Managing the risk of further fungicide resistance

Over the past four decades, fungicides have become an integral input in the production of cereals in Ireland. They are critical to help us battle fungal cereal diseases that thrive in our wet and mild climate. The 1980s and early 1990s were regarded as the golden era for fungicide development, with some of the most widely recognised fungicides developed during this period, most notable of which include a suite of azoles and strobilurins.

Since then, increased costs of development, coupled with increasing regulations on usage, have significantly reduced the flow of new fungicides making their way from the point of innovation through to commercialisation.

This unfortunately restricts the availability of new fungicide molecules and, more importantly, new fungicide families which place greater pressure on those already available as they become increasingly relied upon for disease control.

The targets of these fungicides are fungal pathogens. These undergo numerous lifecycles throughout a growing season, resulting in astronomical numbers of offspring being produced.

In this type of scenario, an over-reliance on a select group of fungicides will inevitably result in the selection for fungicide resistance.

It is therefore critical to look at what the implications of this may be in the short term. For instance, what issues may lie ahead for the 2022 or 2023 seasons?

We must also try to look slightly further ahead. Is there a crunch point when these diseases outpace fungicide development – if so, what then? Undoubtedly, we do not ever want to reach this point and so we need to address how we approach disease control to reduce such a “cliff edge” scenario from ever happening.

What is in front of us for 2022?

After an exceptionally mild winter and spring, it is no surprise that most crops have some level of disease present.

Septoria is visible on the lower leaves of almost all winter wheat crops, with mildew, rust and rhynchosporium evident in winter barley. There is also plenty of mildew and septoria present in winter oats.

While each of these diseases will pose their own challenges and require careful consideration in devising control programmes, it is septoria on winter wheat and ramularia on barley which get the most attention at this time of the season and rightly so.

Septoria has demonstrated an ability to adapt to our control programmes, while the loss of chlorothalonil in 2020 brought ramularia control very much back into focus. It is therefore timely to have a review on the sensitivity status of each disease to the main fungicides now used for their control.

Do older azoles and SDHIs still have a role?

Yes, but septoria and ramularia have developed some level of resistance to both the older azoles and SDHIs. Trials have demonstrated that this has significantly impacted on the levels of efficacy we can expect from these fungicides. For both pathogens, multiple mutations/resistance mechanisms are now present throughout Irish populations.

These actives may play a crucial role against other diseases (eg mildew or yellow rust) and, as such, they may still be the first choice in a programme.

However, it is important to understand that the efficacy they provide against either septoria or ramularia is significantly reduced and this should be reflected in where and how they are used in fungicide programmes.

In reality, it is only possible to build fungicide programmes based on information relating to the disease risk. Is the variety sown known to be highly susceptible to either disease, or indeed other diseases such as yellow rust or mildew? Is where the crop is sown placing it at higher or lower risk?

For example, a crop of barley sown in a field that is known to hold water following heavy rain may increase its exposure to stress and potentially increase its susceptibility to ramularia. Is the forecasted weather conducive to the spread of the disease?

Perhaps the combined risks are going to be high and an intensive fungicide programme is required. However, there is no doubt but that there will be situations where the older azoles and SDHIs, when combined with a multisite, will provide sufficient disease control. Know your crop.

Septoria: protecting new actives

In 2020 and 2021, two new fungicides were registered for the control of septoria on wheat. Revysol from BASF was first in 2020. As an azole, Revysol did not herald the arrival of a new fungicide family, but it brought levels of efficacy against septoria and ramularia not seen from the azoles for some years.

Closely following this in 2021, Inatreq arrived in the market from Corteva to give us the first new fungicide family for cereals since the strobilurins. Belonging to the QiI fungicides (Quinone inside Inhibitors), Inatreq introduced a family of fungicides that until then were restricted in activity to diseases such as late blight.

Like Revysol, levels of efficacy against septoria from Inatreq are on par with the SDHIs prior to the emergence of resistance in the mid-2010s. However, unlike Revysol, Inatreq is only registered for use on wheat.

With two new actives in wheat for septoria control, the question is rightly asked, how we can best use them to maximise their strengths but delay the inevitable emergence of and selection for resistance?

Realistically, as both actives are target-specific and likely to be at risk of resistance developing, they are no different in terms of how we might best protect them. This means pretty much following the same guidance as for almost all other major single-site fungicides.

The evidence from the older azoles and SDHIs is clear and applies equally to the newer actives.

When resistance emerges, the more fungicides we apply – whether as increased doses or increased applications – the quicker resistance spreads and affects field performance.

As the development and commercialisation of new fungicides slows, we cannot afford for this to happen to either Revysol or Inatreq as we depend on them to protect each other. Therefore, the simplest and most effective anti-resistance measures for both is to minimise their exposure by maximising the diversity of fungicides used across the programme.

To preserve their long-term efficacy, it is important to ensure Revysol/Inatreq are only applied once per season on any individual crop and when they are applied they need to be mixed with an additional systemic fungicide and a multisite (eg Folpet).

Reducing reliance on fungicides

Were any of our current chemistries to either suffer further reductions in sensitivity or to suffer complete resistance, such as what happened to the strobilurins, the control of diseases such as septoria and ramularia will become very difficult in wetter seasons. It is therefore essential to ask if there is anything we can do to alleviate the reliance we have on fungicide inputs?

To do this, we must first step back and understand why we are actually applying fungicides.

Like all diseases, cereal diseases such as septoria or ramularia need a number of factors to align at just the right time in what is known as the disease triangle

At its simplest, we can view them as risk management tools and, with this in mind, their application must match the risk posed by the different diseases.

Like all diseases, cereal diseases such as septoria or ramularia need a number of factors to align at just the right time in what is known as the disease triangle. Firstly, we need the presence of the pathogenic agent (the fungus); secondly, there needs to be a susceptible host (wheat or barley depending on the disease) and; thirdly, these must coincide with the correct environmental conditions (wet and mild weather) for the pathogen to infect and develop.

There is very little we can do about the weather, chances are there will be rain during May, June and early July, to drive these diseases and these are the critical periods for grain fill. However, are there things we can do with regard the other corners of the triangle? Can we remove the pathogen? Can we remove or alter the host?

Taking measures to alter either of these are the basic components of integrated pest management (IPM). We must also be realistic however, as manipulating either may also adversely impact on yield or on farm profitability. Getting that sweet spot where all objectives are achieved to our benefit is the difficult part.

Removing/reducing pathogen load

Unlike seed-borne or soil-borne diseases, which can be easily manipulated or managed by removing infected seed or through crop rotation, removing the airborne inoculum responsible for the majority of foliar diseases is not a realistic goal.

However, we do know that – in the case of septoria – delaying sowing in the autumn can reduce the amount of initial infections that may overwinter and this will reduce the inoculum load in the environment coming into spring. But if sowing is delayed too far, the risk of poor weather for planting brings the risk of not getting some crops sown late in the season.

The advice must be to sow as late as is possible in the season for your system and your soils.

Similarly, trials have demonstrated that the manipulation of canopy density, be it through seeding rate or N fertilisation, etc, can either alleviate or exacerbate disease pressures. However, it is really only at the extremes of these canopy ranges that we see these differences having any significant effects on disease levels and, again, these can impact yield and overall margins.

Such husbandry relationships to help reduce ramularia pressure in barley are still being researched, so we are still in the process of teasing them out.

Manipulating the host – resistant varieties

There is no doubt that the simplest and most effective control strategy would appear to be the need to sow resistant varieties. But, let’s be honest, if it was that simple we would be doing it already.

Being located at the western edge of Europe does have its advantage when it comes to crop production, with an abundance of water and temperatures that never really fluctuate too much.

For a variety to be successful in our conditions, it needs to have a complete agronomic package

In addition to disease control, these conditions, unfortunately, also pose serious stresses on our crops including increased lodging, straw breakdown and sprouting, to name but a few.

For a variety to be successful in our conditions, it needs to have a complete agronomic package especially for those traits that struggle to be managed by other means (eg fungicides).

For septoria, there are a range of varieties available that have some exceptional levels of resistance but suffer from some other weaknesses that can render them unsuitable for wide-scale cultivation here in Ireland.

Grain quality is often a factor and the closer to the cliff edge we get in terms of the market specs, the more these factors must come into focus. Therefore, we continuously need to assess how we can incorporate such varieties that have some good and some bad traits into our production systems.

That said, the identification and understanding of resistance and resistant varieties/sources is only in its infancy and there will be some way to go before it is likely to make a tangible difference in our production systems.

Although weather models are increasing in their accuracy, the chances of being able to predict what the weather will be when winter wheat crops have their third last leaf fully out is probably a little bit of a stretch.

Predicting what it might be at flag leaf at this stage in the year is guesswork. And, even if we could, knowing the weather at either of those stages will only determine when you might be able to travel through the crop.

What happens between now and then, and after, in terms of rainfall, temperature, etc, are really what will be key to disease development. However, as highlighted earlier, for a disease to develop, other factors need to align in parallel.

I expect our ability to integrate each of these to predict conditions that are favourable for disease development and hence to indicate overall disease risk is likely to greatly improve in the coming years.

Even with that ability, we will still have to ascertain how best to utilise this information to enable it to guide husbandry practice for farmers in individual crops and fields.