The practical implications of selection pressure

For many decades, systemic fungicides have worked very effectively for broad-spectrum disease control on many important crop diseases. A good, well-timed fungicide treatment is expected to give a moderate to high level of disease control, with efficacy levels in the range of 75% to 95% usually observed and control levels of over 90% have been quite common.

This type of fungicide efficacy is only achievable where the fungicide application is well-timed and applied in good spraying conditions. If there is suboptimal spraying conditions or timing, then lower levels of efficacy will be observed.

Modern systemic fungicides are typically site-specific in their mode of action and they work very effectively by blocking an essential biochemical or enzyme pathway at cell level, which is essential for the completion of the life cycle in the pathogen.

In the past - highly sensitive disease strains!

When a fungicide treatment is applied to a highly sensitive pathogen population, this will generally result in a high level of efficacy, even if the spraying conditions or application timing are less than ideal. This was the general observation in the 1980s and 1990s when very effective landmark fungicides such as Tilt, then Folicur and then Opus were introduced. High efficacy was the norm back then and reduced-rate approaches were often very effective. But this was all based on a very sensitive pathogen population.

The continued use of actives against any natural population will very often result in a gradual slow selection process for less-sensitive pathogen strains over time. The timeline can be years or decades, but it will result in lower fungicide efficacy over time. This process also adds a requirement for higher fungicide doses, plus multi-fungicide mode of actions combined, to achieve a good performance.

This approach has been quite successfully used with triazole and other chemistry in recent decades. There is an ongoing selection process for increased resistance in the fungus population and the final outcome will be a more resistant and less controllable pathogen, but the problem looks manageable.

The resistant mutant

However, another scenario is also possible. Sometimes a new pathogen strain emerges, a highly resistant strain to a particular fungicide family or mode of action. It usually emerges from a sexual reproduction event between mating types in the fungal population.

It is to be expected that the frequency of the mutant in the fungal population is very, very low. Fractions of fractions of 1% is probably where the new strains start from and, in nature, this must also compete with the dominant wild fungal strain.

The opportunity for the mutant to increase in prominence in the population, and to become widespread, is provided by using potent fungicides which provide high levels of control of the sensitive wild or normal pathogen strains. When a fungicide treatment is applied twice in a season and provides 90% control each time, this is equivalent to a 100-fold selection event.

If application of this potent fungicide is repeated in successive seasons, this process can push what was a barely detectable strain at minimal frequency up to 1% to 2% of the fungal population. At this frequency, the resistant strain may still not cause the next fungicide treatment to show reduced efficacy, but it may be the last time the fungicide works.

In the scenario presented, it is likely that the next selection event, and a following one maybe in the next season, may be enough to make the mutant strain dominant and quickly very dominant, with 90% to 100% of the pathogen population. This is what happened with the G143A mutation which took out strobilurin fungicide efficacy on septoria tritici across northwest Europe over a 12-month period from summer 2002 to summer 2003. This was as dramatic as it looks in Figure 1.

This was a high-selection scenario and similar issues have occurred many times across the world in recent decades.

The fear now is that it could be repeated for the SDHI chemistry, also on septoria, especially following the detection of mutant strains, such as the C-H152R strain detected by Teagasc in 2015 and also at UK sites.

Can we minimise this selection pressure?

The only way to minimise the selection pressure is to stop using the chemistry. The next best way is to use SDHis as infrequently as possible – once per season in preferable to twice. However, it’s not that simple for practical reasons. We must continue to use more than one type of chemistry to help with the risk of resistance problems in both SDHI and triazole chemistry. The other approach is to avoid high disease-pressure scenarios and, when you are using the chemistry, to use very well-timed applications and in robust mixtures with other chemistry. For SDHI chemistry, this is a mixture with a high rate of triazole and a contact fungicide.

This does not guarantee to minimise resistance risks, but it does guarantee the best fungicide performance possible, regardless of emerging resistance issues. Also, it is a good approach to reducing the risk of the worst-case resistance scenario and to keep the likelihood of fungicide failure in the field down to a very low level.

Strength becomes weakness

It is an unfortunate scientific fact that the high potency of some modern actives, such as the strength of the strobilurins 12 to 14 years ago and also the leading SDHI chemistry today, becomes their weakness in an emerging resistance scenario. This happens because their high potency can easily translate into a high to very high selection-pressure scenario in the field if the mutant or highly resistant strains are becoming widespread. And this can happen rapidly, even if the resistant types are at a very low frequency in the fungal population to begin with.

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