Abstract: The aim of the study was to evaluate the effect of sowing date on the growth and development of the open pollinated (‘Contact’), composite hybrid (‘Kaszub’) and restored hybrid (‘Kronos’) cultivars of winter oilseed rape. Higher degree of blackleg (Leptosphaeria maculans, L. biglobosa) and black spot (Alternaria spp.) symptoms were observed during the three years of investigations. Trace symptoms of infection with Erysiphe cruciferarum, Sclerotinia sclerotiorum and Botrytis cinerea were also noted. The occurrence of blackleg on winter oilseed rape was influenced by both experimental factors investigated. Significantly lower levels of infection of Leptosphaeria spp. were noted in optimal and late sown crops. Significantly, in each year of research, the cultivar with lowest levels of blackleg was ‘Kaszub’. ‘Kronos’ the restored hybrid cultivar was significantly less susceptible to infection and the cultivar with the highest levels of blackleg was ‘Contact’, the open pollinated winter oilseed rape cultivar. There was no effect of the studied factors on black spot occurrence on any of the cultivars.