Aphid monitoring, which was introduced into the SPCS in 1992 to encourage growers to control aphids developing on their crops, has proved very effective at encouraging the management of the potato colonising aphids that transmit leaf roll.  It has not proved as successful in assisting with the control of the non-colonising aphids that transmit non-persistent viruses.  For 2018, with leaf roll remaining at a very low level and with only moderate activity of M. persicae populations during 2017, the level of virus inoculum is not considered to be sufficiently high to merit the reintroduction of aphid monitoring during 2018 crop inspections.  A balanced approach to the management of both leaf roll and non-persistent viruses is required for 2018, with attention paid to varietal susceptibility and the development of aphid populations as the season progresses.

Inspection of the 2017 Scottish seed potato crop revealed an incidence of crops containing virus affected plants of 4.8% of the total area entered for classification (down from 6.7% in 2016).  This is the lowest level of virus incidence recorded since the instigation of the Scottish seed Potato Classification Scheme.  The incidence of mosaic symptoms (i.e. excluding leaf roll) decreased to 4.5% from 6.4% in 2016.  Within this 4.5%, the incidence of PVY was recorded at 3.6%, precisely the level that we predicted in March 2017.  Unfortunately without the Elgin trap operating in 2017 a revised model, based on catches from just the Dundee and Edinburgh suction traps, using the three most abundant cereal aphids which have been most closely linked with PVY transmission in Scotland, provides a retrospective prediction of 4.0%.  This same model is predicting that PVY will decrease from 3.6% of the SPCS crop area containing plants with symptoms to 2.8% in 2018.  The incidence of crops containing any leaf roll decreased from 0.26% in 2016 to 0.25% in 2017.  The model based on aphid data before the introduction of aphid monitoring in 1992 predicted an increase in leaf roll to around 0.7-0.8% of the crop area for 2017.

Following the 2017-18 winter, when temperatures were slightly below average, the SASA forecasts for the first suction trap catches of peach-potato aphids are 21 June for Edinburgh and 23 June for Dundee, some 6 weeks later than in 2017.  Following moderate activity of this species in 2017, a slight increase in the level of leaf roll (to 0.7%) is expected for the 2018 crop.  Following record low levels of virus in 2017, the 2018 crop is predicted to contain even less virus.

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An estimate of the likelihood that aphids will transmit non-persistent potato viruses (e.g. PVY) in the field can be made using the data collected by the aphid suction traps. This estimate, the aphid vector pressure, is calculated by summing the total catch of each aphid species, after multiplication by a factor estimating the efficiency of that species as a vector of PVY. Details of the vector efficiencies used in these calculations are available via the Potato Council Aphid Monitoring webpages. The vector pressure is a very coarse measure of the likelihood of virus transmission. Numerous factors will influence virus transmission, including complex interactions between aphid species and the strain of virus they transmit.

In most years, the aphid vector pressure in Scotland is largely dependent upon the activity of cereal aphids, although peach-potato aphids and potato aphids may also be important.

2017 Summary

Following a busy early season and a very quiet summer period, flight activity is now returning to average levels for this time of the year. Although a late migration of cereal aphids may occur this poses little to no risk to the potato crop as harvest is well underway in most areas.  The total number of aphids known to vector potato viruses that have been caught in the Dundee and Edinburgh suction traps up to the week ending 27 August is 2551.  This figure now ranks ninth of the last 10 years. As recently as 27 June, the total of individual aphids of species known to vector potato viruses was the highest we had recorded over the last 10 years.  Time will tell whether the exceptionally early aphid activity has been more important in vectoring potato viruses, or whether the scarcity of vector activity during July will result in less virus in the 2018 Scottish seed crop.

The total of 2551 aphids includes 373 Rose-Grain aphids (Metopolophium dirhodum) and 540 Bird Cherry-Oat aphids (Rhopalosiphum padi) (making up 15% and 21% respectively of the total aphid vectors caught).  73% of the total number of aphids have been caught in the Edinburgh trap and only 27% at Dundee.

Peach-Potato aphid activity has reduced after relatively high numbers earlier in the season.

Variety has a very important effect on the incidence of virus symptoms observed at classification inspections. The term ‘varietal propensity’ has been adopted to describe whether symptoms observed within a variety are above or below the average across the whole Scottish seed crop (i.e. Propensity = % of diseased crops of variety /% of diseased crops of all varieties).
 
The table below summarises varietal propensity information collected over the period 2009-2012 using data on symptom expression at crop inspection (Mosaics and Leafroll) and laboratory virus diagnoses on leaf samples submitted to SASA from plants exhibiting virus symptoms during crop inspections (PVYN, PVYO/C, PVA and PVV). Values greater than 1 indicate that a virus/symptom is more likely to be found in that variety and values less than 1 indicate that it is less likely to be found in that variety.  The table shows values which are significantly greater than 1 at the p<0.01 level shaded in red; values which are significantly greater than 1 at just the p<0.05 level shaded in orange; values which are significantly less than 1 at the p<0.01 level shaded in dark green; and values which are significantly less than 1 at just the p<0.05 level shaded in light green.  Values that are not significantly different from 1 at the 0.05 level are left clear.  Sample size has a marked effect on the likelihood of significant departures from 1, both for varieties where few crops have been inspected and for viruses/symptoms where the incidence is low (e.g. PVA, PVV, Leafroll).
 
Propensity values can be used to rank varieties in relation to any particular virus/symptom.  However, they should not be used to make quantitative comparisons between viruses/symptoms. As the reliability of propensity data depends upon the inspection and sampling of an extensive number of crops, it is less reliable for varieties with relatively few crops which are only grown over a relatively small area e.g., new varieties.  For these reasons, propensity data are only presented for varieties with over 100 crops grown over 2009-2012.
 
Varietal resistance scores, e.g. those provided on the AHDB Potatoes' Potato Variety Database, relate to resistance to PVYO/C whereas propensity values generally relate to the strains of viruses that are present in the field (e.g. the PVYEU-NTN serotype of PVYN is the dominant virus within the Scottish classification scheme). Therefore, there may not be a straightforward relationship between the two values.
 
Consideration of varietal propensity should be an important part of any virus management programme. Whether a variety has a propensity to leafroll or to PVY can be used to determine the appropriate means of protecting the crop through a control programme for the appropriate aphid vector species. 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.