Methods for assessment of light leaf spot (Pyrenopeziza brassicae) on winter oilseed rape (Brassica napus) in the UK

Light leaf spot (Pyrenopeziza brassicae) was assessed as % plants with light leaf spot, % leaves with light leaf spot or % leaf area with light leaf spot in winter oilseed rape field experiments done at different sites (Rothamsted, Hertfordshire; Boxworth, Cambridgeshire; near Aberdeen, Scotland), with different cultivars (e.g. Bristol and Capitol), different fungicide treatments, on plants sampled at different dates. Regression analyses on data from these experiments showed that there were consistently good relationships between % leaves with light leaf spot and % plants with light leaf spot for plants sampled during the autumn and winter, until the % plants with light leaf spot approached 100%. The slopes and positions of regression lines were sometimes affected by cultivar, fungicide treatment or sampling date, but not by site. The relationship between % leaf area with light leaf spot (square root-transformed) and % leaves with light leaf spot was less consistent than that between % leaves with light leaf spot and % plants with light leaf spot and was sometimes affected by cultivar, fungicide treatment or sampling date but not by site. The relationship between % leaf area with light leaf spot (square root-transformed) and % plants with light leaf spot was also inconsistent and was sometimes affected by cultivar, fungicide treatment, sampling date and site.