L. maculans occurs throughout the world and causes leaf spotting and stem rot in vegetable brassicas, swede and radish and stem canker or blackleg in oilseed rape
Stem infections can reduce seed yield if the infection becomes severe enough to cause early senescence or lodging
The disease is commonly seed-borne; seed infection rarely exceeds 1% but might result in disease outbreak.
Symptoms
Leaf spots: These spots can be rather confusingly variable depending on host resistance and age. Initially, L. maculans causes pale brown spots, which may be surrounded by chlorotic tissue. As the spots enlarge and age, they become round to irregularly shaped lesions, dirty white or pale grey in colour, and they may contain tiny dark specks, which are pycnidia (see Morphology) within which the asexual spores (conidia) develop. Alternatively, the centre of the spot may break or drop out completely;
Stem lesions: often develop from infected leaves via the petioles, are often oval, long, irregular in shape and grey or cream in colour, and can appear as long black transverse streaks in the cortex;
Pod lesions: develop from rain-splashed pycnidiospores and wind-borne ascospores, are round to irregular in shape, with pycnidia, and the pod is often constricted;
Stem cankers: develop from stem lesions at the root collar or crown of the plant. The brown lesions become darker and crack open. When severe, the stem is weakened considerably and this may result in disruption of water transport, lodging and death of the plant;
Root infection: occurs either by spreading from the crown region into the roots or directly from the soil through wounds. Taproots have blackened areas and may be constricted, and lateral roots appear blackened with little or no new root growth.
Commercial importance
Currently, in Europe, North America and Australia, total devastation of the crop can occur due to seedling death, although this is rare. In harvested crops, yield losses are usually 0-10%, but can often reach 30-50% in individual fields
