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.