Virus management in seed potatoes, May 2019

Protecting seed crops from the various aphid-borne potato viruses is essential to maintain the quality of Scottish seed potatoes, particularly in light of the shrinking availability of aphicides for effective virus management. This is the last season that the active substance pymetrozine can be used.

Maintaining high quality seed health now will help protect future crops when we lose the current armoury of aphicides by reducing the level of virus inoculum available to aphids. There was an unwelcome increase in the incidence of crops containing potato leaf roll virus and potato virus Y last season, which has increased the risk of virus infected stocks being planted in 2019 and subsequent spread of virus into the 2019 seed crop.

A conservative approach to maintaining high health seed stocks is recommended through 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 cautious approach, particularly later in the growing season when mature plant resistance may also be occurring, but be aware of the aphid (and virus) pressure and the propensity of some potato varieties for both leaf roll and virus Y.

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 the 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 leafroll 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-virus-infection

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 the non-persistent virus are flying. Risks from outwith the crop should be addressed by attempting to isolate seed crops from potential external sources of the 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 the 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 the 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 the 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://ahdb.org.uk/aphid-news
https://insectsurvey.com/aphid-bulletin
http://www.sasa.gov.uk/wildlife-environment/aphid-monitoring/aphid-bulletins

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 the 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.

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.