Although the “window of opportunity” for fungicide applications is now closed for some diseases, it remains wide open for others. It’s important to continually evaluate how well your fungicide applications perform, but it’s equally important to take the time to understand how you can help keep them performing well.

There are many reasons why a fungicide might fail to control disease development. Often, fungicides “fail” because they are applied too early, too late, or not frequently enough. However, for those times when you know you have done everything right yet the performance is less than stellar, consider that you may be dealing with a fungicide-resistant pathogen population. In the world of turf and ornamentals, most documented fungicide resistance problems occur in turf. However, don’t be deceived; fungicide resistance can develop in pathogens of trees, shrubs, and flowering plants as well.

Let’s step back and address a general question about pesticide-resistant pests: Where do they come from? Genetic diversity through mutations allows resistant pests to be present at extremely low numbers in particular species. When a pesticide effectively controls the majority of susceptible members of a species, only those that possess a resistance trait can survive and reproduce for future generations. In addition, since different pests have various ways to move from one place to another (via airborne spores, seeds, insect vectors, and so on), resistance traits can also be “imported.” It is important to remember that resistant populations (sometimes called biotypes, or strains) start out in extremely small numbers. It may take years for you to even notice resistant populations; you may only perceive the problem as shorter-than-expected duration of fungicide protection.

Resistance Terminology

The terms regarding fungicide resistance can be confusing. Following are the key terms you should become familiar with.

FRAC: Fungicide Resistance Action Committee, an international, industry-based committee that issues guidance and anti-resistance strategies for different fungicide groups. FRAC organized the existing fungicide active ingredients into target site groups and gave each group a specific code number. For example, FRAC group 3 includes the well-known DMI fungicides. FRAC code numbers are beginning to appear on some fungicide labels, which will help pest managers in resistance management planning.

Resistance: A change in the pathogen that results in decreased sensitivity to a fungicide. Because the term “resistance” often has a negative connotation or may be confused with host-plant resistance, plant pathologists sometimes refer to “insensitivity” or “tolerance” instead of resistance. However, some experts argue that “insensitivity” should be reserved for describing a fungus that is not inherently controlled by a fungicide (e.g., DMI fungicides never did control pythium or phytophthora; the pathogens did not change). Whatever term is used, be cautious when interpreting reports of fungicide resistance. Just because a pathogen population is deemed resistant to a fungicide, it does not necessarily mean it will completely fail to control the pathogen. As with host-plant resistance, there are degrees of fungicide resistance.

Cross-resistance: Where pathogens are resistant to different fungicides, especially within the same FRAC group. For example, you would suspect cross-resistance if you switched from propiconazole to myclobutanil and did not control your DMI-resistant pathogen population. Both of these fungicides are DMIs and belong to FRAC group 3.

Multiple resistance: The resistance of pathogens to fungicides from different FRAC groups. For example, you would suspect multiple resistance if you treated the previously mentioned DMI-resistant population with azoxystrobin (a FRAC group 11 fungicide) and found that you still could not control the disease.

Resistance Management Strategy

The development of a fungicide-resistant pathogen population can be summarized by the following principle that applies to all pests and pesticides: The appearance of pesticide-resistant pests is the consequence of using the same (or similar) site of action year after year, or of repeating applications of a pesticide during the growing season to kill a specific pest species not controlled by any other pesticides or in any other manner. Specific recommendations for reducing the potential for fungicide resistance development include these:

1. Integrate the use of fungicides into an overall disease and pest management program that includes appropriate cultural practices, host-plant resistance, and scouting.
2. Apply fungicides preventively (when conditions or predictive models suggest disease is likely to occur) rather than curatively, and always use an effective dose.
3. Avoid using an active ingredient (or members of the same FRAC group; see Table 1) more than once per season. For example, there are many different DMI fungicides, but they all have the same site of action. Thus, exclusive use of different active ingredients within the same FRAC group is a poor rotation strategy that will promote development of resistant pathogens. If multiple applications are necessary, alternate or tank-mix effective active ingredients from different FRAC groups.
4. Monitor the efficacy of all fungicides used, and record other factors that may influence fungicide performance and/or disease development.

Is It Really Resistance?

How do you know if you are truly dealing with a fungicide-resistant pathogen population? Several criteria may be used to diagnose the problem correctly:

• The disease was controlled effectively with this fungicide in the past.
• All other causes of fungicide failure have been eliminated (timing, frequency, environmental, or misapplication problems and so on).
• Other diseases on the fungicide label (besides the one in question) were controlled effectively.
• The site has a history of continuous use of the same fungicide or other fungicides within the same FRAC group.

If you answer yes to several or all of these criteria, you should suspect resistance; report it to the fungicide manufacturer representative and to your local University of Illinois Extension office (http://web.extension.uiuc.edu/cie2/offices/findoffice.cfm).

By understanding the fungicides you use and executing a sound fungicide resistance management plan, you can greatly reduce the chances that fungicide-resistant pathogen populations will become a problem.

Sources

1. Locke, Tom, et. al. “Fungicide Resistance.” Fungicide Resistance Action Group – UK. August 2001. <http://www.pesticides.gov.uk/uploadedfiles/Web_Assets/RAGs/FRAG_UK_leaflet.pdf>. Accessed July 15, 2004.
2. Brent, Keith, J. “Fungicide Resistance in Crop Pathogens: How can it be managed?” Fungicide Resistance Action Committee. April 1995. <http://www.frac.info/publications.html>. Accessed July 15, 2004.
3. “FRAC Fungicide List 1 (arranged by FRAC Code).” Fungicide Resistance Action Committee. <http://www.frac.info/publications.html>. Accessed July 15, 2004.