Northern Ireland’s seed potato growers have been advised to be on the lookout for aphids and to spray crops to prevent the spread of virus diseases. In this report Robin Bolton, Senior Crops Development Adviser, CAFRE and Dr Stephen Jess, Plant Health and Environment Branch, AFBI, outline the basis behind this advice.

Protecting seed potato crops from aphid-transmitted viruses is an increasing challenge for the potato industry, both locally and internationally. To assist Northern Ireland seed growers in meeting this challenge, crop certification inspectors from DARD have continued to trap aphids at four locations within high grade basic seed potato production areas.

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The samples collected have been analysed by AFBI and the results indicate that, owing to the relatively mild winter, large numbers of virus transmitting aphids have been recorded.

Alerts of high aphid numbers have already been issued to seed potato growers by the crops team at CAFRE, informing them of the need to spray emerged crops urgently with insecticides to prevent the spread of virus and maintain the health of high-grade stocks.

In Northern Ireland, Potato Virus Y (PVY) is the most damaging of the aphid-transmitted viruses. It is a non-persistent virus that is transmitted rapidly when infected aphids probe plants with their mouthparts to feed. Consequently, both colonising and non-colonising aphids may transmit PVY.

Most susceptible at emergence

Potato crops are most susceptible to virus infection at emergence and it is essential that seed crops are sprayed with insecticides during the early part of the season, particularly during mild, dry spells. Do not wait until aphids are seen on leaves. Action must be taken before aphids are found colonising the leaves. In Northern Ireland, PVY is transmitted by aphids migrating across crops and the damage may have occurred before aphids are found on the leaves.

Varieties with high resistance to PVY, for example Navan, are unlikely to benefit from applications of insecticide (refer to Potatoes, Varieties for Northern Ireland 2011 http://www.afbini.gov.uk/12816AFBI PotatoVarieties.pdf). For varieties with low resistance to PVY, for example, Premiere, Maris Piper, Cultra, and Kerr’s Pink, or to viruses PVV and PVA, for example Desiree, the following aphicide programme is suggested:

Apply the first insecticide treatment at 80% crop emergence, when plants are most susceptible.

Repeat at 14-day intervals up to early August or three weeks prior to desiccation, whichever is earliest. Desiccation of seed crops as early as possible helps to limit virus spread.

Beware of resistance

To prevent insecticide resistance developing in the aphid population it is critical that an anti-resistance strategy is adopted. Tank mixtures of two different aphicide groups are recommended to reduce the risk of resistance and to minimise the transmission of virus.

There are two main groups of aphicides:

Rapid knock-down chemicals, usually formulated around the pyrethroids, lambda-cyhalothrin or esfenvalerate, are recommended to rapidly kill aphids.

They also act as a deterrent to aphids feeding on the leaves, which is the main means whereby they can restrict spread of viruses Y, V and A. Examples include Hallmark Zeon (lambda-cyhalothrin), Dovetail (lambda-cyhalothrin + pirimicarb), Sumi-Alpha (esfenvalerate).

Systemic and translaminar chemicals are taken up by the leaf and kill aphids after they feed on the leaf sap. This method of activity effectively controls spread of leaf-roll virus, but is ineffective against spread of viruses Y, V and A.

These products include Actara (thiamethoxan), Aphox (pirimicarb), Biscaya (thiacloprid), Insyst (acetamiprid), Plenum WG (pymetrozine) and Teppeki (flonicamid). Note: the above insecticides should not be used during flowering.

Do not use more than two applications of any neonicitinoid insecticide (Actara, Biscaya or Insyst on any crop.

It is recommended to tank mix a pyrethroid such as Hallmark Zeon or Sumi-Alpha with one of the systemic/translaminar products.

Remove sources

Virus transmission by aphids within seed potato crops can also be reduced by removing the sources of infection. This could mean early selective rogueing within the crop or the removal of other possible virus sources such as ground-keepers and haulm growth on compost heaps.

Ware crops

In ware crops, the virus transmitting activity of aphids is of little concern.

Application of insecticides should only be considered when high infestations of aphids are likely to reduce yield through sap feeding causing the leaves to curl in response to drought stress.

A threshold of five aphids per leaf will justify an aphicide on ware crops. Ideally aphid populations should be monitored weekly to determine when numbers start to rise, as this is the ideal time to target them.

A good method for detecting aphids is to hold a white sheet of card under the leaves and gently shake the foliage.

Aphids can be clearly seen as they fall on to the white card. The insecticides listed are also recommended for ware crops.

Always check product label for details of maximum number of applications, spray and harvest interval.