Following two photographs are of symptoms on cabbage seedlings taken mid-June 2019. Conditions had been unusually favorable for Alternaria leaf spot that spring. This disease normally occurs during fall. The first image is the leaves the day they were removed from the field. The round spots on the leaf on the left are more characteristic of this disease than the others. Numerous spores of the pathogen were visible on all spots when viewed with a dissecting microscope. After incubating the leaves over night in a moist chamber (plastic box with wet paper towel on the bottom), many more spores were present giving the spots a dark brown velvet appearance (second image).
For information about this disease and its management, see main webpage about Alternaria leaf spot on crucifers.
