Fungicides are More Than a Plant Disease Management Tool

Fungicides have long been an important tool in the plant disease management toolbox for agricultural crops. This is particularly true for diseases of corn, soybean, and small grain crops such as wheat and barley. Fungicides are commonly used to manage fungal diseases where hybrid or variety resistance is unavailable, limited, or not effective enough to provide acceptable disease control, as is the case with Fusarium head blight (FHB) of small grains. FHB can devastate wheat and barley crops in the United States and Canada, resulting in yield loss while negatively impacting the health of humans and livestock that consume affected grain. Variety resistance alone is often not enough to manage FHB and associated mycotoxins. The use of demethylation inhibitor (DMI) fungicides, commonly referred to as triazole fungicides, prevent FHB from severely limiting cereal crop production in the United States and Canada (Paul et al. 2018).

There are several additional examples of diseases, such as southern rust and tar spot of corn, where fungicides may be needed to prevent yield loss because many commercial hybrids are susceptible to these diseases. Preventative fungicide applications can help to minimize yield loss due to these and other economically important diseases.

Although fungicides can be used as part of a larger crop disease management plan, repeated or unwarranted applications increase the selection pressure on the fungal population. Such practices may select naturally occurring strains of fungi that are resistant or less sensitive to a particular fungicide class. Over time, these resistant fungal strains can become the dominant population, meaning that fungicides that were once effective at controlling a disease are no longer as effective. When this occurs, it is referred to as fungicide resistance.

Fungicide resistance or shifts in fungicide sensitivity to different fungicide classes is common in horticultural and other cropping systems where multiple applications occur during the growing season. Fungicide resistance in field crops has been reported recently in several foliar fungi of soybean and in the fungus that causes FHB of cereal crops (Price III et al. 2015; Rondon and Lawrence 2019; Spolti et al. 2014; Zhang et al. 2018). Fungicide sensitivity monitoring programs for many important fungal diseases of field crops are ongoing (Figure 1). University Extension programs are often the first to alert farmers and others in agriculture when shifts in fungicide sensitivity occur within a fungal population and fungicide class.

However, many agricultural professionals may not realize that fungicides are not just critical tools for managing plant diseases. Fungicides, which are commonly called antifungals in healthcare, are also used to treat human fungal diseases, including Valley fever, sporotrichosis, histoplasmosis, candidiasis, and aspergillosis, to name a few.

Aspergillosis is caused by the fungus Aspergillus fumigatus, a common fungus in soil, plant residue, compost, and other outdoor agricultural settings. This disease mostly affects people who are immunocompromised or have severe lung disease and is not spread from person to person. Infection in the lungs occurs when spores of this fungus are inhaled. Aspergillosis is often treated with antifungals in the same triazole class as DMI fungicides, which are commonly referred to in the medical field as azoles.

As aspergillosis is a fungal disease, fungicide resistance can occur in A. fumigatus in a similar manner as plant pathogens. Repeated treatments of azole fungicides in humans can select for azole-resistant A. fumigatus. Recently, azole resistance has appeared in humans infected with A. fumigatus who have never previously been treated with azole antifungals. These cases have occurred across the world, including in the United States. Some of these azole-resistant strains are resistant to all medical antifungal treatments, meaning there is no effective treatment for patients infected with those strains of A. fumigatus.

It has been recently reported that azole-resistant strains of A. fumigatus likely acquired their resistance from azole exposure in agricultural settings rather than in a medical environment (Kang et al. 2020). Researchers examined A. fumigatus strains collected from agricultural environments and humans and tested them for resistance to azole fungicides and antifungals, as well as resistance to other fungicide classes that are used solely in agriculture. They found that some strains of A. fumigatus from humans had resistance to not only the azole fungicides and antifungals, but also fungicides that only are used in commercial plant production, meaning that azole resistance likely originated in an agricultural setting before that strain of A. fumigatus infected the patient.

Surveys indicate that azole-resistant strains of A. fumigatus are present, but not currently prominent, in field crop settings and are more prevalent in specific agricultural environments including flower production and compost piles (Burks et al. 2021). Although more research is needed to understand how fungicide applications in field crops may affect non-target fungal populations, delaying fungicide resistance development in plant and human pathogens should continue to be an important goal for the agricultural industry.

Our advice on fungicide use in corn, soybean, and small grain crops remains unchanged for now. We advise using fungicides judiciously and as part of a comprehensive integrated disease management strategy that includes the following:

1. Select crop hybrids or varieties that are resistant or less susceptible to yield-limiting diseases.

2. Use cultural methods where possible to reduce crop disease risk. A few examples are crop rotation, residue removal or methods that encourage residue decomposition, and adjusting planting dates to avoid conditions that favor disease development.

3. Use fungicides in response to disease or a disease threat. Disease monitoring and forecasting systems can help determine if environmental conditions during the season increase disease risk and warrant the use of foliar fungicides. Tools such as the FHB risk tool in the United States or DONcast in Ontario for FHB in wheat, Tarspotter for tar spot in corn, and Sporecaster for white mold in soybean are a few that are currently available to help with fungicide application decisions. The National Predictive Modeling Tool Initiative is currently funding research on corn, wheat, and cotton diseases that will help create additional disease forecasting models to aid farmers in future fungicide application decisions.

4. If a fungicide is needed for disease management, apply the fungicide according to its label. Use fungicide products that contain multiple fungicide classes and/or rotate fungicides of different classes. Rotating fungicide classes can help reduce selection of fungicide-resistant strains of fungi and prolongs the efficacy of individual fungicide classes. Using the recommended label rates is crucial to slow the development of resistance to fungicides. Maximize fungicide efficacy by using appropriate nozzles and carrier volume.

Fungicides and antifungals are powerful tools that can manage both plant and human fungal diseases. Maintaining these products for future use against fungi that infect both humans and plants is important for everyone in the agriculture and healthcare industries.

For more information on this topic, listen to the following podcast: Fungicide Use in Crops Poses Risk to Certain Human Medications

References

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Acknowledgements

Authors

Kiersten Wise, University of Kentucky; Marin Brewer, University of Georgia; Carl Bradley, University of Kentucky; Daren Mueller, Iowa State University; Adam Sisson, Iowa State University; Albert Tenuta, Ontario Ministry of Agriculture, Food and Rural Affairs; Tom Allen, Mississippi State University; Gary Bergstrom, Cornell University; Kaitlyn Bissonnette, University of Missouri; Emmanuel Byamukama, South Dakota State University; Martin Chilvers, Michigan State University; Nick Dufault, University of Florida; Travis Faske, University of Arkansas; Andrew Friskop, North Dakota State University; Heather Kelly, University of Tennessee; Alyssa Koehler, University of Delaware; David Langston, Virginia Polytechnic and State University; Samuel Markell, North Dakota State University; Juliet Marshall, University of Idaho; Alfredo Martinez-Espinoza, University of Georgia; Pierce Paul, Ohio State University; Alison Robertson, Iowa State University; Damon Smith, University of Wisconsin-Madison; Darcy Telenko, Purdue University; and Paul Vincelli, University of Kentucky.

Reviewers

Doug Jardine, Kansas State University and an anonymous reviewer.