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Examining fungicide use effectiveness

By Staff | Feb 27, 2009

CHEROKEE – Speaking to grain producers at the Crop Advantage meeting in Cherokee on Jan. 20, Alison Robertson, Iowa State University Extension plant pathologist presented an update on corn fungicide effectiveness in Iowa.

“This year was very wet,” Robertson began. “The growing season was very much cooler (than usual).”

Conducting replicated trials at research farms near Ames, Crawfordsville, Nashua and Sutherland, Robertson was frustrated by not getting any clear answers created by the unusual weather.

The mean yield was a 3.5-bushel increase. In a third of the trials there was a negative yield response while a third of the trials had a “really good yield response.” Responses varied from a -13.5 to +23 bushels per acre.

Diseases managed by fungicides include gray leaf spot, common rust Southern rust, eyespot, Northernleaf blight.

Stewart’s wilt, physoderma brown spot and anthracanose top die back are not affected. Some corn hybrids do dry from the top down. Robertson said if a whole field had top dry down then it was the hybrid. Group dying signifies anthracanose.

Gray leaf spot thrives on typical Iowa State Fair weather, which is hot and humid – perfect conditions for fungi. Because of Iowa’s cool conditions and low humidity in 2009, weather wasn’t a problem.

Common rust was prevalent, she said, with the incidence high, but surprisingly the severity was low.

Northern leaf blight was seen in 17 percent of the corn with no fungicide; with fungicide just under 17. There just wasn’t a lot of disease present in this year’s trials, she concluded.

When determining the value of spaying a fungicide, Robertson advised evaluating disease pressures and the economics. The cost of the product and application may not cover the expense of applying fungicide.

Weather conditions must be considered. Warm, humid conditions just prior to and during grain fill favor the development of many foliar diseases. Geographical location makes a difference. Southeastern Iowa tends to be hotter and more humid than northwest Iowa consequently has greater disease pressure.

Yet this year, the fungicide applications had a greater response at the Sutherland locations than elsewhere in Iowa. That was completely different from what she expected to see.

To determine a fungicide’s effectiveness, Robertson says it is a good idea to leave an untreated check strip. “Otherwise you have no way of knowing whether the fungicide was a good management decision,” she said.

There are factors that producers can do that will influence the response of fungicide on corn. These include:

  • Hybrid selection. Yield response to a foliar fungicide is usually greater on hybrids that have a low disease resistance score.
  • Previous crop. Corn-on-corn vs. soybean/corn rotation.
  • Surface residue. Many corn pathogens survive in corn debris, thereby increasing the risk of disease in fields with high residue.
  • Application technique and timing. Aerial versus ground spraying. Many trials in Iowa showed the greatest yield response occurred when applied at the VT stage in corn. However, that did not prove true every time.
  • Product additives. Products used in the research were Headline, Stratego, Quadris and Quilt. In Iowa, Quilt had the better response. Nationally, Headline had the better response.

“For now,” Robertson said, “yield benefits from a fungicide application on corn should be determined on an individual basis, grower-by-grower, field-by-field and year-by-year.”

Contact Renae Vander Schaaf by e-mail at renaefarmnews@gmail.com.

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