THE GENETICS OF BLACKLEG [LEPTOSPHAERIA-MACULANS (DESM) CES-ET-DE-NOT] RESISTANCE IN RAPESEED (BRASSICA-NAPUS L) .2. 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 (Brassic a napus L.) were conducted using 49 families derived by intercrosses b etween 14 randomly chosen F2 plants from the cross cv ''Maluka'' (resi stant) cv ''Niklas'' (susceptible), conforming to the North Carolina m ating design II (NCR I-IT). 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, (EI) As for (I), but a ssessed 12 weeks after inoculation for crown-canker development, (III) Wound-inoculated on the stems at growth stage 2.4-2.5 on the Harper a nd Berkenkamp scale and assessed for crown-canker development 5 weeks after inoculation, and (IV) Spray inoculated at growth stage 2.3-2.4 w ith a 10(5) pycnidiospores/ml suspension and assessed for crown-canker development nine weeks after inoculation. A L. maculans isolate posse ssing high levels of host specificity (MB2) was used in all inoculatio ns. Seedling resistance was evaluated using a 0-5 cotyledon-lesion sev erity 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 are a (%II). Quantitative genetic analysis of blackleg resistance using th e NCM-II design revealed significant non-additive genetic variances fo r all measures of disease severity, in all four experiments, indicatin g the presence of strong dominance/epistasis at loci controlling black leg resistance. The resistance to crown-canker development, after woun d-inoculation of the stem, was found to possess the highest ratio of a dditive to non-additive genetic variance. Crown-canker development in mature plants of the NCM-II population was not related to the degree o f cotyledon-lesion development at the seedling stage, indicating the l imited value of the cotyledon test in screening for adult-plant blackl eg resistance. The implications of these findings to breeding for resi stance to blackleg in rapeseed are discussed.