Foliar fungicides may be warranted during years with stressful conditions, high disease pressure, and susceptible seed products. Its important to scout your fields as crops near reproductive growth stages. Assessing disease risk can help you prioritize fields and make timely fungicide applications on your farm.
Fungicide application may reduce foliar diseases depending on corn and soybean product resistance and disease pressure. Strobilurin and triazole fungicides are the primary fungicide groups used for in-season corn and soybean disease management. These fungicide groups are “locally systemic”; however, there is minimal systemic activity, so new plant growth after application will not be protected. Fungicides also have preventative and/or curative activity. If a fungicide is on the plant before infection occurs it may act as a protective barrier preventing infection. Curative activity may occur when the fungicide is present within plant tissue and stops early growth of the pathogen in plant tissues.1 Fungicides with curative activity will not “cure” a plant from disease and are most effective if applied prior to infection or within the first 72 hours after infection.1 Triazole and strobilurin fungicides are often used together to help reduce the risk of fungicide resistance.
Assessing Disease Risk
Scouting and assessing your disease risk can help you prioritize fields and maximize your return-on-investment.
- Corn products that are susceptible or moderately susceptible to diseases should be monitored prior to tasseling. Disease activity at or before tasseling increases your disease risk.
- Continuous corn, no-till, and/or reduced tillage corn fields should be monitored as many foliar diseases survive on corn residue and begin producing spores when wet weather favors disease development.
- Disease history. Fields with greater than 35 percent residue and with a history of foliar disease should be scouted.1
- Weather. Scouting when weather conditions favor disease development is important.
- Management Practices. Other risk factors for disease development in corn include irrigation, late planting, high plant population, and high yield potential.
Cool and humid weather conditions at vegetative growth stages and during grain fill can increase common rust and northern corn leaf blight in some fields.
Warm and humid weather favors gray leaf spot and southern rust. These diseases can develop to levels that will reduce yields if they substantially infect the ear leaf or leaves above the ear during tasseling and pollination.
Risk factors for soybean are similar to corn. This includes susceptible soybean products, disease history, continuous soybean production, irrigation, high yield environments, early planted fields, and early maturing products.
Anthracnose, Cercospora leaf blight, white mold, brown spot, frogeye leaf spot, and soybean rust may be managed with a timely fungicide treatment.
Tips to Manage
Fungicides can work better at preventing rather than curing disease. Lesions may take up to two weeks to become visible after infection. Therefore, it is important to scout when weather conditions favor disease development.
Fungicides are generally considered effective for 21 days. This is due to new, unprotected leaf growth after application, and because fungicides degrade or are lost from leaf surfaces.2 Therefore, timing is important to help maximize potential activity of the fungicide. Always read and follow label directions.
Scouting for lesions at stages early enough to prevent severe infection of ear leaves can help establish timing for an economical fungicide treatment. The ear leaf and leaves higher on corn plants should be protected from disease because they contribute the most energy supplied during grain fill.1 A general disease treatment guideline may be to spray when disease symptoms have developed on the third leaf below the ear leaf, or on leaves above that, on half of the plants during the tasseling stage.3 If fungicide application is warranted, generally the greatest benefit usually comes from a single application at tasseling (VT) through silking (R1) growth stages.2 For more information on fungicides for corn diseases, visit: https://www.extension.purdue.edu/extmedia/BP/BP-160-W.pdf
Fungicide application in soybean differs from corn as there is more leaf development after application. Fungicide applications made prior to the R1 (beginning flower) growth stage, or after the R6 (full seed) growth stage are often not economical.5 A relatively early fungicide application at R2.5 (full flower) growth stage may have yield benefit over fungicide applications at R4 (full pod) growth stage.4 White mold or sclerotinia stem rot requires different timing as infection first occurs at R1 or the initiation of flowering. Therefore, an additional application may be necessary at a later growth stage to control other fungal pathogens.
Weather conditions should be considered when determining fungicide timing in soybean. Response from fungicide treatments are most often beneficial in the absence of host resistance and when conditions that favor disease development are anticipated at R3 growth stage. Subsequent fungicide applications may be needed in environments with high disease pressure.
A compatible herbicide or insecticide may be included if labeled for soybean to manage weeds and insects.
For more information on fungicides for soybean diseases, visit: https://www.extension.purdue.edu/extmedia/BP/BP-161-W.pdf
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1Mueller, D. and Roberston. A. 2008. Preventative vs. curative fungicides. Iowa State University. Integrated Crop Management News. https://crops.extension.iastate.edu/cropnews/2008/07/preventative- vs-curative-fungicides
2Hershman, D.E., Vincelli, P., and Kaiser, C.A. 2011. Foliar fungicide use in corn and soybean. University of Kentucky. PPFS-GEN-12. https://plantpathology.ca.uky.edu/
3Robertson, A., Abendroth, L., and Elmore, E. 2007. Yield responsiveness of corn to foliar fungicide application in Iowa. https://crops.extension.iastate.edu/corn/production/foliarfungicide.html
4Bohner, H. 2014. What is the correct time to apply foliar fungicides to soybeans? Crop Talk. Ontario Ministry of Agriculture, Food and Rural Affairs. http://www.omafra.gov.on.ca/english/crops/field/news/croptalk/2014/ct-0614a4.htm
5Koger T. July 2, 2008. Soybeans reaching critical stage for fungicide. Delta Farm Press. Purdue University. Corn & Soybean FieldGuide. 2008 Edition.
6Mueller, D., Robertson, A., and Pedersen, P. 2006. Asian soybean rust management strategies. PM2028. Iowa State University. https://crops.extension.iastate.edu/
7Navi, S.S. 2014. Efficacy tests of foliar fungicides on soybean diseases and yield during 2012 and 2013 growing seasons in Northeast Iowa. Iowa State University.https://crops.extension.iastate.edu/cropnews/2014/02/efficacy-tests-foliar-fungicides-soybean-diseases-and-yield-during-2012-and-2013 Web sources verified 05/02/18.