With each new crop year, growers must make critical decisions around inputs ranging from seed and fertilizer to financing and insurance. Inevitably, decisions around crop protection products such as herbicides and fungicides must be made before or during the growing season. For many producers, a fungicide application comes down to a "game-time" agronomic and economic decision, wherein a host of current or possible scenarios must be considered:

• What does a fungicide application cost?
• What's the price I can expect for my grain?
• What's my potential return on investment (ROI)?
• What's the risk if I don't apply (am I costing myself yield potential)?
• What crop diseases might show up on my farm?
• What impact will the weather have on my crop over the next 30/60/90 days?

These questions and more arise when the topic of a fungicide application is introduced, especially when considered as part of a no-till or cover cropping system.

First, let's address a few agronomic considerations. For a plant disease to appear, three conditions must be met: pathogen presence, a susceptible host, and the correct environmental conditions. The economic damage from a plant disease depends on how long these three conditions intersect, i.e., how long the right environmental conditions allow a pathogen to spread through host plants. Of course, not all diseases result in economic loss, and some diseases are more economically impactful than others. Holcus Spot, for example, is a bacterial disease often present in corn fields but typically does not result in any noticeable damage (note: fungicides cannot control bacterial diseases). Compare that to Tar Spot, an economically devastating fungal disease that can cause yield losses of 25 to 30% in the Midwest. (1) A fungicide application typically provides the most benefit when a disease is prevented from reaching the point where the cost of yield damage exceeds the cost of control, otherwise known as the economic threshold. Fungicides are most effective when applied with other integrated pest management strategies, such as crop rotation, proper corn product selection, and frequent crop scouting.

In their article on fungicide decision-making (2018), the University of Illinois Field Crop Disease Hub urges growers to consider the following when assessing the need for a fungicide application: (2)

• What diseases are most likely to be encountered?
• What are the disease resistance ratings for the seed products planted?
• Is the same crop as last year being planted, or is a rotation being followed?
• Is this a no-till production system?
• Is irrigation going to be utilized?

A residue management plan should also weigh heavily in fungicide application considerations. While no-till farming practices provide a whole host of short- and long-term benefits, one drawback is that remaining crop residue can serve as a host to overwintering pathogens such as Grey Leaf Spot, Northern Corn Leaf Blight, and Tar Spot in Corn, as well as Cercospera Leaf Blight and Frogeye Leaf Spot in soybean. (3) Many growers have also questioned whether cover crops, like crop residue, can be a host to disease inoculum and increase disease pressure. While studies have shown this may be the case for certain seedling root diseases in scenarios such as planting corn after rye or soybean after other legume cover crops, there is little evidence to suggest higher incidences of common foliar diseases during reproductive stages of corn or soybean development. (4) Regardless, the potential for overwintering pathogens in crop stubble, especially in continuous cropping no-till scenarios, means crop rotation and the proper selection of disease-resistant seed products should be a high priority during the planning phase of every crop year.

In addition to agronomic considerations, growers must also consider the economic implications of a fungicide application. While very little in a farming operation is ever assured, the potential yield benefits should help provide a net return on investment. Will the expected yield gain pay for the product, application, and provide a net return on investment? Risk factors that may increase the economic utility of a fungicide in corn and soybean production include continuous cropping (especially in a no-till system), high planting rates, susceptible seed products , the use of irrigation, and weather that favors the spread of disease. (5) Corn growers should also be aware of the risks associated with late planting; diseases like Southern Rust and Tar Spot can impact late-planted corn at critical reproductive stages, further impacting yield more than typically seen with early planting.

Field scouting is and always should be an integral part of any integrated pest management (IPM} strategy. Growers should frequently scout fields to determine if diseases are present or if conditions are conducive for future disease establishment. If a determination is made that a fungicide application is warranted, growers should scout their crop to help ensure the proper timing of a fungicide application. Studies and field trials have shown that a fungicide application should be made between VT to R1 growth stages (tassel to silking) in corn and the R3 soybean growth stage (pinky fingernail-sized pod in any of the top four nodes with a fully developed leaf) to provide the most economic benefit. (6)

In addition to a fungicide application's agronomic and economic considerations, growers should also keep resistance management in mind. Like various weed species that have developed resistance to herbicides, some fungal diseases could develop resistance to certain fungicides. Therefore, growers must utilize resistance management practices such as using products that provide multiple modes of action (MOA), including products like Delaro® Complete Fungicide from Bayer Crop Science, not using the same fungicide in sequential applications, and implementing other cultural practices such as crop rotation. (7)

When used as part of an effective IPM strategy, fungicide applications can provide a range of protection and benefits to operations across the United States. However, regardless of their geography or crop, growers should always be mindful of the risks posed by yield-robbing diseases while also considering the implications inherent to a fungicide application. Proper planning, combined with weighing agronomic and economic risk factors and benefits, can help ensure that the right decision is made each growing season.

Sources:

1 Valle-Torres, J., Plewa, R.D., Avellaneda, M.C., 2020. Tar spot: An understudied disease threatening corn production in the Americas. Plant disease 104.10 (2020): 2541-2550. 2 Kleczewski, N. 2018. Tips to help you make fungicide decisions. Illinois Field Crop Disease Hub. http://cropdisease.cropsciences.illinois.edu/?p=743. 3 Kirby, H.W. 2018. Conservation tillage and plant disease. A Systems Approach to Conservation Tillage. CRC PressChapter 12, pages 131-135. 4 Robertson, A., Kaspar, T., Acharya, J., Mueller, D., and Leandro, L. 2017. Disease risks associated with cover crops in corn and soybean production. Agroecosystems Management Research: Ames, Iowa. Agricultural Research Service. United States Department of Agriculture. https://www.ars.usda.gov/research/publications/publication/?seqNo115=347781. 5 Hershman, D.E., Vincelli, P., and Kaiser, C.A. 2011). Foliar fungicide use in corn and soybeans. University of Kentucky College of Agriculture, Food and Environment. https://plantpathology.ca.uky.edu/files/ppfs-gen-12.pdf. 6 Collins, A. and Esker, P.D. 2022. Fungicide considerations for corn and soybean. PennState Extension. https://extension.psu.edu/fungicide-considerations-for-corn-and-soybean. 7 Damicone, J., and Smith, D. 2009) Fungicide Resistance Management. EPP-7633. Oklahoma Cooperative Extension Facts Sheets. Oklahoma State University. https://shareok.org/bitstream/handle/11244/334679/oksa_EPP-7663_2009-04.pdf?sequence=1.

Web sites verified 6/5/23.

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