Definition of fungicide resistance

The term fungicide resistance, as used by FRAC, refers to an acquired, heritable reduction in sensitivity of a fungus to a specific anti-fungal agent (or fungicide). This results in a change in the sensitivity of a fungus to a specific fungicide mode of action which is generally brought about by selection pressure being applied by the specific fungicide mode of action on the fungus in question, in time and space (often due to prolonged and wide spread exposure of the fungus to the specific fungicide mode of action). Such changes may be detected in populations or only in single isolates.
The understanding of the process of selection in itself indicates that resistant isolates of a fungus are present in the natural population (i.e. not having been exposed to the specific fungicide MoA). These isolates develop through natural / random mutations and the specific fungicide mode of action selects them through exposure.
FRAC gives clear differentiation between resistance which has been artificially generated under controlled conditions (in the laboratory) and that which is selected in the field following use of the specific fungicide.
Lab resistance, where a fungus with lower sensitivity has been generated using various methods in the laboratory. These fungicide resistance organisms are predominantly of academic importance. The resistance developed in the lab can differ from that found in the field but their occurrence could be extended to the field in time. Reports on such studies should be clearly stated as being laboratory studies.
Field resistance, refers to changes (in sensitivity) that occur in a specific fungus under natural conditions. The first indications of the possibility of field resistance may be found through routine monitoring. Poor performance on its own is however not evidence of resistance and this has to be shown by scientific studies.
Serious field performance problems occur when the fungal population is altered from being predominantly sensitive to being predominantly resistant. In addition to the frequency of resistance, different mechanisms of resistance are possible, each with a particular resistance factor. Both frequency and resistance factor contribute to the field performance of the fungicide. This change in sensitivity will also largely depend on the ability of the resistant isolates to survive and reproduce (i.e. the general fitness of the resistant isolates).
To manage resistance effectively, scientists study fungicide resistance on many different levels including the cellular, organismal or population / field level.  Reports of field failures assumed to be caused by “resistance" (i.e. where growers observed reduced efficacy of a product that has previously demonstrated efficacy against that particular pathogen) must be confirmed by lab studies on the organisms, thereby excluding other factors which may influence fungicide performance.
The development of fungicide resistance is a natural evolutionary process. This can happen relatively rapidly in fungi as their reproduction rate is relatively high. The fungicide exerts selection pressure on the pathogen population by killing the initial (or wild type) population and not affecting the changed (or mutant) population.  When changes are slightly disadvantageous under normal conditions (i.e. in the absence of the fungicide), the frequency of the changed population may decrease when the selection pressure is removed.  This is termed a fitness penalty.  

Resistance Monitoring

Resistance monitoring is crucial to understanding what changes the population may be undergoing.  Before new active ingredients are launched as new products, a baseline should be established.  A baseline describes the sensitivity of a given collection of isolates to a specific fungicide prior to their being exposed to that fungicide.  It is important to establish validated methods to create the baseline as well as to describe the pathogen population going forward as different methods may result in different baseline sensitivities.   The sensitivity of many fungi can be measured in simple amended agar studies whereas obligate pathogens need to be tested on living plant material (often leaf discs to increase throughput).  During these sensitivity tests, multiple doses of the fungicide are used to determine an EC50 which stands for effective control to 50% (i.e. the dose that provides 50% inhibition of the isolate as compared to a non-fungicide-amended control).  These EC50 values are graphed in a frequency histogram to determine the baseline (Fig. 1).  Adequate sampling of the population is necessary to measure the variability inherent in the population’s sensitivity to the fungicide.

Fig. 1A.  Hypothetical example of a fungicide baseline shown as a frequency histogram.  1B. Frequency histogram showing quantitative or shifting-type resistance.  1C. Frequency histogram demonstrating early detection of qualitative resistance.  In this example, a few isolates, which are circled in red, were detected that fell outside the baseline.  1D. Frequency histogram exemplifying qualitative resistance after significant selection pressure has shifted the population to a much higher mean EC50 value.   

Routine monitoring of the field population may then be carried out by collecting new isolates and comparing their sensitivity to the baseline. If the mean EC50 value of the samples collected is statistically greater than the mean EC50 value of the baseline, then the sensitivity of the population has shifted (Fig. 1B, year 5). If selection pressure continues to be exerted on the population, the mean EC50 value may shift further (Fig. 1B, year 10).  This shifting-type of resistance is called quantitative (or multi-step or continuous) resistance.  On average, these isolates are controlled by slightly higher doses of fungicide in the lab bioassay.  In early stages of quantitative resistance, the fungicide in question may still provide adequate control under field conditions at current use rates.  As the mean EC50 value of the population gets larger, greater doses of the fungicide may be needed to provide control.  Growers should always follow manufacturers’ labels when applying fungicides.

Occasionally isolates are found that have significantly higher EC50 values than the mean of the baseline (Fig. 1C in red circle).  These individuals are considered to be resistant since the dose needed to control these isolates is much higher and may not be practical under conditions of the lab bioassay.   The term qualitative (or disruptive or discreteresistance is used to describe this type of resistance.  The larger doses required to control these isolates are even less likely to be practical under field conditions.  If, or when, these isolates become prevalent in the population, a marked loss of activity will likely be observed by growers.  

Where resistance monitoring is not routine practice, resistance issues are generally first identified when growers observe a marked lack of control from previously efficacious products.  Disease samples are then taken from these fields to confirm a reduction in sensitivity under controlled conditions in the lab.  This confirmation step is crucial as there are many reasons why previously efficacious compounds are not efficacious in any given field (e.g. intense disease pressure, incomplete application coverage, inaccurate dosing or application timing, etc.).  If reduced sensitivity is confirmed, the resistant isolates are likely already prevalent in that local field population as a marked loss of efficacy has already been observed.  Sampling of surrounding areas can help growers understand how widespread the resistant isolates are and can provide some guidance on product usage for the next season to manage the resistance.  Routine monitoring is recommended wherever possible with the hope that scientists can identify resistant individuals before they become widespread in the pathogen population. Good resistance management practices will hopefully keep them at low frequency in the population such that the fungicide of interest will continue to provide good efficacy under field conditions.

For more information on fungicide resistance and resistance monitoring:

FRAC Monograph 1: Fungicide Resistance in Crop Pathogens: How can it be managed?

FRAC Monograph 3: Sensitivity Baselines in Fungicide Resistance Research and Management





Dr. Dietrich Hermann

Syngenta Crop Protection AG
Research and Development
WRO 1008-2-12
CH 4002 Basel, Switzerland

Tel: +41 61 32 30983

Dr. Juergen Derpmann
FRAC Communication and Website Officer

Bayer AG, Crop Science Division

Duncan Mackenzie 2

Duncan McKenzie
Scientific Support Officer