The genetics of blackleg [Leptosphaeria maculans (Desm.) Ces. et De Not.] resistance in rapeseed (Brassica napus L.) : II. Seedling and adult-plant resistance as quantitative traits.

Inheritance studies of seedling and adult-plant resistance to blackleg [Leptosphaeria maculans (Desm.) Ces. et De Not.] in rapeseed (Brassica napus L.) were conducted using 49 families derived by intercrosses between 14 randomly chosen F2 plants from the cross cv "Maluka"(resistant) cv "Niklas" (susceptible), conforming to the North Carolina mating design II (NCM-II). Four concurrent experiments were performed, where plants from each family were: (I) Spray inoculated with a 10(5) pycnidiospores/ml suspension 10 days after germination and assessed 2-weeks later for cotyledon-lesion development, (II) As for (I), but assessed 12 weeks after inoculation for crown-canker development, (III) Wound-inoculated on the stems at growth stage 2.4-2.5 on the Harper and Berkenkamp scale and assessed for crown-canker development 5 weeks after inoculation, and (IV) Spray inoculated at growth stage 2.3-2.4 with a 10(5) pycnidiospores/ml suspension and assessed for crown-canker development nine weeks after inoculation. A L. maculans isolate possessing high levels of host specificity (MB2) was used in all inoculations. Seedling resistance was evaluated using a 0-5 cotyledon-lesion severity scale. Adult-plant resistance/susceptibility was evaluated using three separate measures of crown-canker size, i.e. the percentage of crown girdled (%G), external lesion length (E) and internal lesion area (%II). Quantitative genetic analysis of blackleg resistance using the NCM-II design revealed significant non-additive genetic variances for all measures of disease severity, in all four experiments, indicating the presence of strong dominance/epistasis at loci controlling blackleg resistance. The resistance to crown-canker development, after wound-inoculation of the stem, was found to possess the highest ratio of additive to non-additive genetic variance. Crown-canker development in mature plants of the NCM-II population was not related to the degree of cotyledon-lesion development at the seedling stage, indicating the limited value of the cotyledon test in screening for adult-plant blackleg resistance. The implications of these findings to breeding for resistance to blackleg in rapeseed are discussed.