The fungal disease Alternaria black spot is one of the most common microbial agents to infect canola crops in Canada. It is a considerable threat for its ability to infect a canola crop during any stage of development. Infections are more common and detrimental in the latter stages of the canola crops, particularly during ripening. Infection at this stage can lead to early pod shattering, resulting in a significant potential for high crop losses during harvest. The most distinct sign of infection is the production of black necrotic spots which produce spores. These spores can further infect surrounding fields as they are dispersed by the wind.
Symptoms of infection can be seen in all stages of growth, such as brownish-black spots on cotyledons, black spots on leaves of seedlings, and as the plant reaches maturity, it is also possible to find black spots on all structures of an infected plant. In initial stages of infection, the brownish-black spots will have a yellowish ring surrounding them, which is more commonly found on the underside of leaves. In severe cases, the upper portion of the stems and pods will begin to wither and form new lesions that will sporulate later on, thus further increasing the inoculum of the disease under field conditions.
With warm-humid conditions, there is an increased likelihood of infection. Additionally, increased wind during warm conditions will enhance the dispersal of spores, thus heightening the probability of surrounding infections within the crop.