While most broad-acre crops have some tolerance for disease, there is practically a zero tolerance for potato blight in any crop. This disease has an ability to cause havoc in a very short period. Given the right conditions, late blight can complete its lifecycle in as little as three to four days. This is the time from first infection through to the production of spores.
Once spores are produced, they are moved on to produce more lesions, which produce more spores and, very quickly, an unchecked infection can go from zero to widespread. If blight is allowed to develop in a crop, it can be difficult to get on top of it and, if weather conditions favour its development, crops can be severely affected in a matter of weeks.
In addition to killing the foliage, and ultimately hitting the ability of the crop to create tubers, spores can be washed down through the soil to the developing potatoes. Where this happens, the disease can either kill the potato or remain dormant to allow harvesting and storage only to cause rotting later when it is taken from storage. In fact, if storage conditions are not right and moisture is allowed to build up, blight is known to spread even within stores.
For these reasons, fungicide programmes on commercial crops must start almost as soon as the crop is peeping through the soil. And this must continue routinely at seven-day intervals until tuber production is complete and the foliage has been completely burnt off. Unlike cereals, there are a diverse range of fungicide chemistries to choose from for late blight control. However, to maximise their efficacy, they must be used at particular periods of crop growth.
Stop infection
As our current population of late blight shows no evidence of sexual recombination, the survival of blight from one season to the next is dependent on its survival in potato crops, whether in seed or dumps or on volunteers. For this reason, late blight disease control must address the initial source of infection, even before fungicides are considered.
The mantra, “you harvest what you sow”, is most definitely true for potatoes. In addition to virus and bacterial diseases, late blight can make its way into crops through infected seed. This source of disease spread can be difficult to spot and it may not be seen until the disease has undergone a number of lifecycles. By then, it will have produced vast amounts of inoculum and infected surrounding plants.
While commercial crops will receive fungicides, any infections which develop from seed will have to make its way up through the stem leaving the infection to develop beneath the canopy when the crop is growing rapidly. Therefore, routine crop inspections are needed. The easiest way to manage this source of infection is to ensure that only clean certified seed is planted. Even though this season’s seed stock came from 2013 crops where blight pressure was exceptionally low, all it takes is a single infected tuber to start the cycle.
Dumps
As well as from within the crop, initial inoculum can also come from last season’s dumps. These contain tubers which did not make the grade last year, whether due to disease or deformities. They will provide a source of inoculum, either directly or indirectly. Diseased tubers in these dumps may well have been placed there because they were rotten with blight. If they contain blight and they are not destroyed, they will provide a continual source of blight.
Even if dumps do not contain any infected tubers initially, it is inevitable that plants growing there will get blight and, subsequently, act as a source of infection for crops. So, any sprouting tubers or potatoes growing in dumps must be burnt off. As dumps contain a range of varieties, these plants can be at different development stages so keeping dumps in check is a season-long process.
Volunteers
Each season, despite the best efforts of growers, there will always be a certain number of tubers left in the fields after harvest. This is inevitable when growing potatoes. Unfortunately, these will subsequently grow and they can also act, directly or indirectly, as an initial source of blight. So, volunteers must also be kept in check. Often, due to late harvesting, the subsequent crop will be spring sown and specific herbicides should be included in the cereal weed spray to get rid of any emerging potatoes early.
However, because of the variable emergence of these volunteers, complete control is difficult. Indeed, these ‘potato stubbles’ may now be farmed by different farmers. Manual rogueing is not really an option because of the acreage involved. So, growers need to be aware that these volunteers may potentially act as ‘green-bridges’ for blight and be extra vigilant with fungicide programmes in potato crops grown in close proximity to such risks.
A quick walk through cereal crops which were preceded by potatoes can provide an early warning to indicate active blight in an area. Also, any increase in garden growers or allotments is likely to increase the risk of blight in local areas as these plants may not receive intensive fungicide protection. However, commercial growers must make themselves aware of these risks and take extra care of crops in close proximity. This is especially important once weather conditions suitable for the spread of blight occur.
Heed the warnings
As mentioned previously, controlling late blight in commercial crops is a season-long process. While late blight was traditionally not seen until late June or early July in most seasons, the risk of infection is always there as temperatures rise from late April onwards. Fungicide programmes generally start shortly after crops emerge.
This season, possibly due to the mild and damp spring experienced, blight has been reported in the south-east of the country in crops which have been protected from the worst of the elements, either by polythene or fleece.
As well as these sightings, Met Éireann has been issuing blight warnings for some weeks now, which indicate weather conditions suitable for the infection and spread of blight.
Know your tools
With a diverse number of fungicides in the market for late blight control, it is important to know the exact attributes of each so that they can be used efficiently. Fortunately, the majority of fungicides available have been classified by Euroblight (http://euroblight.net/control-strategies/euroblight-fungicide-table/) based on their effectiveness (against leaf blight, stem blight and tuber blight and in new growth) and their mode of action (protectant, curative or anti-sporulant).
These rating have been independently validated in the UK, Netherlands, Denmark and Germany under high disease pressure. Therefore, these ratings should guide how control programmes are devised. In all programmes, fungicides with protective activity should be included in all applications.
Furthermore, the key to successful blight control is ensuring that the fungicide is consistency applied. This could mean having to remove a piece of the crop if it cannot be sprayed consistently, such as around poles.
With blight already present in crops and conditions favouring its spread, all crops should receive some protection as they break ground. This can be regarded as a type of holding spray, as it protects the crop as it becomes established. Once established, the crop will rapidly grow and new leaf material will appear each day. Unless some form of a systemic or translaminar fungicide is included, this new growth will be left untreated.
Unfortunately, resistance to the most active systemic fungicide available, metalaxyl, is present in the Irish population and, so, metalaxyl-based products should only be used once in a crop early in the season and spray intervals need to be tight.
As crop growth begins to slow, translaminar fungicides and protective fungicides will provide sufficient control. Towards the end of the programme, it is essential to ensure that the potatoes underground are protected from any spores that may make their way towards the ground. Fungicides with anti-sporulant activity should be considered for the final applications.
Since 2008 and the emergence of the aggressive strain of late blight, dubbed Blue-13 or 13-A2, programmes have generally been reduced to seven-day spray intervals. Although the frequency of this strain has shown fluctuations, it is still present in the Irish population and intervals should still be kept to seven days. Where blight pressure is low, as was experienced last year, there is a possibility of reducing application rates.
However, if the broken weather experienced at the moment continues, rates must be kept at those recommended. Also, if this broken weather affects the spraying intervals and they get stretched in areas where blight is known to be active, the inclusion of cymoxanil in the subsequent application may be warranted.
Cymoxanil will provide the kickback needed to cure any latent infections that may have taken hold between applications. Where spots or foci of blight are detected in fields, the best thing to do is to burn off the infected area, including what may appear to be uninfected around it. This action must be quickly followed with an application of cymoxanil and a protectant-based product to the entire crop. It is essential that crops are continually monitored to help make these decisions.
Teagasc, in collaboration with UCD and AFBI in NI, will continue to monitor the Irish late blight population in 2014. Previously, this identified the changes in 2009 with the emergence of the aggressive Blue-13 strain and its resistance to the phenylamide fungicides. To help increase the potential of this study to detect any further population changes, Teagasc is seeking the assistance of growers and gardeners. If you have late blight at any stage during the summer, please place blighted leaf samples in an air-tight freezer bag, ensuring air is also trapped inside and send to Dr Steven Kildea, Teagasc Crop Science, Oak Park, Co Carlow.
Further directions on sending samples can be obtained at http://www.teagasc.ie/crops/potatoes/blight.asp or by emailing stephen.kildea@teagasc.ie
Containing a blight epidemic is difficult and, so, the prevention of infection is key.Blight can start from infection coming from a previous dump, volunteers in neighbouring fields or seed infection.A blight cycle can turn over every three to four days when conditions are right.Crop protection must now begin early post emergence.
While most broad-acre crops have some tolerance for disease, there is practically a zero tolerance for potato blight in any crop. This disease has an ability to cause havoc in a very short period. Given the right conditions, late blight can complete its lifecycle in as little as three to four days. This is the time from first infection through to the production of spores.
Once spores are produced, they are moved on to produce more lesions, which produce more spores and, very quickly, an unchecked infection can go from zero to widespread. If blight is allowed to develop in a crop, it can be difficult to get on top of it and, if weather conditions favour its development, crops can be severely affected in a matter of weeks.
In addition to killing the foliage, and ultimately hitting the ability of the crop to create tubers, spores can be washed down through the soil to the developing potatoes. Where this happens, the disease can either kill the potato or remain dormant to allow harvesting and storage only to cause rotting later when it is taken from storage. In fact, if storage conditions are not right and moisture is allowed to build up, blight is known to spread even within stores.
For these reasons, fungicide programmes on commercial crops must start almost as soon as the crop is peeping through the soil. And this must continue routinely at seven-day intervals until tuber production is complete and the foliage has been completely burnt off. Unlike cereals, there are a diverse range of fungicide chemistries to choose from for late blight control. However, to maximise their efficacy, they must be used at particular periods of crop growth.
Stop infection
As our current population of late blight shows no evidence of sexual recombination, the survival of blight from one season to the next is dependent on its survival in potato crops, whether in seed or dumps or on volunteers. For this reason, late blight disease control must address the initial source of infection, even before fungicides are considered.
The mantra, “you harvest what you sow”, is most definitely true for potatoes. In addition to virus and bacterial diseases, late blight can make its way into crops through infected seed. This source of disease spread can be difficult to spot and it may not be seen until the disease has undergone a number of lifecycles. By then, it will have produced vast amounts of inoculum and infected surrounding plants.
While commercial crops will receive fungicides, any infections which develop from seed will have to make its way up through the stem leaving the infection to develop beneath the canopy when the crop is growing rapidly. Therefore, routine crop inspections are needed. The easiest way to manage this source of infection is to ensure that only clean certified seed is planted. Even though this season’s seed stock came from 2013 crops where blight pressure was exceptionally low, all it takes is a single infected tuber to start the cycle.
Dumps
As well as from within the crop, initial inoculum can also come from last season’s dumps. These contain tubers which did not make the grade last year, whether due to disease or deformities. They will provide a source of inoculum, either directly or indirectly. Diseased tubers in these dumps may well have been placed there because they were rotten with blight. If they contain blight and they are not destroyed, they will provide a continual source of blight.
Even if dumps do not contain any infected tubers initially, it is inevitable that plants growing there will get blight and, subsequently, act as a source of infection for crops. So, any sprouting tubers or potatoes growing in dumps must be burnt off. As dumps contain a range of varieties, these plants can be at different development stages so keeping dumps in check is a season-long process.
Volunteers
Each season, despite the best efforts of growers, there will always be a certain number of tubers left in the fields after harvest. This is inevitable when growing potatoes. Unfortunately, these will subsequently grow and they can also act, directly or indirectly, as an initial source of blight. So, volunteers must also be kept in check. Often, due to late harvesting, the subsequent crop will be spring sown and specific herbicides should be included in the cereal weed spray to get rid of any emerging potatoes early.
However, because of the variable emergence of these volunteers, complete control is difficult. Indeed, these ‘potato stubbles’ may now be farmed by different farmers. Manual rogueing is not really an option because of the acreage involved. So, growers need to be aware that these volunteers may potentially act as ‘green-bridges’ for blight and be extra vigilant with fungicide programmes in potato crops grown in close proximity to such risks.
A quick walk through cereal crops which were preceded by potatoes can provide an early warning to indicate active blight in an area. Also, any increase in garden growers or allotments is likely to increase the risk of blight in local areas as these plants may not receive intensive fungicide protection. However, commercial growers must make themselves aware of these risks and take extra care of crops in close proximity. This is especially important once weather conditions suitable for the spread of blight occur.
Heed the warnings
As mentioned previously, controlling late blight in commercial crops is a season-long process. While late blight was traditionally not seen until late June or early July in most seasons, the risk of infection is always there as temperatures rise from late April onwards. Fungicide programmes generally start shortly after crops emerge.
This season, possibly due to the mild and damp spring experienced, blight has been reported in the south-east of the country in crops which have been protected from the worst of the elements, either by polythene or fleece.
As well as these sightings, Met Éireann has been issuing blight warnings for some weeks now, which indicate weather conditions suitable for the infection and spread of blight.
Know your tools
With a diverse number of fungicides in the market for late blight control, it is important to know the exact attributes of each so that they can be used efficiently. Fortunately, the majority of fungicides available have been classified by Euroblight (http://euroblight.net/control-strategies/euroblight-fungicide-table/) based on their effectiveness (against leaf blight, stem blight and tuber blight and in new growth) and their mode of action (protectant, curative or anti-sporulant).
These rating have been independently validated in the UK, Netherlands, Denmark and Germany under high disease pressure. Therefore, these ratings should guide how control programmes are devised. In all programmes, fungicides with protective activity should be included in all applications.
Furthermore, the key to successful blight control is ensuring that the fungicide is consistency applied. This could mean having to remove a piece of the crop if it cannot be sprayed consistently, such as around poles.
With blight already present in crops and conditions favouring its spread, all crops should receive some protection as they break ground. This can be regarded as a type of holding spray, as it protects the crop as it becomes established. Once established, the crop will rapidly grow and new leaf material will appear each day. Unless some form of a systemic or translaminar fungicide is included, this new growth will be left untreated.
Unfortunately, resistance to the most active systemic fungicide available, metalaxyl, is present in the Irish population and, so, metalaxyl-based products should only be used once in a crop early in the season and spray intervals need to be tight.
As crop growth begins to slow, translaminar fungicides and protective fungicides will provide sufficient control. Towards the end of the programme, it is essential to ensure that the potatoes underground are protected from any spores that may make their way towards the ground. Fungicides with anti-sporulant activity should be considered for the final applications.
Since 2008 and the emergence of the aggressive strain of late blight, dubbed Blue-13 or 13-A2, programmes have generally been reduced to seven-day spray intervals. Although the frequency of this strain has shown fluctuations, it is still present in the Irish population and intervals should still be kept to seven days. Where blight pressure is low, as was experienced last year, there is a possibility of reducing application rates.
However, if the broken weather experienced at the moment continues, rates must be kept at those recommended. Also, if this broken weather affects the spraying intervals and they get stretched in areas where blight is known to be active, the inclusion of cymoxanil in the subsequent application may be warranted.
Cymoxanil will provide the kickback needed to cure any latent infections that may have taken hold between applications. Where spots or foci of blight are detected in fields, the best thing to do is to burn off the infected area, including what may appear to be uninfected around it. This action must be quickly followed with an application of cymoxanil and a protectant-based product to the entire crop. It is essential that crops are continually monitored to help make these decisions.
Teagasc, in collaboration with UCD and AFBI in NI, will continue to monitor the Irish late blight population in 2014. Previously, this identified the changes in 2009 with the emergence of the aggressive Blue-13 strain and its resistance to the phenylamide fungicides. To help increase the potential of this study to detect any further population changes, Teagasc is seeking the assistance of growers and gardeners. If you have late blight at any stage during the summer, please place blighted leaf samples in an air-tight freezer bag, ensuring air is also trapped inside and send to Dr Steven Kildea, Teagasc Crop Science, Oak Park, Co Carlow.
Further directions on sending samples can be obtained at http://www.teagasc.ie/crops/potatoes/blight.asp or by emailing stephen.kildea@teagasc.ie
Containing a blight epidemic is difficult and, so, the prevention of infection is key.Blight can start from infection coming from a previous dump, volunteers in neighbouring fields or seed infection.A blight cycle can turn over every three to four days when conditions are right.Crop protection must now begin early post emergence. 
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