Fungicides are effective tools in a plant disease management program. They halt or inhibit infection, growth and/or reproduction of the target fungal pathogens and can be effective in preventing or minimizing the incidence of target plant diseases. Commercial fungicides contain one or more active ingredients each with a specific mode of action. Therefore, selecting the appropriate fungicide for the target plant pathogen, application time and method for full coverage is critical for disease control.
Fungicides are categorized as either protectant or systemic depending on their capacity to be absorbed by the plant tissue. Protectant fungicides provide a protective barrier on the surface of the plant tissue to prevent infection by fungal pathogens, so they are active only on the surface of plants, are susceptible to weathering, and need to be applied frequently to cover new growth. Systemic fungicides are absorbed into the plant tissue. They can move a short distance within the tissue or reach the vascular system and translocated to other parts of the plant. Therefore are less susceptible to weathering. Nonetheless, fungicides are preferable applied before favorable conditions for infection occur to prevent infection and disease development.
Fungicides inhibit fungal growth by interfering with critical pathogen metabolic processes. Their active ingredients have a particular mode of action, which refers to the specific cellular process inhibited. Those fungicides that disrupt cellular functions in multiple places or different metabolic processes in the biology of the fungal pathogen are multisite inhibiting fungicides or have broad-spectrum activity. Fungicides that disrupt only one single site or cellular function are site-specific or have narrow- spectrum activity. Site-specific fungicides are at higher risk of the pathogens to become resistant to it because pathogens are more likely to overcome single site inhibition than multisite inhibition. Most of the new and/or systemic fungicides are site-specific.
Fungicide resistance has become a serious widespread problem in plant disease management. Fungicides become less effective when the pathogen becomes less sensitivity to the fungicide and disease develops even when applied at recommended rates, which were effective previously to the original sensitive pathogen population. Because of the genetic variability in wild-type pathogen population, the probability that the fungicide does not affect a few individuals (strain) is high. With repeated applications of the same fungicide, these individuals will survive and continue reproducing until the resistant strain dominates the pathogen population. Therefore, fungicide resistance in a pathogen population becomes important when the fungicide-resistant population outnumber the fungicide-sensitive population. Then, resistance may become a stable, inheritable adaptation of the pathogen population. Therefore, fungicide resistance is a fungus acquired heritable reduction in sensitivity to a specific active ingredient (or fungicide) and it means reduced or no disease control after a correct application.
Inappropriate use of fungicides with the same mode of action can lead to resistance. Repeated or incorrect use of a fungicide promotes the buildup of the resistant population. Resistance may occur gradually or suddenly depending on how much and for how long fungicides with similar mode of action are used. Therefore, the fungicide mode of action, pathogen genetics and cultural practices influence the development and how fast the fungicide resistance appears within the pathogen population.
The Fungicide Resistance Action Committee (FRAC) is a Specialist Technical Group of CropLife International (CLI; Formerly Global Crop Protection Federation, GCPF) that provide fungicide resistance management guidelines to prolong the effectiveness of "at risk" fungicides and to limit crop losses should resistance occur. Among their main aims are to identify existing and potential fungicide resistance problems and provide guidelines and advice on the use of fungicides to reduce the risk of resistance developing, and to manage it should it occur.
Fungal pathogens that become resistant to one fungicide (active ingredient) likely are resistant to other fungicides with the same or similar mode of action (cross-resistance). Consequently, FRAC developed fungicide group code numbers, referred to as FRAC codes, to educate and facilitate management of pathogen resistance. Fungicides with the same FRAC code have a similar mode of action and could exhibit cross-resistance. All fungicides with one or more active ingredients list the FRAC code(s) for their active ingredients in the front of the label. FRAC updates and publishes the full list of the codes for all fungicide common names (active ingredients), their modes of action and the risk level (low, medium or high) for fungicide resistance development annually and it can be found at https://www.frac.info/.
The FRAC code number on product labels was assigned primarily according to the time of product introduction to the market. Then, letters and numbers were assigned to distinguish the fungicide groups according to their mode of action and cross-resistance behavior. The mode of action code to classify fungicides consists of two parts. A letter (A, B, etc.), which refers to the mode of action in a pathogen's biology and a number, which refers to specific biochemical target sites. The mode of action grouping is according to processes in the metabolism starting from nucleic acids synthesis (A) to other secondary metabolic processes (B to I), e.g. respiration (C). Additional groups include host plant defense inducers (P), those with an unknown mode of action and unknown resistance risk (U), and fungicides with multi-site inhibitors (M). Fungicidal products of biological origin are grouped according to the main mode of action within the respective pathway categories. A more recently introduced category is "Biologicals with multiple modes of action" (BM), which are of biological origin. If available, the biochemical mode of action is given.
FRAC provides information on the mechanism of resistance and the resistance risk. There is increasing evidence that the degree of cross-resistance can differ between group members and pathogen species or even within species. For the latest information on resistance and cross resistance status of a pathogen/fungicide combination, it is advised to contact local FRAC representatives or product manufacturer's representatives.
A resistance management program should integrate resistant varieties, good cultural practices and appropriate use of fungicides. Take in consideration that the strategy to manage fungicide resistance focus at slowing down the development of resistant pathogen populations. Therefore, resistance management plans must be implemented before resistance becomes a problem with at-risk fungicides available for a particular use. The resistance management program is to minimize the use of at-risk fungicides without compromising disease control.
Although specific strategies vary depending on the fungicide FRAC code, the target pathogen and crop, the general approach is similar:
The following practices should be used when fungicides are necessary:
REVISED: September 11, 2020