This guide addresses management of pesticide resistance and describes the Fungicide Resistance Action Committee's (FRAC) classification of fungicides and bactericides registered for agricultural and non-agricultural use in Florida by their modes of action. A cross-reference of common names for active ingredients, along with corresponding examples of their trade names, is also provided.
Fungicide-resistant plant pathogens are not new. Although the first confirmation of fungicide resistance was in 1960, there were few subsequent occurrences until 1970. Since then, fungicide resistance has not been infrequent, especially with the introduction of systemic fungicides. There has been concern that the time it takes for resistance to emerge has been shortening. Sometimes resistance occurs within two years of a new commercial fungicide introduction. The development of rapid resistance is related to the increased specificity of targets and selection pressure on the target population.
Resistance problems are not unique to plant pathogens. Insecticide-resistant insects, herbicide-resistant weeds, and antibiotic-resistant bacteria are well-documented. These resistant pests have two common traits: 1) they have exceptionally large populations and 2) a rapid rate of reproduction. Weeds were the last category of pests to show resistance because there is a very large seed bank in the soil with many individuals that have limited pesticide exposure. In contrast, insects reproduce with multiple generations in a single year, and some bacteria reproduce several times in a single hour. In both cases, there is less of a population reservoir with these organisms.
Where large populations exist, great genetic diversity exists in the population. In these large populations, several individuals will be tolerant of chemical-control measures, perhaps only one in a million or one in a billion. Pests typically become resistant when the same pesticide is used repeatedly within a single year or for several consecutive years. Some individuals believed that selection pressure forced pests to mutate, but this view has been largely discounted through research. However, more accepted reasons for resistance are as follows:
There was always a small resistant population present from natural variation.
When a pesticide is applied, the susceptible population is controlled, but the smaller, resistant population becomes a larger proportion of the population that re-infests the site.
Cross-Resistance versus Multiple-Resistance
Agricultural producers in Florida have the option of using more than 50 different fungicide active ingredients and many more trade products, including mixtures. Many of these active ingredients work in the same way; in other words, they have the same mode of action. Despite the numerous trade products available, these fungicides affect fewer than 20 growth mechanisms.
A plant pathogen is cross-resistant when it cannot be controlled by fungicides affecting the same growth process. An example of a plant pathogen that has cross-resistance is one resistant to fungicides in the triazoles or pyrimidines chemical groups. Both of these are demethylation inhibitors (DMI) that disrupt sterol synthesis.
A more serious concern is multiple-resistance. This phenomenon occurs when a plant pathogen is not controlled by fungicides that affect different growth processes. For example, if a plant pathogen is resistant to fungicides that inhibit both mitosis and protein synthesis, two different fungal growth processes, it would be labeled as having multiple-resistance.
Farmers and crop advisors need to know which fungicides are best suited to combat resistant plant pathogens. Some manufacturers use the FRAC numerical classification of fungicides (listed in Table 1) to support the use of fungicides in a manner suitable for resistance management. The fungicides are classified according to their modes of action, collective and chemical group names, and active ingredient common names. Some examples of popular trade names are provided in Table 2 as a cross-reference.
The tables only include fungicides available in Florida. Those with an intrinsic "high risk" of resistance selection are identified. "High risk" is determined by the following:
The product activity persists over time, or there is the practice of repetitive use or sustained treatments.
There is only one target for the mode of action.
Only one gene needs to be mutated for a fungus to be no longer susceptible.
The product is easily metabolized by the target fungus.
There is cross-resistance with existing fungicides.
There is high genetic variability within the target plant pathogen population.
The target plant pathogens are known to have large populations with rapid multiplication.
The target plant pathogen multiplies rapidly with frequent generations.
Some target organisms possess the ability to metabolize products.
There are no harmful effects on resistant individuals.
FRAC encourages fungicide registrants to use this system to indicate the mode-of-action group in a uniform location on their product labels. Some registrants identify the mode-of-action group on the front panel of their product labels.
Similar systems have been proposed and encouraged for herbicides and insecticides.
Agricultural producers can have difficulty keeping track of which modes of action they use because of the great variety of trade names and package mixtures of fungicides.
Fungicide Resistance Action Committee (FRAC). Accessed September 2015. http://www.frac.info/
OEPP/EPPO. 2002. "EPPO Standard PP 1/213(1) Revision. Resistance Risk Analysis." Bulletin OEPP/EPPO Bulletin 33:37–63.
Rogers, M.E., and M.M. Dewdney. 2012. 2012 Florida Citrus Pest Management Guide: Pesticide Resistance and Resistance Management. ENY-624. Gainesville, FL: University of Florida Institute of Food and Agricultural Sciences. https://edis.ifas.ufl.edu/cg026
Tomlin, C.D.S., ed. 2003. The Pesticide Manual: A World Compendium, 13th edition. The British Crop Protection Council.
FRAC's classification of fungicides registered for use in Florida by FRAC numerical code, mode of action, target site of action, and active ingredient common name with resistance risk indication.
Cross-listing of active ingredient common names with trade products registered for use in Florida.