Effects of light leaf spot (Pyrenopeziza brassicae) on yield of winter oilseed rape (Brassica napus)

The relationship between development of light leaf spot and yield loss in w inter oilseed rape was analysed, initially using data from three experiment s at sites near Aberdeen in Scotland in the seasons 1991/92, 1992/93 and 19 93/94, respectively. Over the three seasons, single-point models relating y ield to Light leaf spot incidence (% plants with leaves with light leaf spo t) at GS 3.3 (flower buds visible) generally accounted for more of the vari ance than single-point models at earlier or later growth stages. Only in 19 92/93, when a severe light leaf spot epidemic developed on leaves early in the season, did the single-point model for disease severity on leaves at GS 3.5/4.0 account for more of the variance than that for disease incidence a t GS 3.3. In 1991/92 and 1992/3, when reasonably severe epidemics developed on stems, the single-point model for light leaf spot incidence (stems) at GS 6.3 accounted for as much of the variance. Two-point (disease severity a t GS 3.3 and GS 4.0) and AUDPC models (disease incidence/severity) accounte d for more of the variance than the single-point model based on disease inc idence at GS 3.3 in 1992/93 but not in the other two seasons. Therefore, a simple model using the light leaf spot incidence at GS 3.3 (x) as the expla natory variable was selected as a predictive model to estimate % yield loss (y(r)): y(r) = 0.32x - 0.57. This model fitted all three data sets from Sc otland. When data sets from Rothamsted, Rosemaund and Thurloxton in England were used to test it, this single-point predictive model generally fitted the data well, except when yield loss was clearly not related to occurrence of light leaf spot. However, the regression lines relating observed yield loss to light leaf spot incidence at GS 3.3 often had smaller slopes than t he line produced by the model based on Scottish data.