Potato late blight continues to be the greatest disease threat to potato production in Ireland and elsewhere. Control generally requires a season-long effort, with any lapse in control often exploited by the disease.
However, our understanding of late blight continues to evolve and so too should our actions to control the disease.
In 2008, the Phytophthora infestans strain dubbed Blue-13, or EU-13_A2, was detected in Ireland for the first time. This was a significant development at the time. Not only was this strain resistant to the phenylamide fungicides, but it appeared to be more virulent and aggressive than any other strains present at that time, based on how it was rapidly becoming the dominant strain throughout Europe and beyond.
Fears at the time
Recommendations to reduce the usage of phenylamide fungicides were well founded as outbreaks of blight were reported following usage, particularly in 2009, when weather conditions were extremely favourable for the disease.
At the time, there was equal concern that Blue-13 was of the A2 mating type, a type that allows for sexual recombination. This in itself was unusual because, until this point, the Irish population and indeed most of population of western Europe was almost exclusively of the A1 mating type.
The detection of Blue-13 in the same fields as strains from the older A1 mating type population, with both often detected in close proximity, meant that for the first the time there was a real possibility that sexual reproduction could occur in the Irish P. infestans population.
Such events would have untold consequences for late blight management in Ireland as the offspring created, called oospores, would have increased genetic diversity with the potential to throw up more strains that could show resistance to fungicides in the future.
In addition to increasing the genetic diversity of the pathogen, the spores are also long-lived and soil-borne. Were such oospores to be produced, they would likely have had serious implications for late blight control programmes.
The fall of Blue-13
Fast forward some 14 years and, having dominated for almost a decade, we now struggle to detect Blue-13 strains in Irish crops. Over the past three seasons, newer and ever more aggressive strains, some of which are fungicide resistant, are now being detected and dominating the population.
An updated blight warning model is soon to be launched by Met Éireann to help growers to more accurately target essential fungicide application.
Although the potential for oospores to emerge still exists, these “newer” strains are clonal in nature and, similar to what happened with Blue-13, they were first detected in continental Europe, followed by the UK and eventually here.
While they appear to be more virulent and aggressive than Blue-13, or indeed its contemporary at the time Pink-6 (EU_6_A1), the knowledge we gained back in the late 2000s has changed our overall approach to controlling late blight.
In essence, taking risks with late blight control can be extremely costly.
In an era of integrated pest management and ever-increasing pressures to reduce our pesticide input, it is important to ask whether it is possible to actually achieve these goals when tackling a beast such as potato late blight.
Knowing the level of risk
Ultimately, late blight control is about preventing infections from getting established.
With an ability to reproduce and produce masses of spores in a matter of days, if you are trying to play catch-up with late blight it is certainly not easy but it is also not impossible.
For most disease pathogen interactions, the first line of defence will be the host variety. Indeed, there are numerous varieties of potato available with exceptionally high levels of late blight resistance and the risk of infections ever taking hold and causing significant losses is often very low with these.
Late blight symptoms in a potato crop.
However, for one reason or another and often relating to quality or market suitability, these varieties are not widely grown.
Most of the crops grown in Ireland are susceptible to late blight and therefore the risk will very much depend on other factors such as inoculum availability and of course the weather.
In the absence of soil-borne oospores, for which there is no evidence that they exist in Ireland or at least do not actually contribute to late blight epidemics, then all late blight outbreaks originate from asexually produced sporangia of strains that already exist in the population. Understanding this is extremely important.
Assuming no potato plants actually survive over the winter months, infections in spring or early summer can only be initiated from one real source – overwintering tubers. Such tubers can of course be in different forms: seeds, volunteers or dumps.
Managing these accordingly can dramatically influence levels of initial inoculum in the following season.
Limiting initial inoculum
When it comes to potato production, the saying “you get what you put in” rings true.
Poor-quality seed from a disease perspective can cause significant disease-related issues throughout the season.
In reality, it is impossible to inspect every seed tuber for potential infection.
However, decisions on what makes a seed crop and how it is managed will determine how clean the seed is. Has blight been detected in the field? How close to the seed crop were other crops that had infection and how bad was this crop? This includes garden plots and even fields that had potatoes in the previous season.
Assuming good disease-free seed is planted, this reduces the main sources of any initial inoculum to both volunteers and discard dumps.
Ensuring any regrowth is removed regularly is essential
As the latter are farm-specific, they really are something that should be easy to manage at a farm level.
Ensuring any regrowth is removed regularly is essential. However, as sporulation can also occur from the tubers themselves, as it is not just limited to foliage, then covering such piles or dumps to prevent inoculum from getting airborne is equally important.
This finally leaves volunteer potatoes from the previous crop or crops. These are often the most difficult to manage, especially where crops are grown in rented land and may be managed by a different grower in subsequent seasons.
Being aware of where these fields are is important and acting to remove the risk within them, if possible, will help. It will be difficult to prevent small tubers from being left in the field but trying to remove as many of them as is possible at harvest is key.
Predicting conditions favourable for disease
The presence of inoculum and a susceptible host does not always guarantee infection. For infection to occur, environmental conditions that favour the development of the disease must also occur.
As most populations are dominated by a small number of strains of blight, it is possible to assess what environmental conditions are optimum for these specific strains and to utilise this information to develop models that can be used to predict conditions favourable to disease development.
In Ireland, these models have been developed by Met Éireann and they have been in operation since the early 1950s.
A late blight lesion showing the typical under-leaf symptoms.
Since then, considerable changes have occurred in the P. infestans population, with multiple displacements in the dominant strains occurring. Production patterns too have changed, with commercial production increasingly confined to smaller areas of the country but with greater intensity.
Furthermore, the availability of efficacious fungicides with different modes of action has greatly increased.
Conversely, the financial risks resulting from serious late blight infections are of a magnitude that is now greater than they were some 70 years ago. Therefore, in 2016, as part of a Department of Agriculture-funded project tasked with establishing a platform for IPM in Ireland, a revaluation of the model used by Met Éireann was undertaken in a collaboration between Teagasc, Met Éireann and Maynooth University.
The overall aim was to determine if improvements to the model should be made and to demonstrate its applicability to current systems of production.
Revisiting the blight
prediction model
Using historical outbreak data from the Teagasc potato breeding programme generated during the period in which Blue-13 emerged in the Irish P. infestans population, the weather conditions that favoured disease development were reassessed.
These identified subtle changes that could be made to the model used by Met Éireann to better forecast periods that are conducive to late blight development. These changes are now being implemented by Met Éireann and will form the basis from which future warnings are issued.
These changes included a decrease in critical relative humidity and changes in the estimation of leaf wetness to include not just rainfall but also relative humidity.
As critical periods for infection are likely to be increasingly encountered following these changes, Met Éireann will provide a real-time overview of late blight risk, calculated as effective blight hours (EBH) – or hours during which the forecasted weather is conducive to late blight – are met. This calculation is then repeated on a six-hour basis.
Figure 1. A Met Eireann colour-coded blight map showing the areas subject to greatest infection risk on Wednesday 2 June 2021. The calculations to produce this image are recalculated on a 6-hour basis and the results are presented as effective blight hours in the categories presented in the legend.
This risk will be represented as:
Very low risk (0-3 EBH). Low risk (4-6 EBH). Medium (7-9 EBH). High risk (10-12 EBH).Very high risk (>12 EBH).The risk levels will be provided on a three- to five-day forecast, allowing users to track periods of forecasted weather conditions favourable for late blight development ahead of time (see Figure 1).
Location-specific
In addition, location-specific meteograms (see Figure 2) will be provided with 10-day forecasts, including those specific criteria used in the model (temperature, humidity and rainfall).
Figure 2. A blight meteogram for Ballyhaise for the seven days between 6 and 12 September 2021. The graphic shows that most of this period had a relative humidity above 88%, air temperature was above 15oC and a rainfall event on 9 September gave rise to increasing EBHs (effective blight hours) from about midday on that date.
The model has been trialled at Oak Park since 2017 across several varieties and presents considerable improvement in late blight predictive power.
When it becomes available on the Met Éireann website, growers should make themselves familiar with the model. It does not provide specific recommendations on spraying, etc, but does provide the information needed to help make informed decisions on disease risk and should be used accordingly.
Ongoing battle with fungicide resistance
As mentioned earlier, the arrival of Blue-13 represented a significant challenge for late blight control, specifically in relation to its resistance to the phenylamide fungicides.
It is worth noting that phenylamide resistance had been with us for almost three decades at that stage. However, unlike previous resistant strains, Blue-13 did not suffer the fitness penalties previously associated with resistance.
While this may not of so much importance now, both Blue-13 and the emergence of resistance in the early 1980s represented considerable shifts or displacements in the Irish late blight populations.
Similar displacements are currently ongoing. In 2018 the fluazinam resistance strain EU_37_A2 was first detected in Ireland, with additional new strains belonging to the genotypes EU_36_A2 and EU_44_A1 since detected.
While there are not believed to be any fungicide resistance issues relating to either EU_36_A2 or EU_44_A1, their detection does need to be monitored closely.
In the case of EU_37_A2, prior to its arrival in Ireland, its sensitivity or lack of same to fluazinam had been flagged as a concern in continental Europe and the UK.
Trials conducted at Oak Park in both 2020 and 2021 have confirmed this is also the case in Ireland, with poor efficacy achieved following application of fluazinam. Subsequent DNA fingerprinting of the developing lesions confirmed the selection of the EU_37_A2 strain.
Up to now, fluazinam has been an extremely effective fungicide for late blight control.
However, it should no longer be relied upon for control as strains of EU_37_A2 are now present in the population, especially later in the season when it was typically used for tuber blight control.
Its detection also serves to highlight the fact that fungicide resistance in P. infestans still remains a possibility and consideration should be given to how best to devise fungicide anti-resistance strategies for late blight.
A range of very effective fungicides are still available to us and these should be utilised throughout a programme where possible. Additionally, mixtures of different modes of action should be applied, where possible.
Potato late blight continues to be a major disease threat to the production of potatoes in this country.The disease has encountered a number of fundamental changes to its population in recent decades.Reduced sensitivity to fluazinam has been confirmed in Ireland, which will force a rethink on the use of this active for blight control.A new blight prediction model is shortly to be introduced by Met Éireann which will increase the accuracy of blight risk prediction in our current Phytophthora populations.
Potato late blight continues to be the greatest disease threat to potato production in Ireland and elsewhere. Control generally requires a season-long effort, with any lapse in control often exploited by the disease.
However, our understanding of late blight continues to evolve and so too should our actions to control the disease.
In 2008, the Phytophthora infestans strain dubbed Blue-13, or EU-13_A2, was detected in Ireland for the first time. This was a significant development at the time. Not only was this strain resistant to the phenylamide fungicides, but it appeared to be more virulent and aggressive than any other strains present at that time, based on how it was rapidly becoming the dominant strain throughout Europe and beyond.
Fears at the time
Recommendations to reduce the usage of phenylamide fungicides were well founded as outbreaks of blight were reported following usage, particularly in 2009, when weather conditions were extremely favourable for the disease.
At the time, there was equal concern that Blue-13 was of the A2 mating type, a type that allows for sexual recombination. This in itself was unusual because, until this point, the Irish population and indeed most of population of western Europe was almost exclusively of the A1 mating type.
The detection of Blue-13 in the same fields as strains from the older A1 mating type population, with both often detected in close proximity, meant that for the first the time there was a real possibility that sexual reproduction could occur in the Irish P. infestans population.
Such events would have untold consequences for late blight management in Ireland as the offspring created, called oospores, would have increased genetic diversity with the potential to throw up more strains that could show resistance to fungicides in the future.
In addition to increasing the genetic diversity of the pathogen, the spores are also long-lived and soil-borne. Were such oospores to be produced, they would likely have had serious implications for late blight control programmes.
The fall of Blue-13
Fast forward some 14 years and, having dominated for almost a decade, we now struggle to detect Blue-13 strains in Irish crops. Over the past three seasons, newer and ever more aggressive strains, some of which are fungicide resistant, are now being detected and dominating the population.
An updated blight warning model is soon to be launched by Met Éireann to help growers to more accurately target essential fungicide application.
Although the potential for oospores to emerge still exists, these “newer” strains are clonal in nature and, similar to what happened with Blue-13, they were first detected in continental Europe, followed by the UK and eventually here.
While they appear to be more virulent and aggressive than Blue-13, or indeed its contemporary at the time Pink-6 (EU_6_A1), the knowledge we gained back in the late 2000s has changed our overall approach to controlling late blight.
In essence, taking risks with late blight control can be extremely costly.
In an era of integrated pest management and ever-increasing pressures to reduce our pesticide input, it is important to ask whether it is possible to actually achieve these goals when tackling a beast such as potato late blight.
Knowing the level of risk
Ultimately, late blight control is about preventing infections from getting established.
With an ability to reproduce and produce masses of spores in a matter of days, if you are trying to play catch-up with late blight it is certainly not easy but it is also not impossible.
For most disease pathogen interactions, the first line of defence will be the host variety. Indeed, there are numerous varieties of potato available with exceptionally high levels of late blight resistance and the risk of infections ever taking hold and causing significant losses is often very low with these.
Late blight symptoms in a potato crop.
However, for one reason or another and often relating to quality or market suitability, these varieties are not widely grown.
Most of the crops grown in Ireland are susceptible to late blight and therefore the risk will very much depend on other factors such as inoculum availability and of course the weather.
In the absence of soil-borne oospores, for which there is no evidence that they exist in Ireland or at least do not actually contribute to late blight epidemics, then all late blight outbreaks originate from asexually produced sporangia of strains that already exist in the population. Understanding this is extremely important.
Assuming no potato plants actually survive over the winter months, infections in spring or early summer can only be initiated from one real source – overwintering tubers. Such tubers can of course be in different forms: seeds, volunteers or dumps.
Managing these accordingly can dramatically influence levels of initial inoculum in the following season.
Limiting initial inoculum
When it comes to potato production, the saying “you get what you put in” rings true.
Poor-quality seed from a disease perspective can cause significant disease-related issues throughout the season.
In reality, it is impossible to inspect every seed tuber for potential infection.
However, decisions on what makes a seed crop and how it is managed will determine how clean the seed is. Has blight been detected in the field? How close to the seed crop were other crops that had infection and how bad was this crop? This includes garden plots and even fields that had potatoes in the previous season.
Assuming good disease-free seed is planted, this reduces the main sources of any initial inoculum to both volunteers and discard dumps.
Ensuring any regrowth is removed regularly is essential
As the latter are farm-specific, they really are something that should be easy to manage at a farm level.
Ensuring any regrowth is removed regularly is essential. However, as sporulation can also occur from the tubers themselves, as it is not just limited to foliage, then covering such piles or dumps to prevent inoculum from getting airborne is equally important.
This finally leaves volunteer potatoes from the previous crop or crops. These are often the most difficult to manage, especially where crops are grown in rented land and may be managed by a different grower in subsequent seasons.
Being aware of where these fields are is important and acting to remove the risk within them, if possible, will help. It will be difficult to prevent small tubers from being left in the field but trying to remove as many of them as is possible at harvest is key.
Predicting conditions favourable for disease
The presence of inoculum and a susceptible host does not always guarantee infection. For infection to occur, environmental conditions that favour the development of the disease must also occur.
As most populations are dominated by a small number of strains of blight, it is possible to assess what environmental conditions are optimum for these specific strains and to utilise this information to develop models that can be used to predict conditions favourable to disease development.
In Ireland, these models have been developed by Met Éireann and they have been in operation since the early 1950s.
A late blight lesion showing the typical under-leaf symptoms.
Since then, considerable changes have occurred in the P. infestans population, with multiple displacements in the dominant strains occurring. Production patterns too have changed, with commercial production increasingly confined to smaller areas of the country but with greater intensity.
Furthermore, the availability of efficacious fungicides with different modes of action has greatly increased.
Conversely, the financial risks resulting from serious late blight infections are of a magnitude that is now greater than they were some 70 years ago. Therefore, in 2016, as part of a Department of Agriculture-funded project tasked with establishing a platform for IPM in Ireland, a revaluation of the model used by Met Éireann was undertaken in a collaboration between Teagasc, Met Éireann and Maynooth University.
The overall aim was to determine if improvements to the model should be made and to demonstrate its applicability to current systems of production.
Revisiting the blight
prediction model
Using historical outbreak data from the Teagasc potato breeding programme generated during the period in which Blue-13 emerged in the Irish P. infestans population, the weather conditions that favoured disease development were reassessed.
These identified subtle changes that could be made to the model used by Met Éireann to better forecast periods that are conducive to late blight development. These changes are now being implemented by Met Éireann and will form the basis from which future warnings are issued.
These changes included a decrease in critical relative humidity and changes in the estimation of leaf wetness to include not just rainfall but also relative humidity.
As critical periods for infection are likely to be increasingly encountered following these changes, Met Éireann will provide a real-time overview of late blight risk, calculated as effective blight hours (EBH) – or hours during which the forecasted weather is conducive to late blight – are met. This calculation is then repeated on a six-hour basis.
Figure 1. A Met Eireann colour-coded blight map showing the areas subject to greatest infection risk on Wednesday 2 June 2021. The calculations to produce this image are recalculated on a 6-hour basis and the results are presented as effective blight hours in the categories presented in the legend.
This risk will be represented as:
Very low risk (0-3 EBH). Low risk (4-6 EBH). Medium (7-9 EBH). High risk (10-12 EBH).Very high risk (>12 EBH).The risk levels will be provided on a three- to five-day forecast, allowing users to track periods of forecasted weather conditions favourable for late blight development ahead of time (see Figure 1).
Location-specific
In addition, location-specific meteograms (see Figure 2) will be provided with 10-day forecasts, including those specific criteria used in the model (temperature, humidity and rainfall).
Figure 2. A blight meteogram for Ballyhaise for the seven days between 6 and 12 September 2021. The graphic shows that most of this period had a relative humidity above 88%, air temperature was above 15oC and a rainfall event on 9 September gave rise to increasing EBHs (effective blight hours) from about midday on that date.
The model has been trialled at Oak Park since 2017 across several varieties and presents considerable improvement in late blight predictive power.
When it becomes available on the Met Éireann website, growers should make themselves familiar with the model. It does not provide specific recommendations on spraying, etc, but does provide the information needed to help make informed decisions on disease risk and should be used accordingly.
Ongoing battle with fungicide resistance
As mentioned earlier, the arrival of Blue-13 represented a significant challenge for late blight control, specifically in relation to its resistance to the phenylamide fungicides.
It is worth noting that phenylamide resistance had been with us for almost three decades at that stage. However, unlike previous resistant strains, Blue-13 did not suffer the fitness penalties previously associated with resistance.
While this may not of so much importance now, both Blue-13 and the emergence of resistance in the early 1980s represented considerable shifts or displacements in the Irish late blight populations.
Similar displacements are currently ongoing. In 2018 the fluazinam resistance strain EU_37_A2 was first detected in Ireland, with additional new strains belonging to the genotypes EU_36_A2 and EU_44_A1 since detected.
While there are not believed to be any fungicide resistance issues relating to either EU_36_A2 or EU_44_A1, their detection does need to be monitored closely.
In the case of EU_37_A2, prior to its arrival in Ireland, its sensitivity or lack of same to fluazinam had been flagged as a concern in continental Europe and the UK.
Trials conducted at Oak Park in both 2020 and 2021 have confirmed this is also the case in Ireland, with poor efficacy achieved following application of fluazinam. Subsequent DNA fingerprinting of the developing lesions confirmed the selection of the EU_37_A2 strain.
Up to now, fluazinam has been an extremely effective fungicide for late blight control.
However, it should no longer be relied upon for control as strains of EU_37_A2 are now present in the population, especially later in the season when it was typically used for tuber blight control.
Its detection also serves to highlight the fact that fungicide resistance in P. infestans still remains a possibility and consideration should be given to how best to devise fungicide anti-resistance strategies for late blight.
A range of very effective fungicides are still available to us and these should be utilised throughout a programme where possible. Additionally, mixtures of different modes of action should be applied, where possible.
Potato late blight continues to be a major disease threat to the production of potatoes in this country.The disease has encountered a number of fundamental changes to its population in recent decades.Reduced sensitivity to fluazinam has been confirmed in Ireland, which will force a rethink on the use of this active for blight control.A new blight prediction model is shortly to be introduced by Met Éireann which will increase the accuracy of blight risk prediction in our current Phytophthora populations. 
This is a subscriber-only article
SHARING OPTIONS: