet Advancement coordinator with the Michigan State Universit Cooperative Extension Service. Th Michigan Sugarbeet Research & Education Advisory Council (REACH) recently published a bulletin titled “Management Guidelines for Controlling Rhizoctonia in Sugarbeets.” It can be accessed through the Agriculture link at www.michigansugar.com

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