Relationships between phoma leaf spot and development of stem canker (Leptosphaeria maculans ) on winter oilseed rape (Brassica napus ) in southern England

Models were constructed to describe the relationships between incidence of phoma leaf spot at different growth stages in autumn/winter or early spring and incidence of stem canker (basal canker or stem lesions) in summer on winter oilseed rape in southern England. Model 1, describing the phoma leaf spot/basal canker relationship, was y(1) = beta (0) + beta (1)x(1) + beta (2)(x(2) - x(1)) if x(2) > x(1), and y(1) = beta (0) + beta (0) + beta (1)x(1) if x(2) less than or equal to x(1), in which y(1) was the incidence (Ic plants affected) of basal canker at harvest,x(1) was the maximum incidence of phoma leaf spot during the period from sowing to growth stage (G.S.) 1,6-1,7 (about 100 days after sowing) and x(2) was the maximum incidence of phoma leaf spot between G.S. 1,7 and G.S. 2,0 (start of stem extension). Model 2, describing the phoma leaf spot/stem lesion relationship, was y(2) = alpha (0) + alpha (1)x(3) + alpha (2)x(4), in which y(2) was the incidence of stem lesions at harvest, x(3) was the incidence of phoma leaf spot at G.S. 3,3-3,5 (flower buds visible) and x(4) was the incidence of phoma leaf spot at G.S. 4,5-5,5 (flower buds opening). Data from field experiments with four winter oilseed rape cultivars at Boxworth or Rothamsted in the 1992/93, 1993/94, 1996/97, 1997/98 or 1998/99 seasons were used to test the models. The values of R-2 for the regression equations testing model 1 for the phoma leaf spot/basal canker relationship were 0.75, 0.93, 0.91 and 0.89 for cvs Apex, Bristol, Capitol and Envol, respectively. The values of R-2 for the regression equations testing model 2 for the phoma leaf spot/stem lesion relationship were 0.58, 0.57, 0.54 and 0.71 for cvs Apex, Bristol, Capitol and Envol, respectively. The phoma leaf spot/basal canker relationship (model 1) could also be fitted to the combined data set for all four cultivars (R-2 = 0.65), whereas the phoma leaf spot/stem lesion relationship (model 2) could not to be fitted to the combined data set for the four cultivars. The relationships between incidence and severity of stem canker were examined and the values of R-2 for the regressions of severity on incidence were 0.91 for basal canker and 0.89 for stern lesions.