CPN-4001 — December 2016
Yes. In fact, researchers in several North Central states have confirmed that the fungus that causes frogeye leaf spot in soybean has developed resistance to the quinone-outside inhibiting (QoI/strobilurin) fungicide group (Figure 1).
Fungicide applications do not cause resistance. Resistant fungal strains are already present in the fungal population. Such resistance is caused by naturally-occurring genetic mutations.
Fungicide applications select for these resistant fungal mutants — the fungicides kill the fungicide-sensitive population and only the resistant mutants survive. Eventually, the population of the resistant fungal strains increases and replaces the sensitive fungal population (Figure 2).
Once the population of the fungicide-resistant mutants is predominant, efficacy of a specific, fungicide active ingredient may be reduced or lost.
Figure 2. This figure demonstrates the selection for resistant (red spots) fungal strains among fungicide sensitive strains (blue spots) with repeated applications of the same fungicide active ingredient.
When fungicide resistance occurs in a fungus, fungicide applications of a specific active ingredient may no longer effectively control the particular disease the fungus causes. Several fungicide active ingredients are at high risk for developing fungicide resistance, especially in the QoI/strobilurin group.
There are multiple fungicide groups available for use on field crops, but the majority of available fungicide products fall into two groups: the QoI group and the demethylation inhibitor (DMI) group (Table 1).
Fungicide group names represent different target sites within specific modes of action. A mode of action is how the fungicide’s active ingredient inhibits fungal development. For example, a fungicide may work by inhibiting respiration in the fungus. A target site is the specific location at which the fungicide works in the fungus.
The Fungicide Resistance Action Committee (FRAC) developed a numerical code for classifying fungicides. Each number represents a specific target site or group name (Table 1). Fungicide labels include these “FRAC Codes.” If a fungus is resistant to a specific fungicide active ingredient, then it may be resistant to all of the fungicide active ingredients that have the same FRAC Code.
Take the following steps to delay fungicide resistance:
The Crop Protection Network (CPN) is a multi-state and international collaboration of university and provincial extension specialists, and public and private professionals who provide unbiased, research-based information to farmers and agricultural personnel. Our goal is to communicate relevant information that will help professionals identify and manage field crop pests.
Find information about identifying soybean diseases and fungicide efficacy from the Soybean Research and Information Initiative at soybeanresearchinfo.com/resourcelibrary.html.
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Carl Bradley, University of Kentucky; Martin Chilvers, Michigan State University; Loren Giesler, University of Nebraska-Lincoln; Daren Mueller, Iowa State University; Adam Sisson, Iowa State University; Damon Smith, University of Wisconsin-Madison; Albert Tenuta, Ontario Ministry of Agriculture, Food and Rural Affairs; Kiersten Wise, Purdue University.
All photos were provided by and are the property of the authors.
Emmanuel Byamukama, South Dakota State University; Anna Freije, Purdue University; Dean Malvick, University of Minnesota; Sam Markell, North Dakota State University.
This publication is a multi-state and international collaboration sponsored by the North Central Soybean Research Program (NCSRP). Contributors to this series come from land grant universities in the North Central states and Canada.
USDA is an equal opportunity provider and employer.