There are many fungicide and insecticide products labeled for use on soybeans. In 2011, Iowa State University Extension specialists conducted a series of field tests to evaluate the performance of these products. The evaluations took place at six locations across Iowa (Figure 1) to determine yield responses to an application made at the R3 stage of soybean growth (beginning pod set).
The following report is authored by Daren Mueller, Alison Robertson, and Stith Wiggs of the ISU Department of Plant Pathology and Microbiology; and Matt O'Neal and Erin Hodgson, of the ISU Department of Entomology.
Materials and methods: The experimental design was a randomized complete block with at least four replications at each location. Details on variety and planting, application and harvest dates 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 six 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 an IPM spray was only necessary at Sutherland. Total seed weight and moisture was measured, seed weight was adjusted to 13 percent and yield was calculated.
Results: Yield varied across locations ranging from 39.4 to 75.9 bu/ac in the untreated control (Table 2). Differences were observed between pesticide treatments and the untreated control at the Sutherland and Ames locations.
Foliar disease did not differ between fungicide and insecticide treatments and the untreated control at the Armstrong, Crawfordsville, Kanawha and Nashua locations. There were foliar disease differences between the fungicide treatments and the untreated control at the Ames location and insecticide treatments and the untreated control at the Sutherland location (Table 2). The two most predominant diseases found were Septoria brown spot and frogeye leaf spot.
Septoria brown spot did not move into the upper canopy before R6 at any of the six locations, thus it likely had minimal impact on yield. The average severity in the untreated control in the lower canopy was less than 3.5% at all locations except Nashua (7.5%) and Ames (6.6%). At both of these locations, fungicides reduced brown spot severity in the lower canopy, but again, disease probably had minimal impact on yield.
Frogeye leaf spot was found in a few locations, but was greater than 1% severity in the untreated control at only the Ames location (4.9%). All fungicides significantly reduced frogeye severity (averaged 1.1%). As expected, insecticides alone did not have any effect on frogeye leaf spot severity (averaged 5.2% severity). There were no significant differences in disease control between fungicide products.
Soybean aphids averaged 320 aphids per plant at the Sutherland location, which exceeded the economic threshold of 250 per plant. Aphids did not reach the threshold at any other location. At Sutherland, the IPM insecticide and insecticide+fungicide treatments were applied at the R4 growth stage on Aug. 3, which was 13 days after the R3 application. IPM treatments were not applied at the other five locations.
Seed moisture ranged from 8% to 11% depending on location, but did not differ more than a few tenths of a percentage amongst treatments within any location.
Summary: The results of this experiment illustrate the benefits of foliar fungicide and insecticide applications for the management of foliar diseases and insects. There were very small amounts of foliar disease across the state of Iowa in 2011 due to high heat and low rainfall amounts in July and August. Also, this was a moderate soybean aphid year across much of the state. At the four locations with very low insect populations and disease severity, there were no significant yield responses to either insecticides or fungicides. However, at the Ames location, fungicides reduced frogeye leaf spot in the upper canopy and the largest yield responses to fungicides were at this location.
Also, only one of the six locations (Sutherland) reached the threshold level to spray aphids and this was the only location where all insecticides had significant responses to insecticides. Using foliar fungicides and insecticides is an effective way to prevent yield losses to foliar diseases and insect pests. Also, only applying pesticides when needed can reduce overall production costs and preserve product efficacy for when severe outbreaks do occur.
Acknowledgments: Thanks to the ISU Research Farm personnel who assisted with application of treatments and Bekah Ritson and Nate Bestor for helping assess insect populations and disease severity. Partially funded by the Iowa Soybean Association and soybean checkoff.
Daren Mueller is an extension specialist with responsibilities in the Iowa State University Integrated Pest Management program. Mueller can be reached at 515- 460-8000 or by email at [email protected]. Alison Robertson is an associate professor in the plant pathology and microbiology department with extension and research responsibilities; contact her at [email protected] or phone 515-294-6708. Stith Wiggs is a research associate in the plant pathology and microbiology department. Erin Hodgson is an assistant professor of entomology with extension and research responsibilities; contact at [email protected] or phone 515-294-2847. Matt O'Neal is an associate professor in the Department of Entomology with teaching and research responsibilities. He can be reached at [email protected] or at 515-294-8622.