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Infection of canola (Brassica napus) by the blackleg fungus (Leptosphaeria maculans) and subsequent development of leaf and stem lesions was influenced by cultivar resistance and weather conditions. Cultivars Dunkeld, Hyola 42, Karoo, Monty, Pinnacle, Scoop and Mystic showed varying resistance to stem and cotyledon infection, but were susceptible to leaf infection. Fewer lesions were observed on the leaves of the highly resistant cv. Hyola 60 than on other cultivars and scanning electron microscopy revealed that pycnidiospore germination and mycelial growth were inhibited on cv. Hyola 60. Infection by natural ascospore inoculum was favoured by periods of increased rainfall, temperature and wind activity and infection was most prevalent early in the growing season, when plants were most vulnerable to leaf infection. Cultivar Hyola 60 developed very little disease and, hence, was unaffected by …