Fungicides have become an integral tool in the production of Irish cereals.
The development of resistance to these fungicides is a threat that continually hangs over their effectiveness. It is therefore critical to understand the reasons for their usage, measures we can take to reduce the number of target diseases and, equally, whether there are things we can do to reduce the risk of resistance development when we use them.
Since our ancestors started collecting and cultivating seeds for their own use, crop diseases have been an ever-present problem. As our means of managing diseases has changed over the decades, so too has the prevalence or importance of the key diseases we’ve had to tackle. Whilst key diseases such as the mildews and rusts have been ever-present in the early 20th century, seed-borne diseases such as bunts and smuts were often the most critical for cereals.
Certifying seeds to reduce
disease
The development of a seed certification scheme made huge strides in reducing the incidences of these diseases. This development coincided with the first fungicide seed treatments, which were mercury-based compounds. The combination of both meant that farmers now had confidence that the majority of the seeds they planted would germinate and emerge. Of course, this led to the proliferation of other diseases, which then thrived on these plants.
Again, while mildew and rust were still a major presence – as their impact is often seen later in the crop’s life – diseases that limited crop development were often more critical to combat, with soil- and stem-based diseases now often the most critical.
Advances in agronomy
Advances in agronomy (including rotation, fertilisation and breeding) meant that, in addition to having the confidence that a crop would establish once planted, farmers also had the confidence that, if planted in autumn, the crop would overwinter well, could be provided with sufficient nutrition to maximise yield, and would stay upright until the end of the season. These advances have been critical to the development of different crops as we now know them.
However, such crops are an ideal breeding ground for foliar diseases. This is where the mildews and rusts would presumably dominate, which in many incidences they did – they are now the key diseases we battle in oats. But they aren’t the only ones.
In wheat, septoria is now our major disease threat, whilst in barley, net blotch, rhynchosporium and (most recently) ramularia are all equally important. Through developments in breeding, mildew has become a lesser threat for wheat and barley, with all recommended spring barley varieties now containing the MLO-based resistance that provides excellent levels of protection to-date against the disease.
Similarly, genetic-based resistance to yellow rust and brown rust in wheat is a key means of its control. Although it needs careful monitoring and management, particularly with ensuring initial risk is limited by limiting the cultivation of susceptible varieties, its overall impact on yield can be kept to a minimum without the need for intensive fungicide interventions. Unfortunately, for those other foliar-based cereal diseases mentioned above, fungicides remain an essential tool in their management.
As foliar diseases, they infect the foliage or upper parts of the stem, which can include the stem, leaves and ears. In doing so, they limit the capacity of the crop to achieve its potential yield. In most instances, they do this by reducing the crop’s ability to capture available sunlight through causing the premature death of its upper-canopy or leaves as the crop is filling the grains that have been set. In the case of barley, the early development of either net blotch or rhynchosporium during tillering can potentially limit the number of grains set, either indirectly, by limiting the number of tillers produced, or directly, by limiting the number of grain sites per ear.
Understanding crop growth to control disease
By understanding the epidemiology (or life cycles) of these organisms that cause diseases, we can try to adapt how we grow crops to limit disease development. For those diseases that are only seed-borne (eg, smuts), we can very successfully limit their development and impact by preventing the planting of infected seeds and/or ensuring seed treatments are applied. Similarly for soil-borne diseases, we can diversify the system through rotation to both reduce the overall risk of disease severely impacting the entire system and also limit the continual build-up of disease in the soil, prolonging the cultivation of the crop. For foliar diseases, this tends to be a bit more difficult. They often only survive between seasons in stubble, trash, volunteers or even alternative hosts. Wind-dispersed spores are then released, often over a prolonged period of time, including from the harvest of one crop up to and beyond the emergence of the next. This limits the capacity to restrict the disease’s development by simply treating the seed or rotating a crop.
Septoria tritici has become less sensitive to many fungicide actives over recent years.
The source of ramularia in crops each season is believed to be a combination of infected seed, stubble and trash from the previous season’s crop, and wild grasses found in surrounding fields. Simply put, it may be extremely difficult, if not impossible, to completely limit the source of infections or inoculum for some of the foliar diseases we now battle.
For some, we can limit the amount of inoculum with agronomic practices, which form the basis of the prevention or suppression of the disease, and are the most basic measure of integrated pest management (IPM).
Delaying the sowing of winter wheat will reduce the levels of septoria over winter, which could potentially reduce epidemics that occur with grain filling. Seed treatments can limit seed-borne infections of diseases, such as net blotch or rhynchosporium, if present in the seed, impacts the crop early on (there’s no evidence that they can limit ramularia infection later in the season).
Controlling disease where
varieties have low levels
of resistance
In the absence of high-level varietal resistance, such as is available for mildew or yellow rust, the control of major diseases in cereals becomes heavily reliant on the application of fungicides. These fungicides primarily protect the crop from infection or limit the development of the disease in crops already infected.
To maximise their effectiveness, it is important to ensure they are applied at crop growth stages prior to periods of infection. In barley, this depends very much on the disease, with protection to rhynchosporium and net blotch potentially required from late tillering, whilst with ramularia protection is needed prior to the ear fully emerging. In wheat, even though we can find septoria in crops from the seedling stage onwards, its impact on yield is during grain filling, when it reduces the area of the upper-canopy available for photosynthesis. The role of fungicides against septoria is to protect the upper-canopy from infection, so their application should be to these leaves.
What fungicides should we use?
The main fungicides we use to control these key diseases fall within a small group of fungicide families. These can be roughly categorised as the single-site chemistries, including the strobilurins, azoles, SDHIs and, most recently, QiIs and the multisites to which folpet and sulphur belong.
The classification of each as ‘single’ or ‘multisite’ indicates the resistance development risk, with those classified as single-site chemistries often being at greater risk. The single-site chemistries tend to be more dynamic, eg providing greater levels of efficacy and flexibility of timing. This is partly due to the nature of the molecules and their inhibitory spectrum. Risk of resistance isn’t just attributed to the fungicide; it’s important to understand it’s a relationship with the specific disease targeted.
Yellow rust has the potential to reduce final crop yields by up to 50%.
For the main foliar diseases, we now battle the specific biological and epidemiological nature of each to lead to each being at medium to high risk of resistance developing towards the different single-site chemistries. This is in part due to the large population size of each and their mixed reproduction status (both sexual and asexual), which when combined mean a large reservoir of strains exist from which resistances can emerge, and once present is easily selected across local to regional scales. Understanding these risks is essential if we are to develop anti-resistance strategies that are to be successful in prolonging the effective lifespans of the different fungicide families.
Anti-resistance strategies
At their most basic level, anti-resistance strategies aim to limit the exposure of fungicides to the pathogen. We often think they relate solely to what we put into or leave out of the spray tank, but any measure that reduces the need to intensively apply fungicides will reduce the exposure of the pathogen to the fungicide, and so reduce the rate of potential resistance development and selection.
From this, we must think of the different agronomic decisions that are made in advance of fungicides being applied. This includes the choice of variety, sowing date, rotation and nutrition. Each component in combination with the prevailing weather conditions will determine overall disease pressures and the intensity of fungicide required, eg early sowing of a susceptible variety will require more fungicides than a resistant variety. Accepting that attention has been paid to each to minimise disease pressures and that fungicides are still required to protect potential yields, decisions on how we use fungicides can either alleviate or accelerate resistance development and spread.
To limit exposure, we can look at three major aspects: doses of fungicide applied, mixing or alternating fungicides, and number of applications. Each influences how much contact the disease has with the fungicide. We must also be aware that the fungicide must come in contact with the disease for it to actually provide control. It’s almost a case of accepting that when we apply a fungicide, we run the risk of resistance developing, although we are trying to reduce the chances of this happening.
Reducing doses applied
Contrary to the view that high rates are needed to prevent resistance, in most of the cereal diseases we tackle, it is the opposite – lower rates reduce selection for resistance. This is because pathogens causing the diseases have huge population sizes, are highly mobile and have demonstrated (with the death of many fungicides) that they are highly adaptable.
Two to three fungicide applications to the upper-canopy will never be sufficient to control the entire population, irrespective of how good the fungicide is. The different strains of the disease are in a battle with each other for resources (leaf-space). By applying fungicides, we tilt this battle in the favour of the resistant strain, its competition. The longer the duration of the control given by a fungicide, which is directly related to dosage, the more this balance is tilted. Of course, we still have to achieve disease-control, but the rate applied must reflect the expected disease pressures that are built into the system from all aspects of IPM.
Mixing and/or alternating
fungicides
Similar to dosage, the aim of mixing and alternating fungicides is to reduce the overall exposure of individual actives to the pathogen. In the case of alternating, this is simple, as the same fungicide or fungicide family are not continually applied. The pathogen is exposed to different fungicides. With mixing, this means that individual components of the mix are not exposed. If one fungicide doesn’t control it, the other will and, hence, will reduce the selection for resistance.
However, this is all based on the premise that the different fungicides in the mix are equally active with a similar duration of activity. Also, at least one of the components should ideally have a lower risk of resistance, eg, a multisite. In reality, it is hard to achieve everything, however, we should where possible think about alternating and mixing chemistries simultaneously across the fungicide programme.
Number of treatments
This is a combination of dose-rate and alternating. The more times we apply a specific fungicide, the more the pathogen is exposed to that fungicide. When alternating, we are effectively limiting the number of applications, yet by restricting the treatment number, we also restrict the total dose that can be applied.
Again, in reality it may be difficult to restrict specific actives, given the diversity of diseases that can be experienced by an individual crop, eg, azoles provide a broad spectrum of control.
However, if this is the case, careful consideration must be given to the impacts this may have on the wider programme and future measures adapted to limit this from happening.
Fungicides have become an integral tool in the production of Irish cereals.
The development of resistance to these fungicides is a threat that continually hangs over their effectiveness. It is therefore critical to understand the reasons for their usage, measures we can take to reduce the number of target diseases and, equally, whether there are things we can do to reduce the risk of resistance development when we use them.
Since our ancestors started collecting and cultivating seeds for their own use, crop diseases have been an ever-present problem. As our means of managing diseases has changed over the decades, so too has the prevalence or importance of the key diseases we’ve had to tackle. Whilst key diseases such as the mildews and rusts have been ever-present in the early 20th century, seed-borne diseases such as bunts and smuts were often the most critical for cereals.
Certifying seeds to reduce
disease
The development of a seed certification scheme made huge strides in reducing the incidences of these diseases. This development coincided with the first fungicide seed treatments, which were mercury-based compounds. The combination of both meant that farmers now had confidence that the majority of the seeds they planted would germinate and emerge. Of course, this led to the proliferation of other diseases, which then thrived on these plants.
Again, while mildew and rust were still a major presence – as their impact is often seen later in the crop’s life – diseases that limited crop development were often more critical to combat, with soil- and stem-based diseases now often the most critical.
Advances in agronomy
Advances in agronomy (including rotation, fertilisation and breeding) meant that, in addition to having the confidence that a crop would establish once planted, farmers also had the confidence that, if planted in autumn, the crop would overwinter well, could be provided with sufficient nutrition to maximise yield, and would stay upright until the end of the season. These advances have been critical to the development of different crops as we now know them.
However, such crops are an ideal breeding ground for foliar diseases. This is where the mildews and rusts would presumably dominate, which in many incidences they did – they are now the key diseases we battle in oats. But they aren’t the only ones.
In wheat, septoria is now our major disease threat, whilst in barley, net blotch, rhynchosporium and (most recently) ramularia are all equally important. Through developments in breeding, mildew has become a lesser threat for wheat and barley, with all recommended spring barley varieties now containing the MLO-based resistance that provides excellent levels of protection to-date against the disease.
Similarly, genetic-based resistance to yellow rust and brown rust in wheat is a key means of its control. Although it needs careful monitoring and management, particularly with ensuring initial risk is limited by limiting the cultivation of susceptible varieties, its overall impact on yield can be kept to a minimum without the need for intensive fungicide interventions. Unfortunately, for those other foliar-based cereal diseases mentioned above, fungicides remain an essential tool in their management.
As foliar diseases, they infect the foliage or upper parts of the stem, which can include the stem, leaves and ears. In doing so, they limit the capacity of the crop to achieve its potential yield. In most instances, they do this by reducing the crop’s ability to capture available sunlight through causing the premature death of its upper-canopy or leaves as the crop is filling the grains that have been set. In the case of barley, the early development of either net blotch or rhynchosporium during tillering can potentially limit the number of grains set, either indirectly, by limiting the number of tillers produced, or directly, by limiting the number of grain sites per ear.
Understanding crop growth to control disease
By understanding the epidemiology (or life cycles) of these organisms that cause diseases, we can try to adapt how we grow crops to limit disease development. For those diseases that are only seed-borne (eg, smuts), we can very successfully limit their development and impact by preventing the planting of infected seeds and/or ensuring seed treatments are applied. Similarly for soil-borne diseases, we can diversify the system through rotation to both reduce the overall risk of disease severely impacting the entire system and also limit the continual build-up of disease in the soil, prolonging the cultivation of the crop. For foliar diseases, this tends to be a bit more difficult. They often only survive between seasons in stubble, trash, volunteers or even alternative hosts. Wind-dispersed spores are then released, often over a prolonged period of time, including from the harvest of one crop up to and beyond the emergence of the next. This limits the capacity to restrict the disease’s development by simply treating the seed or rotating a crop.
Septoria tritici has become less sensitive to many fungicide actives over recent years.
The source of ramularia in crops each season is believed to be a combination of infected seed, stubble and trash from the previous season’s crop, and wild grasses found in surrounding fields. Simply put, it may be extremely difficult, if not impossible, to completely limit the source of infections or inoculum for some of the foliar diseases we now battle.
For some, we can limit the amount of inoculum with agronomic practices, which form the basis of the prevention or suppression of the disease, and are the most basic measure of integrated pest management (IPM).
Delaying the sowing of winter wheat will reduce the levels of septoria over winter, which could potentially reduce epidemics that occur with grain filling. Seed treatments can limit seed-borne infections of diseases, such as net blotch or rhynchosporium, if present in the seed, impacts the crop early on (there’s no evidence that they can limit ramularia infection later in the season).
Controlling disease where
varieties have low levels
of resistance
In the absence of high-level varietal resistance, such as is available for mildew or yellow rust, the control of major diseases in cereals becomes heavily reliant on the application of fungicides. These fungicides primarily protect the crop from infection or limit the development of the disease in crops already infected.
To maximise their effectiveness, it is important to ensure they are applied at crop growth stages prior to periods of infection. In barley, this depends very much on the disease, with protection to rhynchosporium and net blotch potentially required from late tillering, whilst with ramularia protection is needed prior to the ear fully emerging. In wheat, even though we can find septoria in crops from the seedling stage onwards, its impact on yield is during grain filling, when it reduces the area of the upper-canopy available for photosynthesis. The role of fungicides against septoria is to protect the upper-canopy from infection, so their application should be to these leaves.
What fungicides should we use?
The main fungicides we use to control these key diseases fall within a small group of fungicide families. These can be roughly categorised as the single-site chemistries, including the strobilurins, azoles, SDHIs and, most recently, QiIs and the multisites to which folpet and sulphur belong.
The classification of each as ‘single’ or ‘multisite’ indicates the resistance development risk, with those classified as single-site chemistries often being at greater risk. The single-site chemistries tend to be more dynamic, eg providing greater levels of efficacy and flexibility of timing. This is partly due to the nature of the molecules and their inhibitory spectrum. Risk of resistance isn’t just attributed to the fungicide; it’s important to understand it’s a relationship with the specific disease targeted.
Yellow rust has the potential to reduce final crop yields by up to 50%.
For the main foliar diseases, we now battle the specific biological and epidemiological nature of each to lead to each being at medium to high risk of resistance developing towards the different single-site chemistries. This is in part due to the large population size of each and their mixed reproduction status (both sexual and asexual), which when combined mean a large reservoir of strains exist from which resistances can emerge, and once present is easily selected across local to regional scales. Understanding these risks is essential if we are to develop anti-resistance strategies that are to be successful in prolonging the effective lifespans of the different fungicide families.
Anti-resistance strategies
At their most basic level, anti-resistance strategies aim to limit the exposure of fungicides to the pathogen. We often think they relate solely to what we put into or leave out of the spray tank, but any measure that reduces the need to intensively apply fungicides will reduce the exposure of the pathogen to the fungicide, and so reduce the rate of potential resistance development and selection.
From this, we must think of the different agronomic decisions that are made in advance of fungicides being applied. This includes the choice of variety, sowing date, rotation and nutrition. Each component in combination with the prevailing weather conditions will determine overall disease pressures and the intensity of fungicide required, eg early sowing of a susceptible variety will require more fungicides than a resistant variety. Accepting that attention has been paid to each to minimise disease pressures and that fungicides are still required to protect potential yields, decisions on how we use fungicides can either alleviate or accelerate resistance development and spread.
To limit exposure, we can look at three major aspects: doses of fungicide applied, mixing or alternating fungicides, and number of applications. Each influences how much contact the disease has with the fungicide. We must also be aware that the fungicide must come in contact with the disease for it to actually provide control. It’s almost a case of accepting that when we apply a fungicide, we run the risk of resistance developing, although we are trying to reduce the chances of this happening.
Reducing doses applied
Contrary to the view that high rates are needed to prevent resistance, in most of the cereal diseases we tackle, it is the opposite – lower rates reduce selection for resistance. This is because pathogens causing the diseases have huge population sizes, are highly mobile and have demonstrated (with the death of many fungicides) that they are highly adaptable.
Two to three fungicide applications to the upper-canopy will never be sufficient to control the entire population, irrespective of how good the fungicide is. The different strains of the disease are in a battle with each other for resources (leaf-space). By applying fungicides, we tilt this battle in the favour of the resistant strain, its competition. The longer the duration of the control given by a fungicide, which is directly related to dosage, the more this balance is tilted. Of course, we still have to achieve disease-control, but the rate applied must reflect the expected disease pressures that are built into the system from all aspects of IPM.
Mixing and/or alternating
fungicides
Similar to dosage, the aim of mixing and alternating fungicides is to reduce the overall exposure of individual actives to the pathogen. In the case of alternating, this is simple, as the same fungicide or fungicide family are not continually applied. The pathogen is exposed to different fungicides. With mixing, this means that individual components of the mix are not exposed. If one fungicide doesn’t control it, the other will and, hence, will reduce the selection for resistance.
However, this is all based on the premise that the different fungicides in the mix are equally active with a similar duration of activity. Also, at least one of the components should ideally have a lower risk of resistance, eg, a multisite. In reality, it is hard to achieve everything, however, we should where possible think about alternating and mixing chemistries simultaneously across the fungicide programme.
Number of treatments
This is a combination of dose-rate and alternating. The more times we apply a specific fungicide, the more the pathogen is exposed to that fungicide. When alternating, we are effectively limiting the number of applications, yet by restricting the treatment number, we also restrict the total dose that can be applied.
Again, in reality it may be difficult to restrict specific actives, given the diversity of diseases that can be experienced by an individual crop, eg, azoles provide a broad spectrum of control.
However, if this is the case, careful consideration must be given to the impacts this may have on the wider programme and future measures adapted to limit this from happening.
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