Propensity of the top 30 varieties to Blackleg
Variety has a very important effect on the incidence of disease 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. Propensity is expressed as (% of disease within crops of a specific variety) / (% of disease in crops of all varieties). The propensity score also indicates by how much a variety is above or below average, and can therefore be used as an indication of how one variety may compare with another.
The table below summarises varietal propensity information collected over two periods 2009-2017, as well as just for season 2017, using data on the incidence of blackleg symptoms observed at crop inspection (see also attached spreadsheet). Data are presented for the top 30 varieties entered for classification within the SPCS in 2017; these top 30 varieties making up c. 75% of total SPCS area in 2017. By taking the maximum value for blackleg incidence across all inspections for each crop and multiplying this by the area of the respective crop, a quantitative estimate of the infection can be made. Thus a crop of 4ha with a 3% incidence of blackleg contains 0.12 ha of blackleg infection. Values greater than 1 indicate that blackleg is more likely to be observed in that variety and values less than 1 indicate that symptoms are less likely to be observed. The table also shows values which are significantly greater or less than 1 at the 0.05 confidence level. The cells in the table are shaded in red when the propensity is significantly higher than expected (i.e. 1) and shaded in green when values are significantly lower than expected. Values that are not significantly different from 1 are left clear. Sample size has a marked effect on the likelihood of significant departures from 1, hence the 2009-17 column shows a greater number of statistically significant departures from 1 than does the column for 2017 by itself. As the variance associated with the data for varieties with a high propensity is far greater than for those with a low propensity, fewer varieties have a propensity significantly greater than 1 and more varieties have a propensity significantly less than 1.
The interpretation of these data should take into account that the management of blackleg, e.g. heavy roguing, will have an effect on the disease incidence observed during inspections. However, with approximately 4,000 seed crops entered for classification each year, similar husbandry practices would need to be extended across a high proportion of crops to have a marked influence on propensity.