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Virus management in seed potatoes 2018

18 May 2018

 

Protecting seed crops from the various aphid-borne potato viruses is essential to maintain the quality of Scottish seed potatoes.

Multiplication of early seed potato generations will require a robust approach to aphid-borne virus management to avoid viruses entering the seed stock – this will require a comprehensive aphicide programme as detailed below. Later seed potato generations can take a more relaxed approach, particularly later in the growing season when mature plant resistance may also be occurring. 

There are various sources of virus that aphids can pick up the virus from, and these include:

  • The mother seed stock – i.e. the seed crop planted in the field
  • Other seed and ware crops in proximity to the crop
  • Groundkeepers/volunteers in potato and non-potato crops
  • Potato dumps where haulm growth is present

Aphids will acquire virus from the above sources if they contain virus infected plants, and both non-potato colonising and potato colonising aphids play a significant role in virus transmission.

There are two groups of aphid-borne viruses that are of concern in seed potatoes; persistent viruses (such as Potato leaf roll virus - PLRV) and non-persistent viruses (e.g. Potato virus Y – PVYO, PVYN, Potato virus A – PVA, Potato virus V – PVV).

The persistent virus PLRV resides in the phloem sap of plants and is acquired from infected plants by aphids feeding on the plants and ingesting sap. The ability of an aphid to transmit PLRV is then delayed for several hours because the virus has to pass through the digestive system of the aphid and enter its saliva before transmission can occur. Consequently only potato colonising aphids will be transmitting PLRV. Once the virus has been acquired by an aphid, it remains infective for the rest of the aphid’s life.

The non-persistent viruses PVYO, PVYN, PVA and PVV are rapidly acquired when an aphid feeds on an infected plant because these viruses, which reside in most plant tissues including the leaf epidermal cells of plants, are carried on the aphid mouthparts. Non-persistent viruses can be passed on to another plant within a few minutes during aphid feeding. Consequently, winged aphids which briefly probe plants to determine whether they are suitable host-plants and then move onto another plant, have the potential to spread these viruses quickly within the crop. These winged aphids may be non-potato colonising or potato colonizing aphids, and several non-potato colonising aphid species are involved in PVY virus transmission.

Varietal propensity for virus

The term ‘varietal propensity’ has been adopted to describe whether virus symptoms observed within a variety are above or below the average across the whole Scottish seed crop. Whether a variety has a propensity to PLRV or to PVY can be used to determine the appropriate means of protecting the crop through an aphicide programme for the appropriate aphid vector species. For example, aphicide programmes could be less intensive for varieties that have a low propensity for virus infection. Propensity should also be considered in any planting programme as there will be advantages in ensuring that varieties with a propensity to say, PVY, are planted away from crops which are considered a likely source of inoculum for that virus.

Full details on varietal propensity can be found at https://www.sasa.gov.uk/seed-ware-potatoes/virology/varietal-propensity-...

Prioritising virus management

Aphicide treatments should be prioritised towards high priority crops, e.g. field generations 1 to 4, and varieties with a high propensity to prevalent viruses. Crops which will not be used for further seed production, particularly field generations 6 and 7 could be considered as low risk.   Another factor to consider would be to also prioritise crops in which virus symptoms were recorded in the previous growing season. Growers should have access to this information, particularly if they had grown the crop in the previous season.  If a seed stock has been purchased from another producer, then that producer’s permission will be required to access crop inspection data for the parent crop.

Removing sources of virus

Growing seed crops in an environment in which all sources of aphid-borne viruses are kept to a minimum is a key consideration in the production of healthy seed potatoes. This includes sources within the seed crop as well. High quality virus-free seed should be sourced to minimise the risks of spread from infected plants within the crop, supported by roguing of any virus infected plants and groundkeepers at an early stage – preferably by the beginning of June - before aphid vectors of non-persistent virus are flying. Risks from outwith the crop should be addressed by attempting to isolate seed crops from potential external sources of virus, and by minimising the sources of virus available for aphids to pick virus up from. Adjacent fields should be checked for the presence of infected groundkeepers/volunteers and action taken as appropriate. Neighbouring ware crops should be as free from virus as possible - the use of untested home-saved seed to grow ware can increase the risk. Preventing haulm growth on dumps is also important in limiting the opportunities for flying aphids to pick up virus and carry it into seed crops.

Roguing of groundkeepers/volunteers from non-potato crops, and potato plants exhibiting virus symptoms from ware and seed crops is an essential component of virus management in seed potatoes – aphids have to pick up virus from somewhere.

Information on aphids

The cumulative appearance of aphids throughout the season can be monitored from the AHDB network of aphid water traps (http://aphmon.fera.defra.gov.uk/) and the UK network of aphid suction traps at the following links:-

https://cereals.ahdb.org.uk/monitoring/aphid-news/aphid-news-archive-(2016-2018).aspx

http://resources.rothamsted.ac.uk/insect-survey/bulletins

http://www.sasa.gov.uk/wildlife-environment/aphid-monitoring/aphid-bulle...

This information can be used as an early warning system for general areas and growers are encouraged to use data from both the water traps and the suction traps to gauge the threat from aphids and virus in their area.

Aphids that carry and transmit non-persistent viruses include non-colonising as well as potato-colonising species, and they can acquire and transmit virus to potatoes if they land and probe on a potato leaf without producing colonies on the leaf. These non-colonising aphids include the bird cherry–oat aphid, the grain aphid, the rose–grain aphid, the apple–grass aphid, willow-carrot aphid. The aphids that can colonise and multiply on potatoes include the peach–potato aphid, potato aphid and glasshouse–potato aphid.

Consequently, the numbers of key aphids caught in the aphid water traps and suction traps should be used as a guide to the risk of aphid movement into potato crops, and the beginning of the aphicide treatment programme should be based on the appearance of the key aphids below, rather than specific potato–colonising aphids. 

Key aphids

Once any of the following aphids begin to appear in local water traps or suction traps then there is risk of transmission of non-persistent viruses.

  • Myzus persicae - Peach–potato aphid
  • Acyrthosiphon pisum - Pea aphid
  • Rhopalosiphum padi - Bird cherry–oat aphid
  • Aphis nasturtii - Buckthorn–potato aphid
  • Metopolophium dirhodum - Rose–grain aphid 
  • Brachycaudus helichrysi - Leaf-curling plum aphid
  • Rhopalosiphoninus latysiphon - Bulb and potato aphid 
  • Myzus ornatus - Violet aphid 
  • Myzus ascalonicus - Shallot aphid 
  • Macrosiphum euphorbiae - Potato aphid 
  • Aulacorthum solani - Glasshouse–potato aphid
  • Hyperomyzus lactucae - Currant–sowthistle aphid
  • Aphis fabae – Black bean aphid 
  • Sitobion avenae - Grain aphid 
  • Brevicoryne brassicae - Cabbage aphid
  • Cavariella aegopodii – Willow-carrot aphid

Note that those in bold are the aphid species that can colonise potatoes, but all the aphids listed above can transmit non-persistent viruses.

Aphicide treatments

There are now several aphicides that can play a role in virus management in seed potato crops (see Table 1). However, peach–potato aphids resistant to pyrethroid aphicides (esfenvalerate and lambda-cyhalothrin), are now in the majority in Scottish potato and vegetable crops, so correct choice of aphicide is crucial to minimise the risk of aphicide-resistant aphids developing on crops and consequently transmitting virus. A spray programme that alternates differing chemical groups (and in a tank-mix when key aphids appear) is recommended to minimise the transmission of virus by aphids, and reduce the risk of resistance.

There is evidence that the use of mineral oil (e.g. Cropspray 11E, Headland Fortune) alone or in a mixture with some aphicides reduces the transmission of non-persistent viruses. Reliance on mineral oil alone as an aphicide would be at growers’ risk. Note that mineral oils are not compatible with the potato blight fungicides Ranman Top, Shirlan, or the aphicide Biscaya. Trials have demonstrated that the mixture of oils with the blight fungicides Invader, Percos and Revus improved levels of blight control.

  • Mineral oil can be applied up to tuber initiation (Cropspray 11E, Headland Fortune).
  • A rapid knock-down pyrethroid component (lambda-cyhalothrin or esfenvalerate) is recommended before peach-potato aphids or grain aphids appear as the chemical has a rapid kill of aphids and also acts as a deterrent to aphid probing of treated leaves, thus reducing the risk of virus being acquired and transmitted. This aphicide specifically targets the transmission of non-persistent viruses (i.e. PVYN, PVYO, PVA, PVV). When peach-potato aphids or grain aphids appear on crops, relying on pyrethroid aphicides alone is not recommended, as these aphids may well be resistant to this group of aphicides.
  • A translaminar component (acetamiprid, pymetrozine, flonicamid, thiacloprid, thiomethoxam, spirotetramat) which is taken up by the leaf and kills aphids that feed on the leaves is recommended, particularly when peach-potato aphids or grain aphids appear.

No more than two applications of a neonicotinoid product (thiamethoxam, acetamiprid, thiacloprid) can be applied in a season, and are not to be used in consecutive sprays.

Of the aphicides listed above, thiamethoxam, acetamiprid, pymetrozine, flonicamid, thiacloprid and spirotetramat are the only products to which peach–potato aphids and grain aphids have not shown any resistance, so they are an essential component of an anti-resistance strategy against these aphids, and should be used when peach-potato aphids or grain aphids have been found in aphid traps.

Aphid monitoring surveys in peach chards in France and Spain have revealed that some populations of the peach-potato aphid (Myzus persicae) have developed high levels of resistance to neonicotinoid insecticides. To reduce the risk of resistance arising in the UK no more than two applications of a neonicotinoid product (thiamethoxam, acetamiprid, thiacloprid) should be applied over a season to an individual seed crop, and that the first sprays in a programme targeting peach-potato aphids should use a non-neonicotinoid such as flonicamid, pymetrozine or spirotetramat.

 

The following aphicides are approved for use on seed potato crops in 2018:

Aphicide group

Active ingredient(s)

Products

Max  No. of applications of each product

Pyrethroid

esfenvalerate

Barclay Alphasect, Clayton Cajole, Clayton Slalom, Clayton Vindicate, Gocha, Kingpin, Sumi-Alpha, Sven

4

Pyrethroid

lambda-cyhalothrin

Clayton Lambada, Clayton Lanark, Clayton Sparta, CleanCrop Argent, CleanCrop Corsair, CM Lambaz 50 EC, Colt 10 CS, Dalda 5, Eminentos 10 CS, Euro Lambda 100 CS, Hallmark with Zeon Technology, Hockley Lambda 5EC, IT Lambda, Karis 10 CS, Kendo, Kusti, Laidir 10 CS, Lambda 100 CS, Lambda-C 100, Lambda-C 50, Lambdastar, Life Scientific Lambda-Cyhalothrin, Life Scientific Lambda-Cyhalothrin 50, MAC Lambda Cyhalothrin 50 EC, Markgate 50, Martlet, Ninja 5CS, Ozmoze 50 CS, Reparto, RouteOne Lambda C, RVG Lambda-cyhalothrin, Seal Z, Segeant Major, Sparviero, Stealth, Triumph, Triumph CS, Warrior

4

Neonicotinoid

thiamethoxam*+

Actara, Leptom 250 WDG

2 (1 on ware)

Neonicotinoid

acetamiprid*

Insyst, Persist

2 (1 on ware)

Neonicotinoid

thiacloprid*

Biscaya, Pintail, Scabiya, Standon Zero Tolerance, Zubarone

2 (1 on ware)

Azomethine

pymetrozine*

Plenum WG, Quorum, Rozine, Standon LBW

3 (2 on ware)

Pyridine carboxamide

flonicamid*

Teppeki

2

tetramic acid

Spiroteramat**

Movento**

4

*  These aphicides have translaminar activity

** Application only after flowering in those varieties which produce flowers

+ On 27 April 2018, the Standing Committee on Plants, Animals, Food and Feed met to vote on further amending the conditions of approval for three neonicotinoid active substances, imidacloprid, clothianidin and thiamethoxam. Following conclusions delivered in a report by the European Food Safety Authority on 28 February, the Committee voted to adopt restriction of the three substances to greenhouse use only. All outdoor use will be banned. CRD will publish the details of the changes to the authorisations on their website in due course.

The first aphicide treatments

The first treatments should begin when key aphids (see those listed above) appear in traps or on the crop. Growers should regularly check the aphid trap websites listed above for the presence of the key aphids in local water traps and suction traps.

Note that it is predicted that the first catch of peach-potato aphids in 2018 in the Dundee aphid suction trap is 23rd June, and at Gogarbank (Edinburgh) on 21st June.

Non-colonising aphids such as the cereal aphids may appear from mid-May.

Mineral oils have been shown to be effective at reducing virus transmission but note that the use of these oils is at growers' risk.

The pyrethroid aphicides can be used alone (or with mineral oil) up to the point when peach-potato aphids or grain aphids appear on crops or in traps, after which they should be used in a programme every 7 days, with a tank-mix of thiamethoxam, acetamiprid, pymetrozine, thiacloprid, flonicamid or spirotetramat every 14 days. Note that thiamethoxam, acetamiprid, pymetrozine, thiacloprid, flonicamid or spirotetramat should not be used in consecutive sprays. i.e. thiamethoxam followed by thiamethoxam and so on.  Also note that spirotetramat cannot be used before or during flowering.

To combat potential neonicotinoid resistant peach-potato aphids it is recommended that when peach-potato aphids begin to be found in traps or on the crop, flonicamid, pymetrozine or spirotetramat be applied early in the spray programme before any of the neonicotinoid products.

Bear in mind that no more than two applications of a neonicotinoid product (thiamethoxam, acetamiprid, thiacloprid) can be applied over a season to an individual seed crop.

Treatments should be maintained at 7 day intervals (pyrethroid ± oil) with pyrethroid + thiamethoxam, acetamiprid, pymetrozine, thiacloprid, flonicamid or spirotetramat at 14 day intervals, and continue until burning down. Be aware of any regrowth after burning down as this may be attractive to aphids and potentially infected with virus, so aphicide treatments on regrowth are recommended.

Ware crops

Virus management on ware crops is generally not necessary unless the ware crops are sited near to seed crops where they may act as a source of both virus and aphids. The removal of virus affected plants is recommended, and aphid control is suggested once aphids begin to appear. On ware crops isolated from seed crops, aphid control is only necessary when aphid numbers on the crop show a significant increase between two monitoring dates; e.g. 4 per leaf on one day and 12 per leaf the following date of monitoring. If aphid numbers remain relatively static or low then no treatment is necessary. Any of the aphicides listed above can be used on ware crops, but it is preferable to avoid use of pyrethroid insecticides on ware crops due to aphicide resistance in peach-potato aphids.

Summary

•          Assess propensity of variety to PLRV or to PVY https://www.sasa.gov.uk/seed-ware-potatoes/virology/varietal-propensity-...

•          Early roguing (by early June) of potato plants exhibiting virus symptoms from ware and seed crops, and groundkeepers from non-potato crops is an essential component of virus management in seed potatoes

•          The cumulative appearance of aphids throughout the season can be monitored from the AHDB  network of aphid water traps (http://aphmon.fera.defra.gov.uk/) and the UK network of aphid suction traps:

https://cereals.ahdb.org.uk/monitoring/aphid-news/aphid-news-archive-(2016-2018).aspx

http://resources.rothamsted.ac.uk/insect-survey/bulletins

https://www.sasa.gov.uk/wildlife-environment/aphid-monitoring/aphid-bulletins

•          This information can be used as an early warning system for local areas, and growers are encouraged to use data from both the water traps and the suction traps to gauge the threat from aphids and virus in their area

•         Aphicide treatments should begin when key aphids are found in traps, and continue every 7 days until burning down of the crop.

•          When peach-potato aphids or grain aphids appear in traps or crops, include thiamethoxam, acetamiprid, pymetrozine, thiacloprid, flonicamid or spirotetramat in your programme every 14 days

•          Consider flonicamid or pymetrozine as the first partner to a pyrethroid when peach-potato aphids appear in traps or crops

•          Monitor regrowth after burning down and consider an aphicide to prevent virus transmission

Guidelines issued by the Scottish Aphid-Borne Virus Working Group, May 2018

The working group that has provided this advice contains representatives from: SRUC, Scottish Agronomy ltd., SASA, James Hutton Institute, AHDB and McCain.

 

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