Virology

Potato-infecting viruses cause significant damage worldwide and represent a significant threat to seed potato industries. The incidence of virus in seed potatoes can have a significant impact on crop quality (both seed and ware). Virus infection can result in seed crops not meeting the official standard for a class (downgrading) or even for certification as seed potatoes (failure).  As the cultivation of potatoes involves vegetative (asexual) reproduction, virus infection in the parent tuber is generally passed on to the next generation.  Virus free crops are at risk from transmission of virus from sources of infection outwith the planted crop.  The greatest risk is presented by virus infections in neighbouring potato crops, in groundkeepers originating from previous potato crops that have continued to grow in the soil, and from virus reservoirs in the environment, e.g. weeds.
 
Work carried out at SASA using data from the Scottish Seed Potato Classification Scheme has shown that crops grown from a parent crop in which symptoms of infection had been seen at the previous year’s classification inspections have a far greater likelihood of exhibiting virus symptoms than crops grown from a parent crop in which no virus had been seen during inspection.  For example, over the period 2009-2011, whilst virus symptoms were observed in 16% of the crops grown, the virus incidence was 52% for stocks grown from parental material, and 13% for crops grown from parent stock in which no symptoms had been visible.  These data indicate a four-fold difference in the likelihood of mosaic being seen in a daughter crop depending upon whether virus had been observed in the parent crop.
 
The location where crops are grown has an effect on the likelihood of a crop acquiring infection, with the probability increasing when crops are grown in areas where more commercial stocks are in cultivation.  The area of ware potato crops is likely to have a more significant effect than the area of seed crops.
 
Potato viruses are transmitted by a number of vectors.  Whilst over 75% of the virus infections seen in Scottish seed potato crops are transmitted by aphids (e.g. Potato Leaf Roll Virus – PLRV, potyviruses such as Potato Viruses Y, A and V), other viruses may be transmitted by nematodes (e.g. Tobacco Rattle Virus - TRV), fungi (Potato Mop Top Virus - PMTV) or be transmitted by physical contact (e.g. Potato Virus X).  Under Scottish field conditions the symptoms of virus infection are not usually seen during the growing season in which the transmission takes place.
 
Aphid transmitted potato viruses may be transmitted in a persistent (e.g. PLRV) or a non-persistent manner (e.g. Potato Viruses Y, A and V).  Persistently transmitted potato viruses infect the vector aphid for its lifetime and any plants on which such an aphid then feeds will be at risk of acquiring the virus.  Non-persistently transmitted potato viruses can only be transmitted immediately after aphids have fed on an infected plant.  Non-colonising aphid species, such as cereal aphids, that do not use potato as a host but alight on potato plants and probe the leaves, can transmit these viruses.
 
Details of the virus tolerances for classified seed potatoes in Scotland are set out in the SPCS in Scotland leaflet.
 
See also:

Aphid Monitoring of Seed Potato Crops

Aphid monitoring, which was introduced into the SPCS in 1992 to encourage growers to control aphids developing on their crops, has proved 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.  Although leaf roll levels increased during 2018 and higher than average activity of M. persicae is predicted for 2019, the risk of further transmission is not considered to be sufficiently high to merit the reintroduction of aphid monitoring during 2019 crop inspections.  A balanced approach to the management of both leaf roll and non-persistent viruses is required for 2019, with attention paid to varietal susceptibility and the development of aphid populations as the season progresses. 

Inspection of the 2018 Scottish seed potato crop revealed an incidence of crops containing virus affected plants of 9.4% of the total area entered for classification (up from 4.8% in 2017).  The incidence of mosaic symptoms (i.e. excluding leaf roll) increased to 7.3% from 4.5% in 2017.  Within this 7.3%, the incidence of PVY was recorded as having increased from 3.6% to 6.9%, contrary to the decline to 2.8% that we predicted in March 2018.  Without the Elgin trap operating in 2017 and 2018 we are relying on 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, to provide a prediction of 8.0% in 2019. 

The incidence of crops containing any leaf roll increased from 0.25% in 2017 to 3.1% in 2019.  The model based on aphid data before the introduction of aphid monitoring in 1992 predicted an increase in leaf roll to around 0.7% of the crop area for 2018.  As a result of above average temperatures during winter 2018-19 winter, the SASA forecasts for the first suction trap catches of peach-potato aphids are 1 June for Edinburgh and 9 June at Dundee, around 1 to 3 weeks earlier than in 2018. Following moderately high activity of this species in 2018, a further increase in the level of leaf roll to 6.5% is expected for the 2019 crop based on data from Dundee where Myzus persicae activity was high in 2018 and to 4.5% based on data from Edinburgh where Myzus persicae activity was relatively lower in 2018.

Virus Epidemiology

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 AHDB (Potatoes) 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.

2019 Summary

The predicted early start to the aphid flight season due to mild winter temperatures was observed. 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 9 June is now 774.  This figure ranks third of the last 10 years (behind 2017).

The total of 774 aphids includes 449 Willow-Carrot aphids (Cavariella aegopodii), 86 Leaf-Curling Plum aphids (Brachycaudus helichrysi) and 69 Shallot aphids (Myzus ascalonicus) making up 58%, 11% and 9% respectively of the total aphid vectors caught.  60% of the total number of aphids have been caught in the Edinburgh trap and 40% at Dundee.

Edinburgh 2019

The cumulative aphid vector pressure for 2019 (up to 9 June) at Edinburgh now ranks 5th over the last 32 years.  The Willow-Carrot aphid (Cavariella aegopodii) is responsible for 74% of the accumulated pressure, with the Leaf-Curling Plum aphid (Brachycaudus helichrysi) and the Shallot aphid (Myzus ascalonicus) accounting for 5% each.

Dundee 2019

The cumulative aphid vector pressure for 2019 (up to 9 June) at Dundee ranks 4th over the last 32 years.  The Willow-Carrot aphid is responsible for 66% of this value, the Leaf-Curling Plum aphid is responsible for 7% and the Bird Cherry-Oat aphid (Rhopalosiphum padi) accounts for 6%.

Varietal Propensity to Virus Infection

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.

Table showing varietal propensity to virus infection