Integrated Crop Management

Fungicides: Fungicide resistance and the FRAC code

Fifth in a series. See Part 4 [1].

Fungicides that may become an integral part of soybean production are already used in corn and small grain production under certain situations. Since fungal pathogens are often highly variable and may be able to adapt to repeated fungicide sprays, resistance management may become an issue. It is important to protect effective groups of fungicides because resistance may lead to unexpected and costly crop losses to growers, and loss of a valuable product.

Imagine a group of old men rocking on the front porch reminiscing about days gone by and solving today's problems. One mumbles, "fungicide resistance, eh, they just don't make 'em like they used to . . . why back in my day . . ." And he's right.

The first fungicides were sulfur and copper based. Mercury fungicides were developed in the early 1900s and were widely used until it was discovered that they were highly toxic to animals. In the 1940s and '50s, fungicides like Captan® and Maneb® were introduced. All of these fungicides are contact fungicides and only work if applied prior to infection. These older fungicides have another important characteristic: they affect a number of different metabolic sites within the fungus so fungicide resistance has never been a concern.

More recently, highly effective compounds like the triazoles and QoI fungicides (e.g., strobilurins) with specific modes of action have been developed. In other words, these fungicides affect one specific site in one metabolic pathway of the fungus. The exactness of the action of these products means that fungi only have one barrier to overcome. Thus, the problem of fungicide resistance has occurred and is on the increase since growers have relied more and more on these newer fungicides. So, although QoI and triazole fungicides represent marked improvements in performance, including systemic and therapeutic properties, these compounds are prone to resistance because of their specific mode of action.

The Fungicide Resistance Action Committee (FRAC) is a group of professionals whose goal is to provide fungicide resistance management guidelines to prolong the effectiveness of these "at risk" fungicides and to limit crop losses should resistance occur. When growers need to repeatedly spray fungicides to manage a fungal problem, it is best to use fungicides that have different modes of action. The FRAC code helps identify fungicides by their mode of action and informs if they should be used in consecutive sprays.

FRAC code on Quadris label [2]

How do I use the FRAC code?

The FRAC code represents the mode of action of the fungicide. For fungicide resistance management, do not tank mix or alternate fungicides with the same FRAC number in a spray program. Some fungicides are labeled "M," which means that the fungicide acts upon multiple sites and resistance risk is low.

FRAC code for foliar fungicides labeled (or potentially labeled) for use on field crops in Iowa

Chemical Group Common Name Crop FRAC Code
Benzimidazole thiophanate-methyl soybean 1
Demethylation Inhibitors (triazoles) cyproconazole soybean 3
flusilazole soybean
flutriafol soybean
metconazole soybean
myclobutanil soybean
propiconazole soybean
prothioconazole soybean, corn, small grains
tebuconazole soybean
tetraconazole soybean
QoI fungicides (strobilurin) azoxystrobin soybean, corn 11
pyraclostrobin soybean, small grains
trifloxystrobin soybean, corn, small grains
famoxadone soybean
Multi-site contact activity chlorothalonil soybean M
mancozeb corn, small grains
copper small grains
sulfur small grains

Go on to Part 6 [3] of this series.

This article originally appeared on pages 168-169 of the IC-496(15) -- June 12, 2006 issue.

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