Fungicides in Corn: How They Reduce Disease Pressure and When to Apply

Prior to using a fungicide, a basic understanding of what a fungicide is, how it protects plants from fungi, and when to consider an application is important.

Q: What are fungicides? 
A: Fungicides are products that can prevent or reduce the impact of diseases caused by fungi. 

Q: Do fungicides kill the fungi? 
A: They may or may not, depending on the product.  Some fungicides interfere with fungal growth and reproduction while others provide a barrier that inhibits spore germination. Still others may induce changes in the plant that make the plant less vulnerable to fungal infections but are not specifically toxic to the pathogen. 

Q: How do fungicides protect a corn plant from fungi? 
A: Fungicides protect a plant through their specific mode of action and the way each chemistry interacts with the corn plant. There are several modes of action among the many fungicides available. Some hamper energy production, while others stop or inhibit certain metabolic functions. The result can be anything from faulty membranes to improper cell division. Each method of control is considered a mode or site of action. Depending on the pathogen and when the plant is susceptible to infection, fungicides may be applied as seed treatments or to the foliage. 

Q: What are the most important fungicide groups? 
A: There are three primary groups for foliar treatments on corn: 

  1. Demethylation inhibitor (DMI) or FRAC 3 compounds inhibit an enzyme that is necessary for sterol production, which is essential for the development of fungal cell membranes. Inhibition results in abnormal fungal growth and eventual death. 
  2. Quinone outside inhibitors (Qol) or FRAC 11 (Strobilurins) inhibit mitochondrial respiration at complex III, which stops energy production, causing fungal death. These fungicides are effective on germinating spores. 
  3. Succinate dehydrogenase inhibitors (SDHI) or FRAC 7 are also respiration inhibitors, blocking electron transport at complex II. They work primarily be stopping spore germination. 

To slow the development of fungicide resistance in a fungal population, it is important to blend effective modes of action whenever possible with each application. 

Q: What are the differences between a contact fungicide and a translaminar fungicide? 
A: Contact fungicides are applied to plant surfaces, but do not penetrate the plant’s tissues. Contact fungicides must be applied before fungal spores germinate because they provide a barrier to help keep developing pathogens from invading the plant. Contact fungicides can be washed off and degrade from environmental exposure, and residual activity may be as short as 5 days. These are not commonly used for foliar application on corn but are often used as a seed treatment. 

Translaminar fungicides enter plant tissues after application and protect both the top and bottom leaves. Fungicides that interfere with respiration (Qol and SDHI) are primarily preventive, while products interfering with proper membrane construction (DMI) provide preventive protection as well as stop or slow infections that occurred 24 to 72 hours prior to application.  If an active ingredient has systemic movement within the plant, it is limited to acropetal transport, carried with water flow in the xylem. 

Q: Is there a benefit to using a fungicide that is a combination of different modes of action? 
A: Because different modes of action respond to the pathogen and the environment differently, combining modes of action can improve performance consistency. A product with two or more modes of action can offer protection even if a fungus is resistant to one of the chemistries. However, to do so, it is critical that multiple modes of action have activity against each of the important pathogens. Tank mixing different modes of action has been shown to be more effective at slowing resistance development than rotating modes of action in alternating sprays.  

Q: Is it important to know the disease resistance levels of each corn product? 
A: Yes. Each corn product has its own level of resistance to various diseases. One can be highly susceptible to a disease such as Southern Rust (SR) and another highly resistant. If environmental and field conditions are favorable for SR development and a susceptible product is planted, a properly applied foliar fungicide can help reduce the potential SR impact on yield. If the product is highly resistant to SR, a foliar application for the explicit protection against SR may not be warranted. However, other diseases to which the seed product is vulnerable may have the potential to develop. A well-timed fungicide application can provide protection for the spectrum of diseases and help maintain overall plant health. Fungicide resistance management procedures should be followed. 

Q: What fields are considered at high risk for disease development? 
A: Multiple factors such as continuous corn , fields with greater than 35% residue, fields with a previous history of a disease(s), fields near creeks, lakes, or trees , fields with a practice of no-till or conservation tillage, fields that were delayed in planting, those planted to a susceptible seed product, or products with a history of stalk health issues can influence the likelihood of disease development. A proactive fungicide application may be warranted to help decrease the potential for diseases to limit yield potential. 

Q: What environmental conditions are favorable for fungal spore production? 
A: High moisture situations (rain, heavy prolonged dews, irrigation) can provide a favorable environment for spore generation and infection. Temperatures also play a role, as some pathogens favor cooler temperatures, while others can thrive with warmth (SR likes heat and humidity and Northern Leaf Blight (NLB) likes cool, wet conditions). 

Q: Why is field scouting important? 
A: Timely and regular field scouting for disease development can be an early warning system. If disease lesions are identified, then other factors can be evaluated to determine if a foliar fungicide should be applied immediately, after a waiting period, or not at all. Considerations include the product planted, current weather conditions, forecasted weather conditions, and stage of plant development. 

Q: What parts of the corn plant are the most important to be protected with a foliar fungicide? 
A: The ear leaf and the next higher leaves because they are very important for photosynthetic energy production that is needed for grain fill. 

Q: Is there an accepted disease threshold when disease symptoms are found in a field? 
A: One accepted threshold is if the field is planted to a susceptible or moderately susceptible seed product(s), a foliar fungicide may be warranted if lesions appear on the third leaf below the ear or higher on 50% of the plants at tasseling.2

Q: If a foliar fungicide is warranted, what should be the timing for the application? 
A: Consideration should be given to an application from VT (tasseling) to R1 (silking) to help maximize fungicide activity.3 Under severe foliar disease pressure, fungicide applications up to R4 have shown reduction in disease versus untreated. 

Q: Should a foliar fungicide application be made after a late-vegetative growth stage (V7-V9) hail event? 
A: Hail wounds can provide an opening for several corn diseases including common smut, Goss’s leaf blight and wilt, and some stalk rots. These diseases are generally not controlled with a foliar fungicide.4 However, an application may help plants recover quicker by protecting the remaining green tissue from controllable diseases that may be present. Additional leaf tissue lost to disease after hail damage can increase the potential for lost yield. A proactive application in geographies that are prone to hail may help the plant produce a stronger stalk and be protected from controllable diseases.  

Q: Is it beneficial and warranted to apply a fungicide proactively prior to seeing any disease? 
A: Applying fungicides proactively can be beneficial for several reasons: 

  • A proactive application can protect the plant from disease development before usual symptoms appear. Symptoms take several days to appear after initial infection. The time between initial infection and symptom appearance gives a disease such as Gray Leaf Spot (GLS) time to get a foothold and in the process invade plant cells that increase or maintain yield potential. Gray leaf spot has a 2-week latent period from infection to considerable amounts of lesion formation. 

  • Fungicides can help reduce the impact of stress events such as disease, hail, drought, and heat. Plants under stress produce the ripening gas hormone ethylene. When ethylene is produced prematurely, plant growth slows, leaf senescence begins, and kernel abortion can occur, which ultimately and negatively impacts yield potential. Fungicides that contain a strobilurin component inhibit the production of ethylene, which along with other effects, such as an increased production of nitric oxide, helps keep the plant in a heathier state. 

  • During periods of rapid corn growth, the waxy layers on the leaf surface can become thinner, allowing fungi an easier entry into leaf tissue. 

Q: Can adjuvants be used? 
A: Use of adjuvants depends on the fungicide and the timing of application. Fungicide labels should be read to determine if, and when adjuvants can be used. Generally, an adjuvant is recommended for applications from V4 to V8 (number indicates the number of leaf collars visible) and after tassel emergence (VT).  

Sources:

1Smith, D. Field crops fungicide information. Wisconsin Field Crops Pathology. https://fyi.extension.wisc.edu/
2Robertson, A., Abendroth, L., and Elmore, E. 2007. Yield responsiveness of corn to foliar fungicide application in Iowa. https://crops.extension.iastate.edu/
3Mueller, D. and Robertson, A. 2008. Preventative vs. curative fungicides. Integrated Crop Management News. Iowa State University. https://crops.extension.iastate.edu/
4Malvick, D. Do foliar fungicides provide a benefit to corn damaged by hail? County Agriculture Educator. Wright County Extension Office. University of Minnesota. https://local.extension.umn.edu/
5Smith, D. 2015. Corn diseases of 2015 and should I spray fungicide? Wisconsin Field Crops Pathology. https://fyi.extension.wisc.edu/

Additional Source: 
Management of corn diseases. Fungicide efficacy for control of corn diseases – January 2019. The Corn Disease Working Group (CDWG). The Bulletin. University of Illinois. http://bulletin.ipm.illinois.edu/
Voight, Jr., D.G. 2017. When to consider foliar fungicide applications on corn. PennState Extension. The Pennsylvania State University. https://extension.psu.edu/
Dufault. N. 2017. Corn disease management: When to apply a fungicide? IFAS Extension. University of Florida. http://nwdistrict.ifas.ufl.edu/
 

Web sources verified 7/3/19. 5006_Q1