Alternaria black spot (Alternaria brassicae, A. raphani)

Alternaria Black Spot

Alternaria black spot: biology

The fungi causing black spot survive on crop residue or in seed. During the growing season airborne spores are produced on residue and on spots. The disease develops rapidly on pods if plants mature during prolonged wet periods.

Alternaria black spot: damage description

When pods are infected, less seed is produced partially due to pod splitting. Seed weight is reduced and green seed count is increased. Seed germination is reduced when seed is infected.

Black spots occur on leaves, stems and pods. As spots enlarge they may develop a greyish to white centre and may appear like a target. Spots vary from grey to purplish to black and are sometimes surrounded by a yellow zone.

Photo credit: Canola Council of Canada

Alternaria black spot: management

If the disease is present, timely swathing will reduce damage. Spraying a fungicide will reduce yield loss and quality deterioration. Not growing cruciferous crops for several years is recommended, but may not be totally effective because of the widespread occurrence of the pathogens. Treating infected seed will control seedling blight and improve germination but will not protect plants from post-seedling infections. Seed infection is reduced over time so storing seed may improve germination.