Effects of severity and timing of stem canker (Leptosphaeria maculans) symptoms on yield of winter oilseed rape (Brassica napus) in the UK

The relationships between yield loss and incidence (% plants with stems aff ected) or severity (mean stem score, 0-4 scale) of stem canker in winter oi lseed rape were analysed using data from experiments at Rothamsted in 1991/ 92, Withington in 1992/93, Boxworth in 1993/94 and Rothamsted in 1997/98. C ritical point models and area under disease progress curve (AUDPC) models w ere better than multiple point models for describing relationships between yield (t ha(-1)) and incidence or severity of stem canker for the four expe riments. Since yield is influenced by many factors other than disease, % yi eld loss was calculated and critical point models and AUDPC models relating % yield loss to stem canker were constructed. The critical point models fo r % yield loss on stem canker incidence for three of the four experiments w ere similar, but differed from that for Rothamsted in 1991/92. There were a lso no differences between models of % yield loss on AUDPC of both incidenc e and severity for these three experiments. Therefore, general models of % yield loss (L) against AUDPC of incidence (X) or severity (S) of stem canke r from growth stages 4.8 to 6.4 were derived from the combined data sets fo r the three experiments: L = -0.76 + 0.0075X (R-2 = 35%, p < 0.001), L = 0. 26 + 0.53S (R-2 = 37%, p < 0.001). The relationships between % yield loss a nd % plants with different stem canker severity scores at different growth stages were also analysed; the greatest yield losses were generally associa ted with the largest severity scores, for plants assessed at the same crop growth stage, and were also associated with the early development of stem l esions. Further analyses showed that % yield loss was related to incidence or severity of both basal stem cankers and upper stem lesions in experiment s at Boxworth in 1993/94 and at Rothamsted in 1997/98.