There are many fungicides and insecticides labeled for use in Iowa on soybeans. With partial funding from industry and soybean checkoff funds from the Iowa Soybean Association, Iowa State University plant pathologists Daren Mueller and Stith Wiggs evaluated common foliar fungicides and insecticides at seven locations across Iowa in 2012 to determine yield response to an R3 (beginning pod set) application timing.
The evaluations were conducted at ISU's Northwest Iowa Farm at Sutherland, Northern Iowa Farm at Kanawha, Northeast Farm at Nashua, Curtiss Farm at Ames, Armstrong Farm at Lewis, McNay Farm at Chariton and the Southeast Farm at Crawfordsville.
Materials and methods: The experimental design was a randomized complete block with four replications at each location. Details on variety and planting, application and harvest date are listed in Table 1. Treatments (Table 2) consisted of an untreated control, fungicides alone, insecticides alone, fungicides and insecticides in combination and pesticide application based on aphid scouting (IPM).
In applicable treatments, fungicides and insecticides were applied at growth stage R3 (beginning pod) at all seven locations. Disease was assessed when soybeans were at the R6 growth stage (full seed set). Soybean aphid populations were observed between R3 and R6, but soybean aphid populations did not reach threshold at any of the seven locations. Total seed weight and moisture was measured, seed weight was adjusted to 13% and yield was calculated.
Results: Yield varied across locations ranging from 33.9 to 64.6 bushels per acre in the untreated control (Table 2). Differences were observed between pesticide treatments and the untreated control at the Ames and Armstrong locations (Table 2).
Foliar disease did not differ between fungicide and insecticide treatments and the untreated control at the Armstrong, Southeast, Northeast, Northwest and McNay locations. There were foliar disease differences between the fungicide treatments and the untreated control at the Curtiss and Northern location. The most predominate disease found was Septoria brown spot. Septoria brown spot did not move into the upper canopy before R6 at any of the seven locations, so it likely had minimal impact on yield. The average severity in the untreated control in the lower canopy was less than 3%. At some locations, fungicides reduced brown spot severity in the lower canopy, but again, disease probably had minimal impact on yield.
Soybean aphids did not reach the threshold at any location. IPM treatments became an additional untreated control.
Seed moisture ranged from 7.8 – 12.1 across locations, but did not differ more than a few tenths of a percentage amongst treatments within any location.
Summary: "This is the first year we were able to evaluate fungicides under drier weather conditions," says Mueller. "While individual products affected yields at certain locations, in general, all products had minimal significant effects on yield. There were very few diseases (and insects) at all seven locations, so benefits from pest management were not part of the equation. Fungicides are effective for management of diseases. However, in years with very little pest pressure -- most likely from lack of moisture in 2012 -- positive yield responses to fungicides are not consistent."
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