The sights and sounds of airplanes, helicopters and ground sprayers in Illinois corn and soybean fields during July and August usually means one thing: foliar fungicide application season has begun. While fungicides have been common practice in high value crops for many years, the use of these chemistries in row crops like soybean has expanded rapidly during the last 10 to 15 years. There are many excellent articles and sources of data explaining the benefits of using fungicides in corn and soybean, so let’s instead focus on the language of fungicides and a few insights into how fungicides work.
Mode-of-action (MOA) and site-of-action (SOA) are terms that are commonly used to describe herbicides but are also applicable to fungicides. These terms explain how (MOA) and where (SOA) an active ingredient works on the target pest, and they also have implications for managing the potential for resistance to an active ingredient. There are many types of MOA represented within available fungicide active ingredients, but the most used active ingredients for foliar fungicides in soybeans and corn fall into two MOA categories:
MOA: respiration inhibitors
These chemistries inhibit different aspects of fungal respiration (i.e., the conversion of stored carbon to usable energy). Two families of commonly used fungicides are respiration inhibitors, but they work to inhibit respiration at different sites in the respiratory pathways of fungal organisms. These families include the succinate dehydrogenase inhibitors (SDHI) and quinone outside inhibitors (QoI):
- SDHI fungicides: This family of chemistry includes both seed-applied and foliar-applied fungicides. Examples include fluopyram (ILeVO®), fluxapyroxad (Xemium®), sedaxane (Vibrance®) and pydiflumetofen (Miravis®).
- QoI fungicides: These are also commonly referred to as strobilurins or ‘strobis’. Examples include azoxystrobin (Quadris®), pyraclostrobin (Headline®) and picoxystrobin (Aproach®). In addition to their disease control properties, strobilurin fungicides also have what are referred to as ‘plant health benefits’. Strobilurins elicit physiological responses in the plant that can help withstand environmental stresses, ultimately improving growth and productivity.
MOA: sterol biosynthesis inhibitors
Many families of fungicides fall into this category. The MOA impacts the creation of sterols (a type of lipid) in the cell membranes of fungi. The use of demethylation inhibitors (DMI) is widespread in agriculture. You may know these more commonly as the triazole fungicides. Examples include propiconazole, difenoconazole and prothioconazole among many others.
Preventative versus Curative
These terms are commonly used to describe how fungicides work or how they should be used. Preventative fungicides are those that should be applied before the fungal pest has infected the plant. The SDHI and QoI (strobilurin) fungicides are considered preventative in their activity. The term curative can be misleading to some. Curative does not imply that disease lesions will disappear upon application. Fungicides with curative activity, triazoles as an example, work to prevent additional sporulation and continued re-infection of the plant by a fungal pathogen that has previously infected the plant.
Fungicide product selection
There are many branded and generic fungicide products available in the market. Although some active ingredients are available as stand-alone (one active ingredient) products, it is common for foliar fungicides to include two or three active ingredients. Combining strobilurin and triazole active ingredients has been a common approach in the past. This combination provides both preventative and curative activity, and the combination of active ingredients is an important tool to reduce the risk of resistance developing to single active ingredients. SDHI active ingredients are more recent developments, and newer products might include all three types of active ingredients or substitute an SDHI for older strobilurin chemistry.
Another aspect of selecting fungicide products for your farm is to consider the target fungal pathogen and select products that are labeled and efficacious on that pathogen. Fungicide families, and individual members within each family, can vary in their efficacy for specific fungal pathogens. There are several resources available from land-grant universities to help understand efficacy ratings for common soybean fungal diseases like white mold and frogeye leaf spot.
Fungicides are an important component of modern high-yield soybean production systems. You have choices in fungicide products, and additional considerations beyond agronomics will shape your decision. Regardless of the manufacturer that you choose, I encourage you to consider using products that contain at least two active ingredients. These combinations provide the best spectrum of disease control, potentially contribute additional benefits like ‘plant health’, and are the agronomically correct approach to managing fungal resistance to fungicide active ingredients.