Corn fungicides: to spray or not to spray?

Corn fungicides: to spray or not to spray?

Much of Iowa's corn is now reaching V5 to V6 stage; farmers wonder if they should make an early fungicide application.

Much of Iowa's corn crop is now reaching V5 to V6 growth stage and thoughts of making an early fungicide application have crossed some people's minds. With El Niño gaining strength here early in the 2015 growing season, the growing season looks like it will be cool and wet. Thus, Northern Corn Leaf Blight could show up, says Alison Robertson, ISU Extension plant pathologist.

"It's important to scout your young cornfields," she says. "It will be especially important to scout fields this year, particularly those fields planted to corn hybrids that are rated susceptible or moderately susceptible to NCLB."

CORN LEAF DISEASE: It looks like 2015 has a chance to be cool and wet. Since Northern Corn Leaf Blight fungus overwinters in corn crop residue, NCLB could be a disease risk in 2015. It will be important to scout fields, particularly those planted to susceptible hybrids.

Every year, Iowa State University Extension plant pathologists evaluate and compare registered fungicides applied at either V5 to V6 stage of corn growth, or they apply R1 alone, or apply at the V5 to V6 plus R1 growth stage of corn for foliar disease management and to measure the effects on yield. In 2014, ISU tested eight products at six locations in Iowa. A randomized complete block design with four replications was used. At each location, two non-sprayed checks were included.

The 2014 growing season was predominantly cool and wet with above average rainfall in June, August and September. Northern corn leaf blight (NCLB) was reported prior to tasseling in southwest and central Iowa, which was considerably earlier than usual. Over the past decade, NCLB has been reported after silking at various locations across the state. Different corn hybrids were grown at each location in the trial, but all had moderate resistance to NCLB.

Here are results of last year's fungicide trials by location
Following is a summary of results of the 2014 fungicide trials on corn at each of six locations in Iowa. For a look at these results of foliar fungicide applications as reported in the tables, visit extension.iastate.edu/CropNews/2015/0603Robertson.htm

Northwest Research Farm in Sutherland, Iowa: NCLB severity in the upper canopy (ear leaf and all leaves above) two non-sprayed controls were 8.4% and 7.8% (Table 1). Although an application of Priaxor at V6 did not reduce NCLB severity compared to the non-sprayed controls (P less than 0.1), applications of Custodia, Fortix and Stratego YLD at V6 did reduce disease. All treatments that included an application of fungicide at R1 reduced NCLB severity (P less than 0.1). However, applications at V5 followed by R1 were not different from application at R1 only. In general, treatments applied at R1 resulted in greater yields than the untreated check and V5 applications alone (P less than 0.1).

Northern Research Farm in Kanawha, Iowa: NCLB severity in the two non-sprayed controls was 10.5% and 9.3% (Table 1). A reduction in NCLB severity was detected from all fungicides applied at all timings. No difference between application timings was detected, that is, applications made at V5 were as effective as applications made at R1, and at V5 plus R1. This is likely due to the fact that NCLB development started early prior to the crop tasseling. There was significant lodging in the plots due to a wind storm that occurred mid-July. No evidence of an effect of fungicide on yield was detected.

Northeast Research Farm near Nashua, Iowa: NCLB severity was less than 1%. In this part of the state, precipitation was normal to below normal. In general, an application of fungicide at V6 reduced NCLB compared to the non-sprayed controls. Applications of Fortix at R1, Headline Amp at R1, Quilt Xcel at R1, Stratego YLD at V5 plus R1, Stratego YLD at R1 all increased yield compared to the non-sprayed control (P less than  0.1).

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Ag Engineering/Agronomy Farm in Boone, Iowa: In the two non-sprayed control, NCLB severity was 14% and 16% (Table 1). In general, an application of fungicide at V5 did not reduce disease severity, apart from an application of Aproach. An application of fungicide at R1 and double applications of fungicide (V5 plus R1) reduced disease severity. Greater yields occurred with applications of Fortix at R1, Headline AMP at R1, Stratego YLD at R1 and Stratego YLD at V5 plus R1 (P less than 0.1).

Southwest Research Farm in Lewis, Iowa: Disease severity in the two non-sprayed controls was 10.5% and 9.3% (Table 1). No effect of an application of fungicide at V6 on NCLB severity was detected, except for Priaxor (3 ounces per acre) (P less than 0.1).  An application of fungicide at R1 significantly reduced NCLB severity. Double applications of fungicide (at V5 plus R1) were not different from the single application at R1. No evidence of an effect of fungicide on yield was detected.

Southeast Research Farm in Crawfordsville, Iowa: NCLB severity in the two non-sprayed controls was 7.8% and 8.4% (See Table 1). All applications of fungicide reduced disease severity except for applications of Priaxor or Stratego YLD at V5, and Aproach at V5 plus Aproach Prima at R1. Greater yields compared to the non-sprayed control occurred with Fortix, Headline AMP Quilt Xcel and Stratego YLD all applied at R1, and with two applications of Stratego YLD at V5 and R1 (P less than 0.1).

Summing up: All fungicides effectively reduced NCLB at all locations in Iowa in 2014 in the tests. Although applications at either V5 or at R1 reduced disease, foliar fungicide applications at R1 were most effective at reducing disease. No additional disease control occurred with applications at V5 and again at R1.

Northern Corn Leaf Blight could be a disease risk in 2015
With El Niño in effect, it looks like 2015 has a chance to be cool and wet. Since the NCLB fungus overwinters in corn crop residue, NCLB could be a disease risk in 2015. It will be important to scout fields, particularly those planted to hybrids that are rated susceptible or moderately susceptible to NCLB. If disease is present anywhere on more than 50% of the plants in a field, a fungicide application could be a prudent decision. Applications made between VT and R2 should protect the crop. Avoid applications between V12 and V18 since this increases the risk of arrested ear development.

Thanks to managers and staff at ISU research and demonstration farms for managing the trials and applying fungicide treatments at the farms. Alison Robertson is a professor and ISU Extension plant pathologist. She can be reached at [email protected] or by phone 515-294-6708. John Shriver is a research associate in ISU's Department of Plant Pathology.

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