EPIDEMIOLOGY OF BLACKLEG OF OILSEED RAPE

Knowledge of the epidemiology of blackleg of oilseed rape (Brassica napus and B. rapa), caused by Leptosphaeria maculans (anamorph Phoma lingam), has been fundamental to the development of disease control practices, such as field selection, tillage, rotation, seed treatment, and plant resistance. The disease is essentially monocyclic. Primary inoculum consists principally of ascospores from residues of crucifer crops but includes also mycelium from seed and pycnidiospores from crop residue and seed. Ascospores and pycnidiospores from weed plants may also play a minor role in disease initiation. Secondary cycles may be initiated by pycnidiospores produced on infected plants but these secondary infections are not considered to affect yield appreciably. The fungus infects the tap root, the crown, and all aerial parts of the plant. Most losses in yield are caused by crown cankers. which develop principally by infection of young plants from the cotyledon to 8th-leaf stages. Crown cankers often result from neurotrophic colonization of the lamina; systemic, biotrophic colonization of the petiole; and neurotrophic colonization of the crown. Infection of crowns may precede expression of canker symptoms by several weeks during the growing season and symptoms may not progress during the winter. Natural environments suitable for crop growth modify but are unlikely to prevent infection and disease. Disease progress curves rise progressively as the crop develops when disease is measured as severity or incidence of crown cankers but may fluctuate widely when disease is measured as the incidence of leaf lesions. Proposed predictors of the disease are based on the availability of inoculum or the extent of early infection. The epidemiological significance of pycnidiospores, secondary cycles, genetic resistance, and variation in aggressiveness within the fungus needs to be determined.