Managing and Preventing Fungicide Resistance

The use of FRAC numbers was created to help determine the mode of action of the varying fungicides. One class of fungicide may have differing active ingredients and be sold under several different trade names. For example, FRAC group 3 is sold as Bumper, Tilt, Elite, Indar, Laredo, and Quash. Regardless of the trade name or active ingredient, any one of these fungicides within a FRAC group kills the fungal pathogen the same way. In order to prevent fungicide resistant pathogens, back-to-back applications of any one FRAC group should be avoided.

Figure 1: Application of a fungicide
to control a foliar pathogen. The blue
dot represents a resistant fungal
colony to fungicide A.

The formation of fungicide resistance by many pathogens is inevitable as it is basically a game of odds. Making an application of one fungicide kills the majority of the fungi in the environment that are susceptible to that particular fungicide. There are always a few colonies of the targeted fungi that “escape,” and have some degree of resistance to the chemistry (lets say odds of 1/10,000) (Figure 1). This may seem surprising, but this is due to the varying genes found within the same species of fungi (It is no different why you catch a cold, and your coworker remains healthy). Making a second application of the same chemistry provides the environment for the “escaped” population to continue to reproduce and colonize healthy tissues (Figure 2). Eventually, the “escaped” population will become the majority of the population (Figure 3), and attempts to control the pathogen by spraying the same fungicide over and over will not occur (Figure 4A).

Figure 2: After fungicide A application,
the resistant blue colony “escapes” and
begins colonizing other healthy
tissues, increasing in population.

Employing resistant strategies such as fungicide rotation can delay resistance formation. By avoiding back-to-back applications of the same FRAC group, the odds of an “escaped” pathogen occurring is reduced (Figure 4). Using our values from above, if the odds of an escaped pathogen with fungicide A is 1/10,000, and with fungicide B is 1/10,000, then the total odds of an escaped pathogen from both fungicides is 1/100,000,000.

Fungicides also vary in their “odds” of resistance formation. Multi-mode of action fungicides, or broad spectrums, target multiple pathways within the fungus, reducing the chance that one fungus has the ability to be resistant to all targeted modes of action. Examples of multi-mode fungicides include copper (FRAC M1), captan (FRAC M4), and chlorothalonil (FRAC M5). These fungicides generally provide moderate to good control of most fungal pathogens. In contrast are the fungicides that target single pathways. Since only one pathway is targetted, the chance of resistance is high. Examples of fungicide classes within this group include the strobilurins (FRAC 11), and DMIs (FRAC 3). These fungicides usually provide very good to excellent control of most fungi.

Figure 3: A second fungicide is applied.

Often, fungicide cost decreases after the product comes off company patent. If the product and its generics are still effective against the diseases present after patent expiration, then the fungicide will become a cheaper option for use. For example, an off-patent generic strobilurin may sell for $8-$12/acre, while a branded trade name strobilurin sells for $25-$35/acre. Also, having more available active chemistries for the targeted fungi leaves more “tools in the toolbox” when developing a disease management program.

Figure 4: Resulting effect on fungal
populations if fungicide A or B is

Recently, there have been a large amount of fungicides entering the market. Most of these “new” fungicides are combinations of chemistries with currently used mode of actions. Examples include Pristine (FRAC 7/11), Luna Sensation (FRAC 7/11), Inspire Super (FRAC 3/9), Distinguish (FRAC 9/11), Quadris Top (FRAC 3/11) and Quilt Xcel (FRAC 3/11). These fungicides contain multiple compounds in attempts to help reduce the chance of resistance. They provide excellent control of bloom time and summer diseases – including alternaria, rust, and scab. Plan to use these strong fungicides for targeting the optimal timing of scab, rust, or alternaria. After applying one of these fungicides, make sure to rotate away from both of the FRAC groups used. If fungi develop resistance to these fungicides, we will have lost some of the best fungicide chemistries currently available.

Please note that this is not an endorsement for any of the trade names listed, nor does the omission of specific trade names reflect the view of the author. Refer to your local chemical dealer or manufacturer for specific fungicide products available

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