Every year we evaluate commercial fungicides on corn applied at V5 alone, VT-R1 alone, or both growth stages for disease control and effect on yield. The trial was done in collaboration with the farm managers at four Iowa State University (ISU) Research and Demonstration Farms (Table 1). The trials are laid out in a randomized complete block design with four to six replicates. Plot sizes are 10 foot (4 rows) wide and 30-100 foot long. Hybrids varied by location. Trials were also done at the ISU Southeast and Armstrong Memorial Research and Demonstration Farms, but data from those trials are excluded in this report due to nitrogen deficiency and an error in experimental design, respectively.
Effect of fungicides on northern corn leaf blight
In 2015, the predominant disease in the trials was northern corn leaf blight (NCLB), which was present at all locations (Table 1). At growth stage VT-R1, disease severity (percent of upper canopy disease in the plot) at all locations was less than 1 percent. By growth stage R5, NCLB severity ranged from 2.3 to 30 percent in the non-treated control plots. Fungicides reduced mean disease severity, but timing of application affected disease control. For the V5 application, fungicides reduced NCLB severity by 8 percent. Applications at VT-R1 or V5 +VT-R1 reduced disease 32 and 40 percent, respectively.
Effect of fungicides on yield
Average yields in the trial for each treatment ranged from 170.3 bu/A to 232.4 bu/A (Table 2). Mean yield response as a result of a fungicide application varied by timing of application (Table 3).
Effect of hybrid on timing of fungicide application and yield response
At ISU Northeast Research and Demonstration Farm, where a moderately susceptible hybrid was planted (rated 5 on a 1-9 scale where 1 is poor), a V5 + VT-R1 application of fungicide reduced mean disease severity more than an application at VT-R1 alone (15.6% vs 20.2%, respectively). Moreover, mean yield response was greater with the V5 + VT-R1 application compared to the VT-R1 application; 12.8 bu/A vs 7.8 bu/A. An application at V5 did not reduce disease severity, and no yield response occurred.
These data suggest that a double application of fungicide may be prudent on susceptible hybrids and if conditions are favorable for disease development.
At the ISU Northwest Research and Demonstration Farm, where a moderately resistant hybrid was grown (rated 7 on a 1-10 scale where 1 is least tolerance), disease severity and yield response for fungicide applications at V5 +VT-R1 versus R1 alone were not different (NCLB severity: 3.0 vs 3.5%, and yield response 10.3 vs 11.0 bu/A, respectively). Applications of fungicide at V5 did not reduce disease and no yield response was detected.
Consequently, these data suggest that an application of fungicide at V5 on moderately resistant hybrids is not needed, but an R1 application may be needed when conditions are favorable for disease (frequent precipitation and cool temperatures (70s F) for NCLB).
Acknowledgements
Thanks to the managers and staff at ISU Research and Demonstration Farms for collaborating and managing the trials.
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Alison Robertson Professor of Plant Pathology and Microbiology
Dr. Alison Robertson is a professor of plant pathology and microbiology. She provides extension education on the diagnosis and management of corn and soybean diseases. Her research interests include Pythium seedling disease of corn and soybean and Goss's wilt. Dr. Robertson received her bach...