Rhizoctonia Demands More Attention in Upper Midwest ~ By Don Lilleboe

   Though sugarbeet producers in western states have dealt with significant levels of Rhizoctonia crown and root rot for many years, the disease’s development as a serious problem in the Red River Valley and Southern Minnesota growing areas is a more-recent phenomenon.
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      A key factor in Rhizoctonia’s “Upper Midwest expansion” over the past several years is cropping patterns. There now are fewer wheat and barley fields (nonhost crops) and more corn, soybean and edible bean fields (all host crops). That has translated into more disease inoculum present in soils to threaten sugarbeet crops that follow corn or beans. “We think the number-one driver [behind the increased incidence and severity of Rhizoctonia] — if talking about the whole Red River Valley — is soybeans,” says Allan Cattanach, general agronomist for American Crystal Sugar Company. “Whereas corn and soybeans together probably make it worse in Southern Minn, Minn-Dak and the southern end of the Crystal growing area.”

     Other factors believed by American Crystal to have contributed to the increased incidence of Rhizoctonia include: (1) the increasing presence of more-virulent strains of this disease, (2) the fact that common weed species (e.g., pigweed, lambsquarters) are good hosts, (3) very limited usage to date of chemical control options, (4) few good resistant varieties being available until recently, and (5) the presence of environmental conditionsfavorable to the disease’s development.

     As elsewhere, resistant varieties have become a key weapon for those Upper Midwest beet growers encountering moderate to heavy Rhizoctonia infections. For the 2010 growing season, for instance, Southern Minnesota Beet Sugar Cooperative approved five “Rhizoctonia Root Rot Specialty Varieties.” American Crystal Sugar approved 13 varieties for planting by its growers seeking good Rhizoctonia control.

     For growers whose Rhizoctonia problem is slight, the use of a resistant variety may be sufficient to protect against the disease, Cattanach suggests. But “if you have moderate to severe Rhizoctonia, you should be applying a fungicide in addition to using the special variety,” he notes.
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     As elsewhere, Quadris® is the predominant fungicide for Rhizoctonia control, though Proline® also has been shown to be effective on this disease. Last year, about 15,000 American Crystal acres were treated with Quadris. In 2010, however, the cooperative expects upward of 75,000 of its acres to have Quadris applied. Part of the reason is the increased incidence and severity of this disease; hand in hand with that was a significant early order/early pay discount offered this winter by the fungicide’s manufacturer, Syngenta Crop Protection.

Photos by Melvin Bolton, USDA-ARS; Mohamed Khan, NDSU/UM

      American Crystal’s recommendation is to band the fungicide on fourto eight-leaf beets as the soil temperature reaches 65°F. Neither Quadris nor Proline will be effective if they’re applied after infection occurs. “If you wait until 70 degrees, you’re probably a little late applying it,” Cattanach advises. “We think it has to be done by the time [the soil temperature] is around 65.

     “If you’re going to error in application timing, it’s better to go a little early rather than too late.”

     Cattanach adds that daily soil temperature readings from stations located throughout the Red River Valley (and Sidney, Mont.) can be found on the North Dakota Agricultural Weather Network (NDAWN) website: ndawn.ndsu.nodak.edu. American Crystal also has a Rhizoctonia soil temperature advisory system on its website for each NDAWN station.

   How effective are the fungicide treatments? In 2009 Jason Brantner and Carol Windels of the University of Minnesota’s Northwest Research & Outreach Center at Crookston tested various labeled fungicides and new products. Theyninoculated beet plots at either the four- or eight-leaf stage and cultivated immediately after inoculation. (While cultivation is not recommended in Rhizoctonia-infected fields, they did so in order to throw soil onto beet crowns and incorporate the inoculum.)

      Harvested results from the fourleaf treated plots showed a recoverable sugar per acre (RSA) of 9,508 pounds with the label-rate Quadris treatment; 9,044 pounds RSA with Quadris at twice the labeled rate; and 8,583 pounds RSA with a Quadris/ Proline tank mix. The RSA for the “no fungicide” inoculated plots was just 1,922 pounds.

      RSA on the noninoculated plots averaged 8,496 pounds. With Quadris applied at the eight-leaf plant stage, RSA was 9,113 pounds. That compared to a “no fungicide” RSA of 4,533 pounds.

     The 2009 UM-Crookston research also looked at fungicide effect on conventional and Roundup Ready® beets. Brantner and Windels found no significant difference in the performance of Quadris in the two production systems. “Quadris and Proline provide excellent control of Rhizoctonia crown and root rot under both conventional and Roundup Ready systems,” they reported. — More Sugar beet News By Don Lilleboe