Infection of canola (Brassica napus) by pycnidiospores of the blackleg fungus (Leptosphaeria maculans) and subsequent development of leaf and stem infection were influenced by temperature, wetness period and inoculum concentration. On cv. Hyola 42, as temperature decreased the latent period for leaf infection increased exponentially. The greatest number of leaf lesions developed on plants exposed to a day/night temperature of 18/15°C with a 96 h wetness period. Incidence of stem infection was greatest at 23/20°C with a 48–72 h wetness period, whereas no stem infection occurred 42 days after inoculation when plants were incubated at 8/6°C. This is the first report of the effect of temperature and wetness period on infection by pycnidiospores. The number of leaf lesions and incidence of stem cankers were greatest when plants were inoculated with 10⁶ or 10⁷ pycnidiospores/mL, respectively, the highest concentrations used in the experiments. The susceptible cv. Q2 developed significantly more leaf and stem lesions than the less susceptible cvv. Monty and Dunkeld when inoculated with 10⁷ pycnidiospores/mL, but not at the lower concentrations. The moderately resistant cv. Dunkeld developed significantly fewer stem cankers than the other cultivars when inoculated with pycnidiospores at concentrations greater than 10⁴/mL.