Figure 5. Seasonal changes in the severity of stem canker caused by *Leptosphaeria maculans* assessed in June using a disease index (DI) (*y*) over the period from 2002 to 2006 (*x*) on two pairs of *Brassica napus* near-isogenic lines (NILs) (‘Eurol’/’EurolMX’ and ‘Darmor’/’DarmorMX’) without or with the *Rlm6* resistance gene (MX lines) in different genetic backgrounds (‘Eurol’, susceptible;‘Darmor’, quantitative polygenic resistance). Data are from the initial 2002/2003 trial and then from 2003/2004 to 2006/2007 from four series of trials recurrently inoculated in autumn either with stem residues of ‘Eurol’ (a, PHO1), ‘EurolMX’ (b, PHO2), ‘Darmor’ (c, PHO3) or ‘DarmorMX’ (d, PHO4) (see Fig. 1). The DI ranges from 0 (all plants healthy) to 9 (all plants broken down). Results for 2002/2003 are shown as means (SED 0.336, df* *= 9, LSD (5%)* *= 0.761). Lines of best fit from a parallel lines regression analysis for 2003/2004 to 2006/2007 using the factors background (‘Eurol’ or ‘Darmor’) and MX status (without/with *Rlm6*) are shown with solid lines for ‘Eurol’ (open squares) and ‘Darmor’ (open triangles), and with dashed lines for ‘EurolMX’ (closed squares) and ‘DarmorMX’ (closed triangles); (a) two pairs of parallel lines *y *=* *2.95 + 0.44*x* (SE 0.440, 0.115) (‘Eurol’), *y* = 0.35 + 0.44*x* (SE 0.440, 0.115) (‘Darmor’), *y *=* *0.30 + 0.019*x* (SE 0.440, 0.115) (‘EurolMX’), *y* = 0.169 + 0.019*x* (SE 0.440, 0.115) (‘DarmorMX’) *R*^{2}* *= 85.3%, *s*^{2}* *= 0.53, df* *= 58; (b) two pairs of parallel lines *y *= −0.73 + 1.13*x* (SE 0.566, 0.151) (‘Eurol’), *y *= −0.061 + 0.47*x* (SE 0.566, 0.151) (‘Darmor’), *y *= −0.26 + 1.13*x* (SE 0.566, 0.151) (‘EurolMX’), *y *=* *0.41 + 0.47*x* (SE 0.566, 0.151) (‘DarmorMX’), *R*^{2}* *= 64.3%, *s*^{2}* *= 0.91, df* *= 59; (c) four non-parallel lines *y *=* *1.99 + 0.62*x* (SE 0.640, 0.172) (‘Eurol’), *y *=* *0.86 + 0.27*x* (SE 0.640, 0.172) (‘Darmor’), *y *= −0.13 + 0.27*x* (SE 0.640, 0.172) (‘EurolMX’), *y *=* *0.76 − 0.08*x* (SE 0.640, 0.172) (‘DarmorMX’), *R*^{2}* *= 73.4%, *s*^{2}* *= 0.79, df* *= 57; (d) four parallel lines *y *=* *2.48 (SE 0.182) (‘Eurol’), *y *=* *1.03 (SE 0.182) (‘Darmor’), *y *=* *0.95 (SE 0.182) (‘EurolMX’), *y *=* *0.74 (‘DarmorMX’) (SE 0.182), *R*^{2}* *= 46.3%, *s*^{2}* *= 0.53, df* *= 60.

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