The recent decision by the European Commission not to re-approve chlorothalonil for use, while not unexpected, poses serious questions for the future of cereal disease control in Ireland. Chlorothalonil has been the backbone of all cereal fungicide programmes for almost two decades.

Until 2014 its addition was almost solely based on its role in programmes to protect the more vulnerable chemistries from resistance development and spread. Since then it has played a double role, still providing the anti-resistance required but equally, and sometimes more importantly, providing disease control and yield protection.

Risks remain

It will be available for the 2019 season and most likely the early part of the 2020 season but after that we must find alternatives to take up this role. Fortunately, this will hopefully coincide with the much-anticipated commercialisation of the new azole Revysol, from BASF, and the QiI Inatreq (a new mode of action to cereals) from Corteva.

Looking further into the future, pipelines look healthier than in previous years with an additional QiI, a new strobilurin and new SDHIs expected over the coming four to five years. However, each of these are highly specific in nature and, with the exception of the QiIs (which we expect will be no different), diseases such as septoria and ramularia have demonstrated their ability to overcome these modes of action.

Even though this new generation of actives are clearly a step up on where their predecessors are currently, they will each potentially suffer the same fate, although hopefully not for some time. It is therefore essential to take stock now of where we are with cereal disease control in Ireland and to map out the future for the next five to 10 years. To put it simply, it is now time to readjust how we do things to ensure that we can control diseases, ensuring that we can continue to grow cereals profitably in Ireland.

Multipronged approach

To help address this, it is critical to understand what we are actually trying to do when we apply disease control programmes to cereals. First and foremost, cereals are grown primarily for their grain. The more grain the better. Depending on the crop and end market, specific aspects relating to grain quality will come into play. However, the overall amount of grain produced and its weight are what is critical for producers.

In most instances, the more carbohydrates you can get into the grain the better. These carbohydrates are created in the green tissues of the plant through photosynthesis and from there are translocated or moved up to the grains as they develop and fill. To a point, the more grain sites that are created to be filled the greater the yield.

Understanding how diseases interact with these two yield components is therefore important for how we control diseases. Differences also exist between the different cereals in terms of how the components respond to these different influences and this is equally important in terms of how we treat individual crops.

Differences also exist in the diseases we are trying to control. All are trying to survive from one generation to the next and in doing so are looking for a host that can provide them with both the energy source and environment to do so. Each disease has carved out a niche for itself and will exploit weaknesses in plants to ensure its survival.

Diseases react to environment

The prevalence and importance of the different diseases has changed over the decades, reflecting how arable practices have changed during the same time period. For instance, seedling diseases were often the main concern in the early part of the last century, but the advent of seed treatments changed this. Seed treatments and seed testing ensured crops got beyond the seedling stage, at which point root-based problems such as take-all and eyespot became the dominant problems.

Changes in practices such as increased rotation, fertilisation and the movement of arable cropping from marginal lands reduced the burden of these diseases. However, at the same time improvements in genetics, winter cropping and improved fertilisation provided the ideal conditions for foliar diseases to become the dominant threat from the late 1970s onwards.

The principles of arable production have not overly changed since then and we do not expect them to change dramatically in the foreseeable future, so these diseases will likely continue to be the most important in Irish arable crops.

The management influence

So how can we manage these diseases without adversely affecting yield or creating additional problems with other diseases? Fundamentally, we must approach disease control from an integrated approach and, in doing so, not just focus on a single disease or an aspect of control. This concept is referred to as Integrated Pest Management (IPM).

Essentially, this just means using all available control measures and this relates to good agricultural practice. However, with diseases that are extremely weather dependent, it is reasonable to question what impact agronomic practices, other than apply fungicides, can have in managing the most prevalent diseases facing Irish cereal crops. The question can also be approached by asking what practices could actually make the situation worse. We can then work on these to help decrease disease pressure.

Use genetic resistance

The most basic measure that farmers can do is choose varieties that are either able to resist or tolerate disease. However, the first hurdle we have to get over is that most of the varieties we grow or indeed will grow, whether wheat, barley or oats, are not bred specifically for the Irish environment. Varieties we get are bred for other markets, generally the UK.

While disease resistance is undoubtedly one of the traits that are key to deciding whether a variety is suited to the Irish market, weaknesses in standing power or spouting often remove a proportion of potentially resistant varieties that may appear to exist. That said, substantial improvements have been made in recent years, most notably in barley, with the majority of newer winter and spring barley varieties exhibiting strong resistance to diseases such as rhyncho and mildew.

There are some weak ones still available but there is scope to at least choose varieties for locations where both mildew and rhyncho have traditionally being problematic. These must be exploited. January 2019 was a perfect case in point when discussions arose about whether or not to spray some winter barley crops for mildew.

Clearly spraying fungicide in January for mildew adds an additional cost, never mind what it does to the potential longevity of the fungicides. In these instances the reasons for these high incidences of mildew must be addressed. The most obvious solution will be to change variety to one that does not suffer the same level of mildew susceptibility.

Similarly, improvements in septoria resistance have been made over the past 15 years. Although most of the wheat area is still planted to varieties that require robust fungicide programmes, there has been a general increase in overall resistance ratings, reducing the overall risk of yield loss. Unfortunately, as these varieties become widely grown, the disease adapts to the genetic resistance and we can see an increased vulnerability in the crops.

It can be hard to look beyond what has worked consistently. However, it is vital that agronomists and growers take note of the varieties that are coming and how they might be slotted into their cropping systems. This is equally the case for ramularia on barley. This is a difficult disease to screen for in variety trials and resistance ratings do not always reflect what is seen in fields.

However, subtle differences do exist where ramularia is consistently a problem, such as the higher rainfall areas of the country, and it is essential to get whatever information is available on the ability of a variety to tolerate this disease. While this is difficult to get, crop walks and open days often give some indication for local areas. Varietal resistance is the first line of defence and should be utilised as much as possibly within the limitations that exist.

Agronomic decisions

It can be difficult to manage wet weather diseases in a climate that tends to be wet. However, some of our agronomic practices can make a bad situation slightly better or considerably worse. Ideally, we want the former. For diseases such as septoria we take it almost as a given that Irish wheat crops will have some level of disease. In this instance we want to keep the disease to a minimum before we have to intervene with fungicides if at all possible.

We like to think that we have a decent understanding of septoria’s life cycle. We know initial infections are caused by wind-borne ascopsores. Being wind borne, they can travel quite some distance so the management of stubble has a limited role to play. However, we also know that these ascospores are mostly released in early autumn so the later into autumn we get, the lower the inoculum available.

Variety choice and planting date are set to become increasingly important husbandry decisions in the future.

So it stands to reason that delaying drilling as late as is practically possible will lower the levels of disease that will overwinter in a crop. This does not mean that waiting until December will result in no septoria in the following June. Even low levels of infection can explode into severe epidemics in the following spring if the conditions are favourable from February onwards.

However, it does reduce the risk. An early sown crop will always carry more disease as it is exposed to a greater initial inoculum for a longer period and is therefore at a high risk of an epidemic developing. This also holds for most of the other diseases we do battle with. And while the benefits of stubble management may be limited, clearly for other diseases such as net blotch or even tan spot, which is not prevalent in Ireland, it can have a major role to play.

Our knowledge of ramularia development is much less well refined. What we do know is that periods of stress can have massive impacts on disease development. It is difficult to see what we can do about changes in weather (wet, dry etc) but a crop that is sitting in water for prolonged periods of time is undoubtedly under a lot of stress.

Where ramularia is an issue, consideration must be given to where barley is sown on the farm. A crop that is liable to sit in waterlogged soil for considerable periods will have an increased likelihood of ramularia.

Nutrition

The role of fertiliser or crop nutrition is equally important for all diseases. However, it is difficult to determine how we can manipulate fertilisation without adversely affecting crop yield. Where a crop is under stress due to nutrition it is likely also to suffer more from diseases. Crop nutrition should first and foremost be managed to provide a healthy crop.

Fungicides

We often approach fungicides from the perspective of creating yield. However, fungicides only protect potential yields and should be viewed as an insurance against yield loss due to diseases. If the various steps described to minimise disease are taken then the risks of yield losses will be lower and the overall return on fungicide spend might also be lower. However, this should not be viewed as a bad thing.

But doing all these things does not mean that risks disappear. All it takes is a wet May and June and all the hard work can be quickly undone leaving the insurance provided by fungicides to clearly pay their way. In addition, by reducing the overall risks associated with disease, the fungicides will hopefully have a better chance of managing it, even in the most difficult seasons.