Seedling and adult plant resistance to blackleg (Leptosphaeria maculans (Desm.) Ces. et de Not.) in spring rape (Brassica napus L.)

In a glasshouse test with a large number of lines derived by self-pollination of rape plants (Brassica napus L.) randomly selected from each or four spring cultivars, considerable variation in the responses of seedlings to blackleg infection (causal organism Leptosphaeria maculans (Desm.) Ces. et de Not.) was observed between and within cultivars. Although the mean disease scores of lines within each cultivar ranged between the extremes of susceptibility and resistance, a much higher frequency of resistant lines were detected in the cultivar Nosovsky than in Ceska and Zollerngold. The lines used in this initial test provided a basis for developing a susceptible and resistant line from each cultivar, and these were tested together with six other lines previously rated as being resistant in the field, under both glasshouse and field conditions. Responses to seedling infection in four different glasshouse environments showed only three field-selected lines (Wesreo, Mutu and R46) to have adequate seedling resistance. All glasshouse-selected lines were susceptible in these glasshouse tests, and the resistant selections were In all cases slightly more susceptible than the corresponding susceptible selections. In the field test, the field-selected lines as a group were clearly more resistant than any of the glasshouse-selected lines except the Nosovsky resistant selection line. The Ceska and Zollerngold resistant selections, although susceptible in comparison with the field selections, were more resistant than the corresponding susceptible selections. Considerable variation in the mean disease scores of individual plants obtained at maturity was observed within all lines except the two Tower glasshouse selections. The frequency of resistant plants was fairly high in all field-selected lines except 73N22-1 and very low in all glasshouse-selected lines except the Nosovsky resistant selection. Coefficients of correlation between disease scores at maturity in the field and scores of young plants in each of four different glasshouse environments were significant for all but one environment. The highest correlation was observed in the case of the procedure involving a single inoculation 10 days after sowing and growth of seedlings under a daily temperature regime of 25/20�C. The five lines ranked most resistant in this test were also the five most resistant lines in the field.