Diseases in cultivated crops are not a new phenomenon. The root cause of the Irish Potato Famine was a fungal disease, Phytophthora infestans, commonly called late blight. Today, late blight is still a primary disease concern in potatoes; proactive management practices are in place. Genetics, cultural management, and timely, preventative applications of fungicides are critical.

Why Fungicides?

Fungicides are an integral part of efficient food production to not only protect yield, but to maintain the quality of the grain, vegetable, or fruit. A fungicide is not needed in every field, not in every year to manage diseases, but the availability of fungicides is a valuable tool. Equally so, the loss of a fungicide through disease resistance is a problem that affects us all.

At the core of fungicide use is an understanding of the disease triangle. Disease can only occur if all three of these factors are present:

• If a plant is susceptible to the disease-causing organism(s)
• There is a pathogen (fungus) present to infect the host
• There is a proper environment for the crop and pathogen to grow

There is a pathogen (fungus) present to infect the host

There is a proper environment for the crop and pathogen to grow

Disease Management

With disease management and the subsequent use of a fungicide, you must consider: the fungal pathogen life-cycle; a given fungicide’s efficacy on the target pathogen; and the application timing, requirements, and restrictions.

Farmers have excellent resources to address these considerations. The first is the Take Action Fungicide Resistance Management Tool which can be found at iwilltakeaction.com/diseases/about-the-program. The second resource is producted by the Crop Protection Network (CPN) and can be accessed at cropprotectionnetwork.org/resources/articles/diseases.

Using both resources provides a foundation to understand what fungicide active ingredients (a.i.) are contained within a branded product and the efficacy of the a.i. on the crop and disease being managed. Integral to both resources is categorization of the fungicide a.i. by their respective mode of action (MOA).

Understanding MOA

The MOA is the “how” the fungicide controls a susceptible fungus. The “what” is the defined target site of control: the specific process and/or enzyme that the fungicide a.i. interferes with in development of the disease. Each fungicide is assigned an official FRAC (Fungicide Resistance Action Committee) code to keep track of MOA and target sites. Fungicides may have the same MOA but different target sites and, therefore, separate FRAC codes.

Today, there are three primary fungicide MOAs used in corn and soybeans: trizaoles, SDHI, and strobilurin fungicides. The latter two share the same MOA – they both inhibit fungal respiration, but they target different parts of the respiratory system.

Disease resistance to fungicides deserves our collective attention. Understanding fungicides and their respective modes of actions is critical. It is important to note that when a fungal plant pathogen becomes resistant to a specific fungicide, the pathogen is either less or completely insensitive to fungicides in that entire group.

Maximize Benefits from a Fungicide

• Apply according to label recommendations and when the pathogen pressure is high enough to warrant treatment.
• Repeated use of a fungicide with a single FRAC code is not typically recommended.
• Fungicide mixtures that combine two or more FRAC codes provide additional security that one of the a.i. will kill the pathogen even if it is resistant to the other a.i. in the mixture.
• If the pathogen is already resistant to one component of a fungicide mixture, using a fungicide mixture may eventually select for resistance to the other component.
• Be aware that chemical treatments are not the only disease management option.
• Planting highly tolerant or resistant varieties is the first line of defense against many pathogens, and crop rotation often reduces disease.

Recommendations for Management of Fungicides

• Scout; know before you go.
• Use the high label rate of all fungicides; don’t cut rates
• Incorporate application best management practices for equipment, volume, and timing.

To learn more about fungicides and fungicide MOA, please reference your respective state extension service bulletins on these topics.

~Mike Blaine – Field Agronomist