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.