The fungus (Pyrenopeziza brassicae) which causes light leaf spot is able to survive the summer months on debris from the previous oilseed rape crop and to a lesser extent on rape volunteers and vegetable brassicas.
On oilseed rape debris, the fungus enters the sexual stage and develops structures called apothecia (less than 1mm in size), which release wind-dispersed sexual spores (ascospores). These spores can be transported by wind for several miles and can infect newly emerging oilseed rape crops at considerable distances from the spore source.
On infected volunteer oilseed rape and vegetable brassicas plants, asexual spores are produced (conidiospores) which are dispersed by rainsplash and can therefore only infect oilseed rape plants in close proximity.
After the initial infections in the oilseed rape crop in the autumn, the disease spreads by rainsplash-dispersed spores which are produced on the infected plants. Because of this dispersal method, new infections develop around initial primary infection "foci" and the disease appears to be patchy within the field.
During the winter when rape plants are still in the rosette stage, it is thought that some growth tips can get infected which leads to infection of newly emerging leaves and developing floral structures.
During stem extension in spring many of the old rosette leaves senesce. Once again the pathogen enters another sexual stage of development on this dead leaf material, producing apothecia, and these release a further flush of wind-dispersed spores. These also contribute to the infection of new leaf and floral material. Furthermore, the disease is able to spread vertically within the crop by splash-dispersal of spores from lower leaves to the upper canopy during rain events.Biology of the pathogen, Pyrenopeziza brassicae
Stem and pod infections in the summer can be the result of early infections of growth tips in the winter, infection by wind-dispersed spores produced on leaf debris, and vertical splash-dispersal. The amount of stem and pod infection determines the amount of inoculum for winter oilseed rape crops in the next season.