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). To manage resistance effectively, scientists study fungicide resistance on many different levels including the cellular, organismal or population/field level. Reports of "resistance" from the field (i.e. where growers observed reduced efficacy of a product that has previously demonstrated efficacy against that particular pathogen) must be confirmed by studies at the organismal level showing a reduction in sensitivity of the fungal isolate(s) to the specific fungicide. Some scientists use the terms reduced sensitivity or tolerance when referring to smaller reductions in sensitivity which may have little to no impact on fungicide usage in the field, and save the term "resistance" for large reductions in sensitivity of individual isolates which are likely to affect efficacy of a specific fungicide under field conditions if the resistant isolates become widespread in the pathogen population. The term field resistance may also be used to indicate this loss of control under field conditions.
The development of fungicide resistance is an evolutionary process. Fungi, like other organisms, are constantly changing. Occasionally, under certain conditions, these changes provide an advantage or disadvantage in terms of the progeny’s ability to survive and reproduce yet another generation. Advantageous changes allow the individual containing the change to survive and reproduce at greater rates such that their progeny will be a greater percentage of the population over subsequent generations. This can happen relatively rapidly in fungi as their reproductive frequency (i.e. the number of progeny produced from a single individual and the rapidity with which they complete their life cycle is fast) is high. For example, a single Phytophthora infestans lesion can produce thousands of spores and a spore can produce a new sporulating lesion in 3-5 days. The change may be evolutionarily neutral, or even slightly disadvantageous, under most conditions and only be advantageous when certain factors are present. This is the case with fungicide resistance. In most cases of fungicide resistance, the change leading to reduced sensitivity is evolutionarily neutral except when the specific fungicide is applied. The fungicide is exerting selection pressure on the pathogen population since it is killing the initial (or wild type) population but does not kill 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 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 discrete) resistance 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