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 2010-2023 using data on symptom expression at crop inspection (*mosaic and leaf roll) and laboratory virus diagnoses on leaf samples submitted to SASA from plants exhibiting virus symptoms during crop inspections. Data are presented for the most common virus species and serotypes currently found in Scottish seed potatoes i.e. PVY (PVYNserotype), PVA and PLRV. 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.
Propensity values can be used to rank varieties in relation to any virus/symptom. However, they should not be used to make quantitative comparisons between viruses/symptoms. The reliability of propensity data depends upon the ability to see infection during inspection and therefore propensity is of less value for varieties which might carry latent infection or are tolerant to virus infection. Propensity data might be influenced by crop management practices (e.g. heavy roguing), and relies on the sampling of an extensive number of crops; therefore, it is of less value where relatively few crops are grown over only a relatively small area, e.g. new varieties. For this reason, propensity data are only presented for the most commonly grown varieties.
Varietal resistance scores, e.g. those provided on the Potato Variety Database on GB-certified potato varieties https://potatoes.agricrops.org/ relate to resistance to specific virus species (such as PLRV and PVYO strain in the case of PVY), whereas propensity values generally relate to the virus species (serotypes or variants) that are present in the field (e.g. the PVYEU-NTN variant of serotype PVYN is currently the most prevalent PVY variant 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.
Further recommendations on virus management can be found in "The 6 Steps to Effective Virus Management in Certified or Home Saved Seed (2024)".
Note: Leafroll (leaf rolling) disease symptom is caused by Potato leafroll virus (PLRV). Mosaic disease symptoms are caused by a range of potato-infecting viruses for which Potato virus Y (PVYN serotype) is the most prevalent species in Scotland.
SASA