Ramularia leaf spot in barley causes large losses in yield and quality – estimates from Scotland’s Rural College put UK annual losses at over £10m from a £10bn UK barley market. The small spots caused by the disease develop suddenly on upper leaves, stripping away green leaf and subsequently yield and quality.
The disease has been in the headlines this season, as it has developed resistance to the SDHI fungicides and to prothioconazole, which leaves only chlorothalonil (now to be revoked) as the only remaining current tool in the box. So it is timely to think about the disease and what is known about it, and to try to work out how growers can best manage it in 2019.
Research into ramularia
Scotland’s Rural College (SRUC) has led a programme of research into ramularia since it first became a problem in barley crops in the late 1990s and early 2000s. This coincided with the introduction of new systemic fungicides, particularly the strobilurins, which led to a useful extension to green leaf retention in barley and valuable uplifts in yield and quality.
It also coincided with the widespread use of the variety Chariot where ramularia spotting was particularly damaging – in fact, it led to the almost immediate fall of that particular variety. Initial work focused on the field testing of fungicides where a range of products, including the strobilurins, gave excellent control. If only we had realised then that the twin story of varietal weakness and reliance on fungicides would still be our major issue two decades on.
In that time, we’ve done much to unpick the ramularia life story and determine management tools. Initially, it wasn’t even clear if a pathogen was involved and many workers attributed the sudden spotting seen on barley near the end of the season to physiological causes.
Along with other research groups across the globe, the distribution and spread of the problem was traced and it is now known to be caused by the fungal pathogen Ramularia collo-cygni. The name comes from the swan-shaped neck on its distinctive spores, which are now present in all the major barley growing areas in the world. But it is a pathogen like no other.
It isn’t the most aggressive of pathogens, but it has evolved a complicated life cycle and a way of operating which allows it to grow by stealth within the barley host.
It knows to abandon ship when the host dies and to persist in other species when its host is gone. Its final star twist is that it is likely to persist in straw debris in a form that allows it to reproduce sexually.
Many fungal pathogens reproduce clonally, which means that changes in the population are likely to occur fairly slowly, but it is ramularia’s ability to rapidly recombine genes through sexual reproduction that enables it to rapidly evolve and adapt to the tools we have at our disposal to control it.
Present in the seed
By tracking a glow-in-the-dark lab mutant, we have been able to show that it is present in the seed right from the start – carried deep within the young embryo. At this point, it acts almost as an endophyte, growing between the cells and completely below the host barley’s radar. In fact, there is even some thinking that the ramularia and the host have a mutually beneficial relationship at this point.
The ramularia tracks upwards in the plant as it develops, using the moisture between the cell spaces. Surface leaf wetness is one risk factor in this, but there are many other environmental drivers of how well the ramularia moves from the lower to the upper leaves.
Molecular technology tools have enabled us to track the infection and these show its presence in leaves long before symptoms develop – and in the case of lower leaves, we see its presence in leaves that will never show symptoms.
Stress triggers
However, at some point, ramularia will switch suddenly from this benign almost endophytic style of growth to its aggressive and spot-forming alter ego. It is this spot-forming phase that it is a problem for growers. A complex set of chemical signals pass between the ramularia and its host. The ramularia can detect when the host is stressed and in trouble and starts to kill the cells around it and change into its pathogenic and damaging spore-forming phase.
It is when it starts to kill the surrounding cells that visible spots form and the damage to yield and quality occurs. It is often the stress of flowering or the beginning of senescence that tips off the ramularia that it is time to switch into its pathogenic form.
But any stress can have similar effects, so sometimes ramularia spots will form early in the season in response to flooding or drought stress, or deficiencies such as manganese. Bursts of sunshine and extremes of temperature will also stress the crop and can be a factor.
Genome known
We have been able to publish the genome of ramularia, which shows several interesting key features. First is its very near relationship to septoria in wheat – they are the closest of relatives possible. Second is that it has markedly fewer of the genes that allow disease pathogens to degrade cell walls and cause it to be noticed by the host, so it progresses by stealth.
Population studies have shown its huge diversity, even within a short distance in a field, with very few clones present. From this, the likelihood of sexual recombination and its high resistance risk to fungicides was predicted. It was shifted to being categorised as a high-risk pathogen as a result.
Historically, varieties have been weak against the disease. But since ramularia was recognised as a major pathogen, breeders have worked hard to incorporate resistance into breeding stock and improve the crop’s tolerance. This is a long-term game and we started from a very poor base, because the disease had simply not been recognised or incorporated into parent breeding stock.
Some progress has been made – and commercially we’ve certainly moved away from the very weak historic varieties such as Chariot. After a few years of progress, the AHDB levy board in the UK was able to include varietal ratings in spring and winter barley recommended lists, but, unfortunately, because of huge variability between trial sites last year, these have been withdrawn.
This is something that those researchers are trying to fix by re-training site managers and re-calibrating assessment methods. That said, it does illustrate that varietal resistance is not a practical tool for growers for the here and now.
Fungicide breaker
Historically, we have had to rely on fungicides, but this has cost us dearly, as ramularia has developed resistance to all the systemic fungicides including strobilurins, MBCs, SDHIs and prothioconazole, leaving only chlorothalonil as an effective fungicide. However, fungicide responsiveness is patchy globally and it is gratifying that other research groups around the world are keeping in touch and collaborating on latest information.
For example, it appears that in New Zealand they still have sensitivity to azole fungicides, but they have issues with SDHIs. But, given that barley seed is moved around the globe, it would be naive to hope that fungicide resistance would not spread.
Treatments
Fungicides can only work when applied ahead of spot development, so inclusion at T2 treatments (usually booting) is sensible. Inclusion in earlier treatments, ie those applied around the start of stem extension, is more marginal. Because of the many factors that drive ramularia, the link between early fungicide use and what ultimately develops is very weak.
Similarly, we are often asked if seed treatments can help. We have looked at this and we found that some experimental treatments can reduce the amount of ramularia in a seed or young plant in laboratory conditions. But since environment is the main driver of how bad symptoms can become, small reductions at the seedling stage can quickly be obliterated.
Ramularia risk is a function of how well the ramularia can climb the plant and how strongly the spot-forming phase is switched on. So a focus on the actual amount of ramularia seems to be academic.
Similarly, we are often asked if seed testing can help and the answer is pretty much the same – ramularia is present in seed wherever we check for it and even very low levels can go on to produce high levels of symptoms.
Manage stresses
A better approach is likely to be that we accept that ramularia is ever-present in seed and that we act to manage the later stresses that trigger the spot symptoms. Judging which crops are at risk leads us into another difficult area.
There is an increased risk of disease development based on the amount of leaf wetness from emergence up until heading, and there is also a link between environmental stresses such as bursts of heat and sunshine.
While weather factors are outside the grower’s control, all that can be done is to minimise the stresses caused by poor agronomy or harsh chemistry (the morpholines were notorious in this regard). Nutrient deficiencies would be one stress to avoid.
Current work is looking at crop greening agents and biostimulants and it seems likely to yield more misses than hits. But treating crops kindly and minimising known nutrient stresses is likely to be beneficial.
It is also important to recognise that other diseases, such as rhynchosporium and net blotch, are also major stresses to the crop. While fungicides other than chlorothalonil may not help in the control of ramularia any more, they can indirectly help by reducing those other diseases and so stress. Avoid complicated tank mixes though, as they are likely to up the risk of inadvertently stressing the crop.
We’ve worked extensively with other multisites, including folpet. Folpet has shown poor fungicidal activity against ramularia in trials, so its place in a barley programme will be to help manage other diseases or possibly to help with crop greening. Chlorothalonil is the only active where we see direct fungicidal activity in lab and field tests.
Building experience in recognising the disease and the conditions and fields on the farm that show the worst symptoms is useful and will help to target inputs, particularly as chlorothalonil use will soon cease. Building awareness of risk in individual fields is helpful to begin to build a picture of on-farm losses and inform future strategies, so walk crops and watch for symptoms.
SRUC helped the AHDB prepare fact sheets to help with identification and assessment which are available on the AHDB website (https://cereals.ahdb.org.uk/publications/2018/january/11/ramularia-leaf-spot-in-barley.aspx). Leaf wetness may not be the whole story, but it is one risk factor, so if we get a lot of rain, dews or humidity in crops, that increases the risk.
For 2019, having chlorothalonil applied in the booting spray (T2 timing) is absolutely key. Other fungicides in the programme at that timing might be needed to reduce the stress caused by other diseases, but will not help to manage ramularia as such.
For the future, better varieties will come, but we are entering a difficult period now that CTL is set to be removed as a tool.
Professor Fiona Burnett is head of Connect for Impact – SRUC’s knowledge and innovation hub and chair of applied plant pathology at SRUC.
In brief
Ramularia is a relatively recently recognised disease that appears to have very successfully evolved to frustrate many efforts to control it.The life cycle of the disease is now better understood, as are the genetics of the fungus.It has been very successful at developing resistance to all of the single site mode of action fungicides. The loss of chlorothalonil will leave a major gap in our long-term control ability.
Ramularia leaf spot in barley causes large losses in yield and quality – estimates from Scotland’s Rural College put UK annual losses at over £10m from a £10bn UK barley market. The small spots caused by the disease develop suddenly on upper leaves, stripping away green leaf and subsequently yield and quality.
The disease has been in the headlines this season, as it has developed resistance to the SDHI fungicides and to prothioconazole, which leaves only chlorothalonil (now to be revoked) as the only remaining current tool in the box. So it is timely to think about the disease and what is known about it, and to try to work out how growers can best manage it in 2019.
Research into ramularia
Scotland’s Rural College (SRUC) has led a programme of research into ramularia since it first became a problem in barley crops in the late 1990s and early 2000s. This coincided with the introduction of new systemic fungicides, particularly the strobilurins, which led to a useful extension to green leaf retention in barley and valuable uplifts in yield and quality.
It also coincided with the widespread use of the variety Chariot where ramularia spotting was particularly damaging – in fact, it led to the almost immediate fall of that particular variety. Initial work focused on the field testing of fungicides where a range of products, including the strobilurins, gave excellent control. If only we had realised then that the twin story of varietal weakness and reliance on fungicides would still be our major issue two decades on.
In that time, we’ve done much to unpick the ramularia life story and determine management tools. Initially, it wasn’t even clear if a pathogen was involved and many workers attributed the sudden spotting seen on barley near the end of the season to physiological causes.
Along with other research groups across the globe, the distribution and spread of the problem was traced and it is now known to be caused by the fungal pathogen Ramularia collo-cygni. The name comes from the swan-shaped neck on its distinctive spores, which are now present in all the major barley growing areas in the world. But it is a pathogen like no other.
It isn’t the most aggressive of pathogens, but it has evolved a complicated life cycle and a way of operating which allows it to grow by stealth within the barley host.
It knows to abandon ship when the host dies and to persist in other species when its host is gone. Its final star twist is that it is likely to persist in straw debris in a form that allows it to reproduce sexually.
Many fungal pathogens reproduce clonally, which means that changes in the population are likely to occur fairly slowly, but it is ramularia’s ability to rapidly recombine genes through sexual reproduction that enables it to rapidly evolve and adapt to the tools we have at our disposal to control it.
Present in the seed
By tracking a glow-in-the-dark lab mutant, we have been able to show that it is present in the seed right from the start – carried deep within the young embryo. At this point, it acts almost as an endophyte, growing between the cells and completely below the host barley’s radar. In fact, there is even some thinking that the ramularia and the host have a mutually beneficial relationship at this point.
The ramularia tracks upwards in the plant as it develops, using the moisture between the cell spaces. Surface leaf wetness is one risk factor in this, but there are many other environmental drivers of how well the ramularia moves from the lower to the upper leaves.
Molecular technology tools have enabled us to track the infection and these show its presence in leaves long before symptoms develop – and in the case of lower leaves, we see its presence in leaves that will never show symptoms.
Stress triggers
However, at some point, ramularia will switch suddenly from this benign almost endophytic style of growth to its aggressive and spot-forming alter ego. It is this spot-forming phase that it is a problem for growers. A complex set of chemical signals pass between the ramularia and its host. The ramularia can detect when the host is stressed and in trouble and starts to kill the cells around it and change into its pathogenic and damaging spore-forming phase.
It is when it starts to kill the surrounding cells that visible spots form and the damage to yield and quality occurs. It is often the stress of flowering or the beginning of senescence that tips off the ramularia that it is time to switch into its pathogenic form.
But any stress can have similar effects, so sometimes ramularia spots will form early in the season in response to flooding or drought stress, or deficiencies such as manganese. Bursts of sunshine and extremes of temperature will also stress the crop and can be a factor.
Genome known
We have been able to publish the genome of ramularia, which shows several interesting key features. First is its very near relationship to septoria in wheat – they are the closest of relatives possible. Second is that it has markedly fewer of the genes that allow disease pathogens to degrade cell walls and cause it to be noticed by the host, so it progresses by stealth.
Population studies have shown its huge diversity, even within a short distance in a field, with very few clones present. From this, the likelihood of sexual recombination and its high resistance risk to fungicides was predicted. It was shifted to being categorised as a high-risk pathogen as a result.
Historically, varieties have been weak against the disease. But since ramularia was recognised as a major pathogen, breeders have worked hard to incorporate resistance into breeding stock and improve the crop’s tolerance. This is a long-term game and we started from a very poor base, because the disease had simply not been recognised or incorporated into parent breeding stock.
Some progress has been made – and commercially we’ve certainly moved away from the very weak historic varieties such as Chariot. After a few years of progress, the AHDB levy board in the UK was able to include varietal ratings in spring and winter barley recommended lists, but, unfortunately, because of huge variability between trial sites last year, these have been withdrawn.
This is something that those researchers are trying to fix by re-training site managers and re-calibrating assessment methods. That said, it does illustrate that varietal resistance is not a practical tool for growers for the here and now.
Fungicide breaker
Historically, we have had to rely on fungicides, but this has cost us dearly, as ramularia has developed resistance to all the systemic fungicides including strobilurins, MBCs, SDHIs and prothioconazole, leaving only chlorothalonil as an effective fungicide. However, fungicide responsiveness is patchy globally and it is gratifying that other research groups around the world are keeping in touch and collaborating on latest information.
For example, it appears that in New Zealand they still have sensitivity to azole fungicides, but they have issues with SDHIs. But, given that barley seed is moved around the globe, it would be naive to hope that fungicide resistance would not spread.
Treatments
Fungicides can only work when applied ahead of spot development, so inclusion at T2 treatments (usually booting) is sensible. Inclusion in earlier treatments, ie those applied around the start of stem extension, is more marginal. Because of the many factors that drive ramularia, the link between early fungicide use and what ultimately develops is very weak.
Similarly, we are often asked if seed treatments can help. We have looked at this and we found that some experimental treatments can reduce the amount of ramularia in a seed or young plant in laboratory conditions. But since environment is the main driver of how bad symptoms can become, small reductions at the seedling stage can quickly be obliterated.
Ramularia risk is a function of how well the ramularia can climb the plant and how strongly the spot-forming phase is switched on. So a focus on the actual amount of ramularia seems to be academic.
Similarly, we are often asked if seed testing can help and the answer is pretty much the same – ramularia is present in seed wherever we check for it and even very low levels can go on to produce high levels of symptoms.
Manage stresses
A better approach is likely to be that we accept that ramularia is ever-present in seed and that we act to manage the later stresses that trigger the spot symptoms. Judging which crops are at risk leads us into another difficult area.
There is an increased risk of disease development based on the amount of leaf wetness from emergence up until heading, and there is also a link between environmental stresses such as bursts of heat and sunshine.
While weather factors are outside the grower’s control, all that can be done is to minimise the stresses caused by poor agronomy or harsh chemistry (the morpholines were notorious in this regard). Nutrient deficiencies would be one stress to avoid.
Current work is looking at crop greening agents and biostimulants and it seems likely to yield more misses than hits. But treating crops kindly and minimising known nutrient stresses is likely to be beneficial.
It is also important to recognise that other diseases, such as rhynchosporium and net blotch, are also major stresses to the crop. While fungicides other than chlorothalonil may not help in the control of ramularia any more, they can indirectly help by reducing those other diseases and so stress. Avoid complicated tank mixes though, as they are likely to up the risk of inadvertently stressing the crop.
We’ve worked extensively with other multisites, including folpet. Folpet has shown poor fungicidal activity against ramularia in trials, so its place in a barley programme will be to help manage other diseases or possibly to help with crop greening. Chlorothalonil is the only active where we see direct fungicidal activity in lab and field tests.
Building experience in recognising the disease and the conditions and fields on the farm that show the worst symptoms is useful and will help to target inputs, particularly as chlorothalonil use will soon cease. Building awareness of risk in individual fields is helpful to begin to build a picture of on-farm losses and inform future strategies, so walk crops and watch for symptoms.
SRUC helped the AHDB prepare fact sheets to help with identification and assessment which are available on the AHDB website (https://cereals.ahdb.org.uk/publications/2018/january/11/ramularia-leaf-spot-in-barley.aspx). Leaf wetness may not be the whole story, but it is one risk factor, so if we get a lot of rain, dews or humidity in crops, that increases the risk.
For 2019, having chlorothalonil applied in the booting spray (T2 timing) is absolutely key. Other fungicides in the programme at that timing might be needed to reduce the stress caused by other diseases, but will not help to manage ramularia as such.
For the future, better varieties will come, but we are entering a difficult period now that CTL is set to be removed as a tool.
Professor Fiona Burnett is head of Connect for Impact – SRUC’s knowledge and innovation hub and chair of applied plant pathology at SRUC.
In brief
Ramularia is a relatively recently recognised disease that appears to have very successfully evolved to frustrate many efforts to control it.The life cycle of the disease is now better understood, as are the genetics of the fungus.It has been very successful at developing resistance to all of the single site mode of action fungicides. The loss of chlorothalonil will leave a major gap in our long-term control ability. 
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