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Fungicides on our Corn Fields: Answers to Case Study Questions

Sally O. Mallowa
Department of Plant Pathology and Microbiology, Iowa State University






1. What production practices in the U. S. Corn Belt increase the risk of disease in a field?
The U. S. Corn Belt is approximately 150 million acres.  Over the past few decades, the use of minimum or no till as a production practice has been increased in an effort to protect against soil erosion and nutrient loss.  Furthermore, more acres have been planted to continuous corn because of better prices for corn as a result of changes in biofuel policies and enlarging markets in Asia.  These practices contribute to larger amounts of surface crop residue across the landscape. This residue serves as an inoculum source for many residue-borne foliar diseases.  Students may talk about this unprecedented large scale corn “monoculture” system and its impact on the environment and/or biodiversity. Alternatives to recommendations that discuss the effects of this system are acceptable.  

2. Foliar fungicides have been marketed for plant health benefits. What are examples of these plant health benefits? Can the potential for these benefits affect fungicide use decisions?
Strobilurin (QoI) fungicides enhance certain physiological processes in plants that are not necessarily associated with disease and fungicide companies have marketed this characteristic as “plant health benefits.” Some effects that have been detected when strobilurins are applied to plants are: 1) improved chlorophyll synthesis and improved water utilization that each positively contribute to photosynthetic and plant antioxidant enzyme activities, 2) reduced production of harmful reactive oxygen species and increase production of antioxidants that could facilitate stress tolerance, and 3) inhibition of ethylene synthesis, due to delayed senesce, this is known as the “greening effect.”  It may contribute to better stalk strength; it does not contribute to yield in corn. Students’ discussions of logical physiological processes that strobilurin fungicides can affect are acceptable.

The benefits include healthier plants, improved standability, and easier harvest.  For growers with large farms, applying a fungicide may enable them to spread harvest over a longer period.   These benefits are attractive since they would lead to stronger, healthier plants with hopefully higher yields. These additional reasons suggest positive effects of fungicide use (including increased stalk strength) even in the absence of disease, which might be considered a form of insurance. 

3. What is fungicide resistance and what are the risks of foliar corn pathogens developing fungicide resistance?
Genetic alterations of pathogens (mutations) can lead to lower sensitivity to fungicides (fungicide resistance).  Repeated application of a fungicide with a particular mode of action can provide selection pressure for the resistant strain. Strobilurin fungicides are popular because of their broad spectrum of activity and benefits other than disease protection. This increases the likelihood of fungicide resistance, but to date, monitoring programs have not yet reported resistant strains of foliar fungal pathogens in corn. Growers are unable to tell if and when pathogens mutate, however precautionary measures to reduce selection pressure for fungicide resistance are recommended. These include using combinations of fungicides with multiple modes of action and selecting hybrids with high disease resistance.  Checking fungicide labels to ensure correct rate and timing of application is also important. 

University of Wisconsin-Extension - Fungicide resistance management in corn, soybean, and wheat in Wisconsin

4. What is the relationship between legislation related to biofuels, corn prices, and fungicide use?
The relationship is complex and not a direct cause and effect.  Generally, students’ responses that take into account factors that drive these relationships should be acceptable. The federal government has set a standard, encouraging renewable fuel production from ethanol made from grains, primarily corn. This has created a steady market for corn at higher prices and consequently more growers are willing to disregard IPM principles and grow high yielding corn continuously. Although the risk of disease is higher, the price of corn recently has been good enough to offset/ justify the use of fungicides. Strobilurins (QoIs) are broad-spectrum fungicides that reduce infection by pathogenic fungi and therefore conserve plant energy and nutrients that could have been used in plant defense (Bartlett et al. 2002). They are effective and hence more widely marketed than their predecessors; claims for yield increase in the absence of disease further enhance their beneficial properties and good corn prices could encourage and justify their use.   

5. Pablo’s position is that the family use high-yielding susceptible hybrids and scout. What is scouting and how is it done? How does scouting influence the perception of disease risk?
Scouting in this context could be described as a systematic evaluation of plants for diseases present while taking notes on other aspects of environment and production that could
pre-dispose the crop to disease. Disease severity in different sections of a field are noted. Further, preliminary examination of suspected diseased samples could be done for symptoms and signs using a hand lens (portable microscope) and a pocketknife. Comparisons between sick and healthy plants are made. In the current season, scouting assessments aid in determining disease risk and the decision to initiate management practices such as applying a fungicide. Scouting is important even if done late in the season, because accurate assessments aid in determining disease risk the following season.

6. How could a grower determine if he will make a profit or a loss with a fungicide application? How could the increasing availability of generic fungicides influence this consideration?
Combinations of factors typically influence profitability when using fungicides. These include the use of susceptible varieties, favorable weather conditions for disease, conditions that increase disease risk like continuous corn, reduced or no-till practices, late planting, irrigation, etc., and most importantly, if disease actually develops. To determine a break-even point, growers take into account the cost of fungicide purchase, cost of scouting, and application cost, as well as grain prices. Research has shown that profitable fungicide use is more likely when disease is a threat to production. Students’ suggestions that give consideration to some variants of these factors are acceptable. The cost of using name-brand fungicides varies from $20 to $30 per acre; in addition to those, there are also low-cost generic fungicides available. Producers are most likely to see returns on fungicide investments if disease is present.  If making insurance application where disease risks are lower, the availability of generic fungicides could further justify fungicide use.  



1. Foliar fungicides have been applied to field corn for many decades on most of the corn acreage in the U. S. Corn Belt.    

A. true; B. false

2. Which of the following increases the risk of disease in a cornfield?  

A. planting continuous corn;  B. zero/minimum tillage;  C. planting susceptible hybrids;  D. all the above 

3. The Markus family member LEAST interested in fungicide application for “plant health benefits” is: 

A. Eric;  B. Brad;  C. Pablo

4. As the price of corn goes up, the yield increase per acre needed to “break even” for a fungicide application would: 

A. go down;  B. go up;  C. not be affected by the price of corn

5. Strobilurin (QoI) fungicides are being marketed for their “plant health benefits.”  This means that they: 

A. will have little or no adverse environmental impacts;  B. will be able to improve plant health by controlling fungal pathogens; C. will increase corn yields and improve standability and stress tolerance even in the absence of disease;  D. will be unlikely to select for fungicide resistance in pathogen populations


1. The most important factor in determining whether use of a fungicide will be profitable or not is:

A. the presence of disease;  B. application costs;  C. the price of corn; D. the weather;  E. all the above

2. Since the Markus farm has a history of foliar diseases and susceptible varieties have been planted this season, I believe they should: 

A. spray a foliar fungicide; B. monitor the weather, scout for diseases and only spray a fungicide if there is likelihood of disease;  C. look at the price of corn and the cost of application and try to predict the likelihood of a good economic return

3. Do you think that corn growers should use fungicides for plant health benefits if it will lead to increased yields? 

A. Yes; B. No; C. Maybe; D.  I don’t know

4. Do you think the “Twitter” communications were a useful part of the case study activity? 

A. Yes;  B. No; C. No opinion

5. Did this case study activity improve your understanding of the issues related to the use of fungicides in corn production? 

A. Yes; B. Yes, somewhat; C. No

6. Did this case study activity give you a better understanding of the use of fungicide applications for “plant health benefits?”

A. Yes; B. Yes, somewhat; C. No


1. Did this case study activity help you learn about the issues related to the use of fungicides in corn production?

2. If so, what aspects of the case study activity contributed the most to your learning?

3. What specific changes would you suggest that might make this case study more effective in helping students learn about the issues surrounding fungicide use on corn, fungicide applications for “plant health benefits,” etc.?