Late Blight Knowledge Hub

The late blight pathogen can reproduce sexually and asexually. Sexual reproduction allows both parents to contribute genes which results in a new strain with characteristics that differentiate it from both its parents. Asexual reproduction requires only one parent meaning all offspring are clones.

Potato late blight populations are mainly clonal in Great Britain, but sexual reproduction of the pathogen leads to the emergence of novel (non-clonal) genotypes that may have traits such as new virulence or fungicide insensitivity that can make them more challenging to control. 

Differences in genotypic diversity have been found between populations of the pathogen in the northern and southern regions of Scotland, with populations in the north-east of Scotland showing greater genotypic diversity. 

The frequency of outbreaks defined as non-clonal is circumstantial evidence that sexual recombination of the pathogen has taken place in this region. These feature a high genetic diversity – where up to 50% of samples tested are novel multilocus genotypes – with a unique combination of alleles across all loci.

The north-east of Scotland is primarily a seed potato-producing region, therefore importation of seed from other countries (that may contain novel genotypes) would not exist.

The high levels of non-clonal outbreaks observed may be the result of climatological factors that favour oospore formation, survival and germination.  Constant humidity and temperatures of 10–15°C are favourable for oospore germination.  Sexual soilborne oospores are likely to be acting as primary inoculum, generating high genotypic variation of the pathogen in this region.  It is important to understand the mechanism involved and its potential implications.

Foliar and stem infection via the asexual cycle

The asexual life cycle, (see diagram) begins with sporangia landing on a crop. Under warmer conditions, 18˚C or more, most sporangia germinate directly (1) forming a germ tube that penetrates the epidermal cells. Under cooler conditions most sporangia differentiate to form 10 to 12 motile zoospores that are released through the apex of the spore (2). They move in the water film on the plant surface and are attracted to suitable infection sites where they encyst (3), germinate and form germ tubes that penetrate the epidermis (4).

Unchecked, the pathogen will colonise the plant tissue, form visible lesions and begin to sporulate. Under humid conditions the pathogen forms spore-producing mycelium called sporangiophores that exit the plant.

[Illustration 1] The asexual life cycle of potato life blight.

This mycelium appears as a white downy growth on the margins of stem or foliar lesions and generates up to 80,000 sporangia per square centimetre of lesion or 4 x 10¹² sporangia per Ha in a single day. Foliar lesions can rapidly defoliate a potato crop, but stem lesions are also particularly damaging, as they are less susceptible to fungicides, can survive dry periods that can check leaf infections and, if they girdle the stem, will kill all the plant tissue above the lesion.

As the life cycle repeats, a focus of blight develops. From this, air-borne spores establish new infections and secondary foci are created. Spores may travel several kilometres and, provided they are not desiccated, or killed by ultraviolet exposure en route, remain viable and can cause infection in other crops.

The length of time from a spore landing on the plant until sporulation begins is termed the latent period. Under optimal conditions the latent period may be as short as three days and if left unchecked explains the explosive epidemic development.

Foliar lesion © Eric Anderson, Scottish Agronomy

Stem blight © Eric Anderson, Scottish Agronomy

Tuber infection

As tubers form, they become vulnerable to infection from sporangia and zoospores. The infection route involves these spores being washed down through the soil, or down the stems themselves (see diagram), or coming into contact with tubers at lifting or grading.

[Illustration 2] Tuber infection

Survival of sporangia in the soil depends on soil type, moisture content and, to a lesser degree, soil temperature. Under natural environmental conditions most sporangia die within 14 days, but they have been found to survive underground for up to 21 days in the absence of green plant material.

Zoospores emerging from sporangia infect tubers and colonise the layer just under the skin. Most infection prior to harvest is through lenticels and eyes leading to tubers with firm chestnut or granular brown lesions just under the skin that spread inwards.

Frequently tuber blight provides an entry route for secondary infection by soft rotting bacteria that convert the flesh to a putrid semi-liquid state.

The main risk factors for tuber infection in the growing crop are the presence of foliar blight, together with high-risk weather to encourage sporulation, followed by lower temperatures to favour the production of a large number of zoospores, combined with wet soil and irrigation or substantial rainfall. Note that for tuber infection, foliar blight does not need to be severe if the other factors are favourable.

Spray intervals of less than seven days will be required where active blight is present until control is regained. This is likely to require alternating products to ensure adherence to the spray intervals specified on the product label. Ensure that haulm coverage is optimised by selecting appropriate nozzles and using a higher volume of water than normal.  To minimise the selection for insensitive strains, alternate products with different modes of action (FRAC codes) and/or use tank mixtures of active substances, or co- formulations, with different FRAC codes.

Where possible avoid irrigating shortly after periods of high-risk weather, i.e. warm and humid conditions. The combination of weather conducive to the production of sporangia followed shortly after by irrigation substantially increases the risk of tuber infection because spores are washed down into the soil.  The decision over whether to irrigate or not can be easier to make if there is good information on the extent of foliar blight in the crop. The implications for tuber infection are clearly different if foliar blight is restricted to one or two small, localised patches compared with it being present throughout the field.

Minimising the passage of blighted tubers into store

Remember that the role of fungicide applications is not just to protect tubers; it is also to reduce the amount of viable inoculum in the crop. Consolidating cracked ridges with a ridge roller after initial haulm desiccation and ensuring complete haulm desiccation to prevent re-growth can reduce the risk of tuber blight.

If test digs ahead of haulm destruction reveal blighted tubers, then the number making it into store can be reduced by delaying harvest and allowing them to rot induced by secondary bacterial infection. However, it should be noted that the success of this approach is greater for soils that are warmer and wetter. Experience suggests that on occasions tuber decay can be limited in free-draining soils, especially once soil temperatures fall significantly.

Under warm and humid or wet soil conditions, a large number of spores can be produced on regrowth, which subsequently infect tubers damaged during harvest. To minimise the risk of this, check that desiccation was as effective as expected and ensure that haulm has been completely dead for at least 14 days.

Where tuber blight is visible it would be prudent to leave crops for at least 21 days. Although the number of viable spores in the soil decreases quickly after desiccation it is many weeks before the number declines to zero.  If blighted tubers get into store and remain moist, this greatly increases the risk of secondary bacterial soft rots. If a crop with tuber blight is harvested before a healthy crop, ensure that the harvester is thoroughly cleaned between crops.

ASSESSING TUBER BLIGHT LEVELS BEFORE HARVEST

• Use hot box with temperature set at 20°C for 12-18 hours.
• Random sample of 300 tubers required to detect 1% infection with confidence.
• Completely peel and cut all tubers to assess potential infection.

Prevention of tuber blight during harvest

• Ensure thorough haulm destruction, and check crops for haulm regrowth, to prevent re- growth that is blighted spreading blight spores onto tubers on the harvester.
• For crops that have, or have had, foliar blight, if the variety is particularly susceptible to tuber blight, then consider lengthening the period between desiccation and harvest for an extra week or two to allow a greater number of viable spores in the soil to die off. Fewer viable spores are required to infect tubers of tuber-susceptible cultivars.
• Don’t let harvested tubers become wet.
• Ventilate and dry tubers immediately after harvest to avoid condensation on tuber surfaces.

Early symptoms of tuber blight© Eric Anderson, Scottish Agronomy

The following factors increase the risk of tuber blight and a combination of several of them is required for tuber blight to occur:

• Haulm with foliar blight where risk increases with rising levels of inoculum.
• High risk weather that maximises spore production on haulm.
• A temperature drop below 11°C that encourages formation of zoospores which are smaller, motile in water and therefore more likely to reach tubers.
• Rainfall or irrigation of 5 mm or more that washes spores into soil.
• High soil moisture content which favours tuber infection – lesion development increases from 40 to 80% soil field capacity.
• Cracked ridges and shallow-set tubers effectively shortening spores’ journey from leaf to tuber.
• Sandy soils which zoospores can travel faster through than clay soils.

The asexual form of Phytophthora infestans will not survive between seasons in the absence of live potato tissue, so fully rotted tubers break the cycle. Conversely, infected tubers that survive can carry primary inoculum to re-start the cycle the following season.

Oospores and the sexual life cycle

The phases of the life cycle described above are the result of asexual (i.e. clonal) reproduction. Oospores can be an advantage to the pathogen as successful combinations of traits remain genetically fixed for months or even years. However, this limits the opportunities for genetic evolution which is a disadvantage over the longer term.

The pathogen has two mating types termed A1 and A2 and, as illustrated in the diagram, these opposing forms must co-infect and meet in plant tissue (6) to reproduce sexually. Until the 1970s this part of the life cycle could not occur in Europe as the A2 type was only found in Mexico, the pathogen’s centre of origin. With the introduction and spread of the A2 mating type in Europe, both types may now co-infect plants and reproduce sexually, generating thick-walled oospores (7). These oospores enter the soil as the infected plant material decomposes, remain viable for many years and then germinate (8) in the presence of a host plant to form sporangia (9) which re-start the life cycle.  Oospores can survive in a field under very different climatic conditions from several months up to four years.  Indications of sexual reproduction, resulting in soil-borne inoculum were first reported from Sweden. There are also observations of primary infections probably arising from oospores in Denmark and Finland.

The diagram combines the asexual and sexual life cycles of the late blight pathogen and shows how they drive disease development in leaves, stems and tubers.

[ILLUSTRATION 3] The full life cycle of potato late blight

 Each germinating oospore is genetically distinct, and such ‘re-shuffling of the genetic pack’ generates new combinations of traits. Through a process of natural selection those genotypes that are more aggressive, fitter, resistant to fungicide or more capable of overcoming host resistance than the existing pathogen population, will be more likely to spread and cause crop disease that is more difficult to manage.

The implications of oospores acting as a source of inoculum are potentially serious. In other parts of Europe, such as in the Nordics and The Netherlands, early infections from soil-borne oospores have proved difficult to manage.

In British crops, however, despite the prevalence of the A1 and A2 mating types, oospores have not yet been found.  It is almost certain that they are there, but we just haven’t looked for them.  There is an increasing body of evidence to show that oospores could be an important source of primary infection.  In the North of Scotland, the presence of “Other” or “Miscellaneous” genotypes in Fight Against Blight reporting is evidence enough of sexual recombination occurring.   

An important factor is the length of the rotation between potato crops. In the absence of a susceptible crop the viability of dormant oospores declines, and the reduced inoculum load thus decreases disease risk. Maintaining long rotations is therefore advisable for managing late blight as well as other soil-borne pests and pathogens.

Nonetheless, growers should remain alert to the signs of soil-borne oospore inoculum, particularly in crops grown in fields that suffered severe blight infection when potatoes were last grown. Warm, wet conditions after planting stimulate co-ordinated oospore germination that can cause patches of sudden infection of the lower leaf canopy of the emerging crop.

Signs to look for include the early appearance of multiple blight lesions on leaves in contact with the soil and patches of young plants dying-off from stem blight that appears to start from the stem base or below the soil. This may occur in low-lying areas of the field where wet soil conditions favour pathogen activity.

Late blight infection and spread is strongly influenced by the weather with periods of warm wet weather being optimal for the pathogen. There is a strict requirement for high humidity or leaf wetness on the host surface for pathogen spores to infect with temperature influencing the rate at which such infection occurs. Temperatures of 15 to 18^oC are considered optimal for pathogen infection and growth.

Once the infection hyphae have gained entry to the plant, dry cool conditions can be tolerated as the pathogen has access to water and nutrients from its host and can remain in a quiescent state. Subsequent lesion growth and disease spread will then occur with a return to warmer and wetter conditions. Once lesions have formed, the pathogen produces abundant spores with sporulation promoted by cool wet conditions, typically overnight. Drying of the crop the next morning promotes spore release and local dissemination and re-infection to form a disease focus. Longer distance spore dispersal from tens of metres to kilometres depends on wind speed. Cloud cover is an important factor in viable spore dispersal as exposure to ultraviolet light kills sporangia.

A set of meteorological conditions considered optimal for blight infection and spread were defined in 1946 by Beaumont and modified by Smith in the 1950s. These conditions were reviewed and updated in 2016 by the James Hutton Institute. These ‘Hutton Criteria’ predict the infection and spread if on each of two consecutive days the minimum air temperature is above 10^oC and the relative humidity is greater than 90% for more than six hours on each day.

The potato late blight pathogen Phytophthora infestans is capable of rapidly generating billions of spores and spreading as genetically uniform genotypes. Genetic fingerprinting tools have allowed the tracking of such genotypes and shown some to have persisted for decades. The success of these genotypes is due to their traits of aggressiveness; ability to infect and colonise host plant tissue and fitness; ability to spread within and between seasons. In a polycyclic disease such as potato late blight even slight changes in these traits can have a significant effect on their competitive ability.

Virulence profiles within a clone are highly complex with multiple biological profiles known within clones for virulence and aggressiveness traits.    Differences in pathogenicity traits between clones are roughly correct at the population level but are absolutely non-significant at the isolate/individual level.   Genotypes are a poor predictor of phenotype pathogenicity.  For population monitoring phenotyping is still cumbersome and slow, but absolutely needed.  Further research is required on predictive markers to screen large number of samples as factors other than pathogenicity are important for fitness, such as the ability to overcome cultivar resistance genes.

The ability to overcome the resistance of commonly grown potato cultivars or having reduced sensitivity to important fungicides also shapes the population and affects the success of blight management. The industry-sponsored ‘Fight Against Blight’ campaign has supported blight sampling by disease scouts in British since 2003, along with the identification of genotypes so the makeup and development of the blight population can be tracked and growers advised as to how the threat to crops is evolving.

Results of the EuroBlight potato late blight monitoring, which tracks the emergence and spread of strains across Europe, are published annually at euroblight.net. The findings serve as the basis for developing an integrated management plan and actions to reduce the risk of further fungicide resistance.

Integrated Crop Management (ICM) can be defined as a sustainable system for food production that is efficient, and environmentally and economically viable.

A requirement of the Sustainable Use Directive on Pesticides is that growers must have an Integrated Pest Management Plan (IPMP). Farm assurance schemes such as Red Tractor and LEAF can request proof of the IPMP during an audit.

An IPMP for late blight control would consider:

• Control of primary inoculum sources, e.g. volunteers and potato dumps.
• Use of blight resistant cultivars where possible.
• Use of disease forecasting and decision support systems (DSS) to target fungicide application.
• Setting robust thresholds for intervention according to region or varieties.
• Prioritising the use of biological, physical or other non-chemical methods in favour of chemical methods.
• Pesticides applied should be specific for the target and have minimal side effects on non-target organisms and the environment. The use of appropriate nozzles to maximise efficacy and reduce off-target spray to the environment is often essential to comply with LERAP restrictions.
• Where the risk of resistance has been identified, strategies which include the use of multiple pesticides with different modes of action are required.
• Effective control of tuber blight to avoid storage losses and the carry-over of primary inoculum to the following season via infected seed.
• Strategies should be reviewed to evaluate the success of the measures employed and identify areas for improvement.

While accepting these principles are the basis for an IPMP, some have little application in the control of potato blight. In view of the speed of disease spread and its devastating impact on crop output and the need for chemical intervention, methods which focus on disease prevention are vastly preferred to curative options.

© Eric Anderson, Scottish Agronomy

Given the importance of preventing disease establishment, actions which inhibit or suppress blight by good hygiene and variety resistance are critical elements of control. In basic seed stocks the current tolerance for blighted tubers is 0.5% so the use of certified seed offers good protection from introducing blight on seed.

The key to good hygiene is preventing the carryover of disease from one year to the next, either in volunteers or waste potato dumps. In both scenarios, any blight infected mother tubers may initiate a disease outbreak which spreads to neighbouring potato crops. Even if the mother tubers are not carrying blight, the plants they generate are growing without fungicide protection and provide an easy entry point for disease and its rapid spread.

[Illustration 4] Sources of inoculum

The importance of waste potato dumps as potential foci for potato blight is widely recognised. In the Netherlands, national blight control regulations were introduced shortly after the turn of the millennium. As part of these measures, growers are required to cover dumps before 15 April with black plastic sheet throughout the growing season.

Similar recommendations are made in the UK and growers are encouraged to minimise the quantity of potatoes going into outgrade piles by diligent on-farm grading, better extraction of small tubers from any soil waste and keeping piles shallow to increase the chance of frost damage. If plastic sheeting is not an option, then dumps should be covered with enough soil to stop potatoes emerging.

Volunteer or groundkeeper potatoes are self-set tubers which have survived the winter from a previous crop and are then growing as a weed. Hard winters where the ground freezes to 10 to 20m depth greatly reduces the survival of tubers and avoiding deep cultivations keeps more unharvested tubers exposed on the surface.

Volunteer potatoes in a following carrot crop © Eric Anderson, Scottish Agronomy

In some situations, farmers have been able to graze livestock on fields after harvest to clean up unharvested tubers. The viability of unharvested tubers can also be markedly reduced by a chemical approach using maleic hydrazide (processing and ware potatoes only). Applied to the growing crop, it reduces the incidence and vigour of sprouting, but permission should be sought before use as it is not permitted in all market sectors.

Long rotations enable better control of volunteers and should not be tighter than one-in-five to prevent the development of many potato diseases. In the Netherlands and Nordic countries, despite acceptable volunteer control, close rotations of two to three years particularly in starch production areas are linked with a higher frequency of sexual reproduction and infection commonly appears to originate from resting oospores.

Unfortunately, either mild winters or burial by ploughing ensure that volunteers often persist for many seasons and there is an increasing risk of emerging potato plants being carriers of blight and acting as a primary infection source.

While volunteers are usually well controlled in sugar beet, they are often neglected in cereal crops.  Furthermore, most potato fields are rented for one year.  Volunteers are a particular threat and a frequent origin of disease outbreaks in nearby potato crops. Volunteer control is made even more difficult as they can emerge over several weeks from spring to early autumn.

Volunteer potatoes emerging in the post-harvest stubble of the following cereal crop  © Eric Anderson, Scottish Agronomy

In field crops, volunteer potato control is best achieved by either mechanical inter-row cultivation or a targeted herbicide programme. Several sprayer manufacturers offer systems that enable automated spot application for the control of volunteers in row crops, such as the SKAi system from SoilEssentials.

Another key cultural approach to reducing disease ingress and spread is varietal resistance. Since the arrival of blight in Europe, blight resistance has been a target of many variety-breeding programmes. The relative resistance of a variety to blight may change with the spread of new genotypes, so although breeders have introduced new sources of resistance, few have persisted.

The level of blight resistance provided by commercially dominant potato varieties ranges from low (1-3) medium (4-6) and high (7-9). Breeders publish resistance scores for each variety, but the emergence of strains with more aggressive characteristics means resistance ratings need to be regularly revised and this is not being conducted independently.

The potential for significant damage, even complete crop loss, means few growers are willing to use a variety’s resistance as a means to extend spray intervals or reduce fungicide rates. Not only is it complicated to manage a range of blight control strategies but there is also a lack of reliable variety resistance information in relation to prevalent genotypes.

A pragmatic approach is to use resistance ratings as a fallback under periods of high disease pressure or when spray intervals have been stretched. It then makes sense to give priority to the spraying of varieties with the weakest resistance first and likewise target these with the strongest available fungicide combinations.

The potato late blight pathogen Phytophthora infestans is capable of rapidly generating billions of spores and spreading as genetically uniform genotypes. Genetic fingerprinting tools have allowed the tracking of such genotypes and shown some to have persisted for decades. The success of these genotypes is due to their traits of aggressiveness; ability to infect and colonise host plant tissue and fitness; ability to spread within and between seasons. In a polycyclic disease such as potato late blight even slight changes in these traits can have a significant effect on their competitive ability.

Virulence profiles within a clone are highly complex with multiple biological profiles known within clones for virulence and aggressiveness traits.    Differences in pathogenicity traits between clones are roughly correct at the population level but are absolutely non-significant at the isolate/individual level.   Genotypes are a poor predictor of phenotype pathogenicity.  For population monitoring phenotyping is still cumbersome and slow, but absolutely needed.  Further research is required on predictive markers to screen large number of samples as factors other than pathogenicity are important for fitness, such as the ability to overcome cultivar resistance genes.

The ability to overcome the resistance of commonly grown potato cultivars or having reduced sensitivity to important fungicides also shapes the population and affects the success of blight management. The industry-sponsored ‘Fight Against Blight’ campaign has supported blight sampling by disease scouts in British since 2003, along with the identification of genotypes so the makeup and development of the blight population can be tracked and growers advised as to how the threat to crops is evolving.

Results of the EuroBlight potato late blight monitoring, which tracks the emergence and spread of strains across Europe, are published annually at euroblight.net. The findings serve as the basis for developing an integrated management plan and actions to reduce the risk of further fungicide resistance.

Blight Monitoring

Blight monitoring and forecasting systems have made a positive impact on blight control decision making and the sophistication of support systems continues to progress. In the UK, growers can receive warnings of warnings of expected blight pressure based on the Hutton Criteria while samples can be submitted for genotyping and analysis through the Fight Against Blight service.

Such blight warnings are a useful aid to understanding historical disease pressure, but more proactive advice is needed to ensure crops are protected in advance of risk periods. To achieve this, several commercial providers offer Decision Support Systems (DSS) which interpret local weather forecasts to provide a five-day view of both blight pressure and spray windows.

DSS are useful in guiding the timing and interval between blight sprays. In seasons with high and prolonged blight pressure, there may be periods when failure to make spray applications at five-to-seven-day intervals is critical to successful control.

Hutton Criteria

Temperature and relative humidity are critical to determining the risk of infection by late blight. The Hutton Criteria are a set of conditions which facilitate the spread of late blight, these are:

•         Two consecutive days with a minimum temperature of 10^oC, and
•         At least six hours of relative humidity at 90% or above

In some years, low blight pressure may persist over much of the growing season and there may be ample opportunity to use less expensive fungicides and possibly reduce rates. Importantly, DSS must foresee changes in local disease pressure and give opportunity to practically react. Blight pressure can change in hours and the ability to respond at any stage of the growing season is critical.

While blight has demonstrated a tremendous ability to evolve and overcome varietal resistance it has also been effective in developing resistance to fungicides, especially where these have been repeatedly applied as a single mode of action.

Growers across Britain have typically favoured tank mixing or alternating of blight sprays to prevent over reliance on a single active substance or group of actives. Alternating blight products allows greater flexibility to tighten intervals under periods of highest blight pressure and enables different modes of action to be exploited to build up protection in the canopy.

•         For guidance on developing a resistance management policy, see the next section: Characteristics of late blight fungicides. 

The success of your IPM strategy should be reviewed at the end of the season and fed into the farm audit report. The key elements of this review should be blight pressure, fungicide usage and blight incidence.

Blight fungicide inputs should be positively related to the blight pressure experienced over the growing season. Periods of the season with high blight pressure should have more intensive applications and some years will be markedly different to others. Any outbreaks of blight should be diagnosed in relation to both disease source and any inherent weakness in the fungicide control programme. This repeated process is essential to monitor the success of crop protection measures and identify areas for improvement.

Fungicides play a key role in the integrated control of late blight. The threshold for late blight is zero so blight control strategies are primarily preventive by spraying fungicides when weather conditions are conducive to blight and the crop is no longer fully protected by the previous spray.

The protection conferred by a spray decreases as its active substances degrade over time and as new unsprayed leaves grow. Spray timing and interval therefore depend on the characteristics of the fungicide, the growth of the crop and the variety determinacy, weather conditions and disease pressure.

EuroBlight, the potato late blight network for Europe, produces the EuroBlight Fungicides Table which summarises fungicides’ characteristics to provide agronomists and growers with an independent scientific basis for selection.

This section explains the characteristics listed on the EuroBlight table (www.euroblight.net) and thereby provides the understanding needed to select fungicides for blight control strategies.

Leaf blight

Protection of leaves against blight infection by either direct contact or via translaminar or systemic activity.

Leaf blight © Eric Anderson, Scottish Agronomy

Tuber blight

Activity against tuber infection as a result of fungicide application after infection of the haulm, during mid- to late-season, i.e. where there is a direct effect on the tuber infection process. Three characteristics are important in preventing tuber blight:

• Killing spores that are washed from the leaves to the tubers.
• Reducing lesion size causing less spores to be formed.
• Anti-sporulant efficacy to reduce the number and viability of spores.

Tuber blight © Eric Anderson, Scottish Agronomy

New growth

The ratings for the protection of new growth indicate the protection of new foliage by systemic or translaminar movement or the redistribution of a contact fungicide. New growth consists of growth and development of leaves present at the time of the last fungicide application and/or newly formed leaves that were not present. Besides translaminar fungicides some new contacts that are taken up in the leaf wax layer can protect new growth as they are effective at low concentrations providing applications are maintained at seven-day intervals.

Stem blight

Protection of stems by either direct contact or via translaminar activity.

Stem blight © Eric Anderson, Scottish Agronomy

[Illustration 5] Biological efficacy of fungicides

Protectant

Spores killed before or upon germination or penetration. The fungicide has to be present on or in the leaf or stem surface before spore germination or penetration occurs.

Curative

The fungicide is active against the pathogen during the immediate post infection period but before symptoms become visible i.e. during the latent period.

Anti-sporulant

Lesions are affected by the fungicide decreasing sporangiophore formation and/or decreasing the viability of the sporangia formed.

Contact

Contact fungicides are not taken into the plant and are therefore more vulnerable to erosion by wind, rain and sunlight. Some new contacts are taken up in the leaf wax layer. They protect where the spray has been deposited and can protect new growth.

[Illustration 6] Contact fungicide

Translaminar

Translaminar fungicides are taken up by the leaf and show limited redistribution from one leaf surface to the other, e.g. from upper sprayed surface to lower unsprayed surface. They are only transported locally within the plant and can provide partial protection of new growth.

[Illustration 7] Translaminar fungicide

Systemic

Systemic fungicides are taken up by the foliage and redistributed upwards by the xylem vessels. They therefore have the potential to protect new foliage growth formed between fungicide applications.

Often the mobility of a fungicide is used to justify its positioning in a strategy. However, biological efficacy should be the main driver of product choice. In this it is important to realise that all fungicides, whether they be contact, translaminar or systemic, are all protecting the plant. The contacts cannot penetrate plant tissues and therefore do not have curative or anti-sporulant efficacy.

[Illustration 8] Systemic fungicide 

A more detailed appreciation of the characteristics and properties of fungicides is needed to design an effective blight control strategy that is well adapted to the conditions of the particular crop. The EuroBlight  blight ratings provide such information. Efficacy to control leaf and tuber blight is tested in EuroBlight field trials.  Ratings of the other characteristics are decided by the Fungicides Sub-group – independent scientists and representatives from the crop protection industry – on the basis of available data.

The risk of resistance development is a combination of the inherent pathogen risk, the agronomic risk and the inherent fungicide risk. The Fungicide Resistance Action Committee (FRAC) rates the inherent risk of P. infestans developing resistance to fungicides as medium and the agronomic risk as high because of numerous sprays per season.

The inherent risk of the phenylamide fungicides (e.g. metalaxyl-M) is rated as high. Shortly after their introduction in the 1980s, the late blight pathogen developed resistance.  Resistance management at the time consisted of three important measures; restricting the use to only several sprays per season; co-formulating with contact fungicides; and limiting the use to protectant sprays and not using it as an eradicant.

Depending on the mode of action of fungicides, resistance risk can be rated low or high (see table).

FRAC codes and resistance risk of the most important active substances for protection against late blight

Source: FRAC and updated where necessary to reflect latest advice

Fungicides with a medium to high resistance risk need a resistance management strategy that involves mixing modes of action and avoiding the sequential application of products containing the same mode of action to prevent the development of resistance.

Anti-resistance strategy

The late blight population in Great Britain and Europe has proved to be adept at evolving to:

•         Overcome host resistance genes in potato cultivars (virulence)
•         Reproduce faster or under a wider range of environmental conditions (Aggressiveness)
•         Overcome fungicides (resistance) 

There are 12 modes of action available for late blight control and products vary in the way they protect against infection.

Some protect new growth whereas others have differing protectant, curative and anti-sporulant properties. Despite a range of modes of action available for late blight control, implementing resistance management strategies can be difficult, particularly where product options to target a specific stage of the lifecycle are limited, for example when seeking to protect against tuber blight.

To reduce the risk of resistance developing, the Fungicide Resistance Action Committee (FRAC) makes recommendations for use according to the mode of action group.  For late blight control in potatoes, growers are heavily reliant on two mode of action groups: Carboxylic Acid Amides (CAA) and Oxysterol Binding Protein Homologue Inhibition (OSBPI). A third group, Quinone inside Inhibitors (QiI) are also widely used but the risk of resistance is of less concern.

For CAA fungicides, such as those containing either benthiavalicarb, dimethomorph, mandipropamid or valifenalate the following recommendations apply:

•         Apply CAA fungicides preferably in a preventive manner.
•         Apply a maximum of 50% of the total number of intended applications and no more than six in total for late blight control.
•         Apply CAA fungicides always at recommended dose rates.
•         Apply CAA fungicides using not more than two consecutive applications.
•         Apply CAA fungicides preferably in mixtures with effective partners belonging to different modes of action.
•         Alternation with fungicides having other modes of action is recommended in spray programmes.
•         Good agricultural practices must be considered to reduce source of inoculum, disease pressure and resistance risk, e.g. consider planting  resistant varieties and refer to disease forecasting models.

The emergence of mandipropamid-resistant strain 43_A1 in Denmark in 2018 highlighted the need to protect CAA fungicides by applying in a mix with another mode of action partner. CAA fungicides typically account for more than half the products used in typical season, so their efficacy must be protected.

For OSBPI fungicides, such as those containing oxathiapiprolin, the following recommendations apply:

•         Apply oxathiapiprolin only preventatively and in mixtures with effective fungicides from different cross-resistance groups.
•         Oxathiapiprolin products should not be use more than three times in sequence but can be used in alternation with fungicide of a different mode of action.
•         Fungicide programmes must deliver effective disease management.
•         Apply oxathiapiprolin only preventatively and in mixtures with effective fungicides from different cross-resistance groups.

For QiI fungicides, such as those containing either cyazofamid or amisulbrom, users are advised to follow resistance management protocols by ensuring both products are mixed with a suitable partner belonging to a different mode of action group.

•         Apply QiI fungicides in a preventative manner.
•         Apply a maximum of 50% of the intended applications per crop cycle.
•         Do not apply for more than three consecutive applications.
•         Alternate with fungicides belonging to a different mode of action group.

Zoxamide use recommendations

•         Do not exceed a maximum of six applications or 50% of the intended number of applications per crop cycle whichever is lower.
•         Apply no more than two consecutive applications zoxamide-containing products.

Ametoctradin use recommendations

•         Apply ametoctradin-containing products in a preventative manner.

Fluopicolide use recommendations

•         Apply a maximum of four fluopicolide-containing sprays but not more than 50% of the intended number of sprays per season.
•         Do not apply more than two consecutive applications.

A blight strategy is a plan of how you to intend to approach the protection of a potato crop against blight. It should contain IPM actions – based on the eight principles described in the section 2: Integrated Pest Management – and crop protection options to be deployed as the season unfolds. Here we focus on how to select crop protection options appropriate to crop growth stage and blight risk.

Previously the industry talked about a ‘blight programme’ rather than a ‘blight strategy’. The change of phraseology acknowledges that coming up with a precise plan of sequenced sprays before seeing what the season brings is inflexible and potentially risky and/or wasteful.

Think of a blight programme as the result of a blight strategy. A programme is a record of the crop protection sprays applied that were selected from the blight strategy. It is a useful record to review in the light of the outcome to refine future seasons’ strategies.

Potato crops grow through three distinct phases each of which has different needs for protection. In this section these needs are described, and the development of blight strategy is discussed with reference to the characteristics of blight fungicides explained in section 3: Characteristics of late blight fungicides.

Nozzle choice

The need to control drift has led to the advent of nozzles featuring drift reduction technology (DRT). Sadly, the lack of clarity around application practices, the inconsistency in requirements – the same active substance can incur different buffer requirements depending on the crop being sprayed – and the fact that most advice for spraying potatoes is extrapolated from data generated in the spraying of cereal crops, increase the risk that operators will fail to comply with regulation.

Drift reduction nozzle © Eric Anderson, Scottish Agronomy

Certain plant protection products have an aquatic buffer zone requirement when applied by horizontal boom or broadcast air-assisted sprayers. If you want to reduce this aquatic buffer zone, there is a legal obligation to carry out and record a Local Environment Risk Assessment for Pesticides (LERAP). For horizontal boom sprayers it is only possible to reduce buffer zones of 5 metres; buffer zones of greater than 5 metres cannot be reduced.

Some products specify use of DRT recognised as having LERAP three-star low-drift status and a buffer zone of 6, 12 or 18 m (as necessary for each crop) as a condition of authorisation for horizontal boom spraying. The specified distance must be recorded in Section A of the LERAP record form.  Authorisations issued under these arrangements also specify a second buffer zone of 30 m, beyond which use of DRT is not required. This is necessary to protect watercourses from higher rates of drift arising from use of standard spraying equipment and procedures. These distances cannot be reduced under the LERAP scheme.

Additional guidance can be accessed at the HSE website here.

Most plant protection products suggest a minimum LERAP three-star nozzle is used. These ratings, however, typically only apply at pressures below field rates. Consequently, the general advice is to trade up to a four-star nozzle, which at field rates – usually between 2.1 and 4.0 bar – are rated as three-star for drift reduction.

An up-to-date list of LERAP three- and four-star nozzles can be found on the Health & Safety Executive website here.

Early crop canopy development is from emergence to the end of the rapid growth phase when the canopy is complete. The crop then moves into the mid-season phase. When this ends is less clear-cut and depends on variety and target lifting date. Generally, the transition from mid- to late-season is at the point where the crop is likely to need just two or three more sprays. Late season is from this point up to complete cessation of green stems or haulm.

Early season protection

[Illustration 9]

Previously the advice was to begin protection when plants met along rows or when the first warning of risk occurred, whichever was the earlier. With the possibility that blight populations may now be active at temperatures below the 10°C Hutton criterion and that infection can begin without a Hutton Period occurring we have to be alert to local conditions from emergence. All it may take is soil moisture or humidity to initiate early season sporulation.

A major factor in successful blight control is to start early enough. The first spray should be applied either at the rosette stage, when there is sufficient crop to intercept the spray.  Ideally, an in-field weather station will be in place to provide live data on potential blight conditions in the crop.  Where live data is not available, a Hutton Period from a nearby synoptic weather station should serve as an indication of blight risk.

At the rosette stage, 90% of a field’s surface area is bare soil so for the first spray it is appropriate to go for an economical protectant.

Rosette stage  © Eric Anderson, Scottish Agronomy

If there is a risk that blight has been introduced on seed, then it is appropriate to use a fungicide with activity on zoospores, e.g. fluazinam or cyazofamid in a mix with mancozeb or ametoctradin. A blight-infected seed stock carries a higher risk of non-emergence (blanking) or earlier expression of blackleg symptoms.

Early black leg symptoms  © Eric Anderson, Scottish Agronomy

Thereafter, it will be necessary to protect rapid canopy growth with sprays that have good ratings for protection of new growth.

Products used at this stage should be systemic or alternatively bind strongly to the leaf wax layer so as the buds open and develop, they carry the active substance to new growth.

If forced to apply shortly after a high-risk period, it is advisable to add a tank mix partner with anti-sporulant activity. It is also important to use angled nozzles to maximise crop canopy spray penetration and coverage.

[Illustration 10]

Crops are most able to tolerate disease during mid-season, so this is the time to take advantage of periods of low risk to make savings with some economical protectant products. However, it is important not to reduce rates and to stick resolutely to seven-day intervals. At this time, consideration should also be given to countering the risk of tuber infection. This begins at tuber initiation which can be from just two to three weeks after emergence. Even through a low-risk mid-season period, including at least one spray with tuber blight activity will start building in tuber protection and pay dividends later.

If visual blight on stems or leaves is found in a crop, or one nearby your crop, products with anti-sporulant activity should be selected. Early infections on the lower stem are often overlooked but once spores from a stem lesion infect leaves, the disease becomes obvious.

The late blight fungicide table located on the Euroblight website, lists those active substances and active substance combinations with good ratings for foliar blight and anti-sporulant activity.

The aim is always to prevent the onset of foliar blight for as long as possible while acknowledging that prevention is unlikely in a high-pressure season and at some stage infection will get into the crop and have to be dealt with. Typically, this occurs during the mid-season phase of growth but in lower pressure years it can be staved off until late season.

Fungicide products vary in their curativity (also described as ‘kickback’). In general, the curativity period will be shorter in susceptible varieties where temperature is higher – 20 to 23°C rather than 10 to 15°C – and/or infection is caused by a more aggressive genotype.

With the arrival of 6_A1 and 13_A2 blight genotypes in the 2004 and 2005 respectively, together with 36_A1 in 2017, curative activity was observed to be reduced compared with what was seen historically because these strains exhibited a faster life-cycle than was seen in the strains they displaced. In practice, the curative activity may be less than 24-hours.

In a small study the curative fungicide activity of cymoxanil was found to be reduced when an isolate (13_A2) with shorter latent and infection periods was compared with a less aggressive genotype (8_A1). This observation served to demonstrate that some mixing partners, namely cymoxanil which is popular among growers for its curative activity, is limited in its practical value. It is a similar situation for dimethomorph. It has an even shorter eradicant period than cymoxanil but is persistent in the canopy for seven days compared with three for cymoxanil. Neither cymoxanil nor dimethomorph should be applied unless in a mixture with a partner belonging to another mode of action group.

Most product labels do not allow for an interval of less than seven days, however, by alternating products, intervals can be reduced to three to five days. The best advice is to maintain shortened intervals and alternate products with different modes of action until actively sporulating blight lesions dry up.

During the late season phase of growth, the priority is protection of tubers so for the last two to three sprays, products with good tuber blight ratings should be used. If blight escalates and is active in the crop anti-sporulant activity is needed in addition to tuber blight activity to reduce the amount of viable inoculum in the crop. The Euroblight fungicide table assigns a rating to those active substance combinations that deliver good anti-sporulant with tuber blight activity.

The stewardship conditions enforced by some buyers limit the application of certain active substances. This is the case for propamocarb, so it is worth checking with your buyer before the season starts as such conditions can limit the number of applications that can be used to less than that stated on the product label.

Continuing to keep crops growing when they have active foliar blight puts daughter tubers at a high risk of infection. The rare exception is when there is no significant rainfall until after target tuber sizes are achieved and haulm is completely desiccated.

Blighted progeny tubers are more likely to rot away prior to harvest, especially if infection is earlier in the tuber bulking phase of the growing season and conditions favour secondary bacterial activity, i.e. soils are wet and warm. If blight infection of tubers is late in the growing season, then the risk of secondary bacterial soft rot in store will generally be higher.

The harvesting process can be a very effective way of introducing tuber infection as the mechanical mixing of soil and tubers rubs spores into them. Key precautions to minimise risk of infection at harvest are:

•         Wait until haulm has been dead for at least 14 days.
•         Ensure there is no re-growth after desiccation; it is particularly susceptible to blight infection.
•         Maintain appropriate spray intervals until all haulm is completely dead.