Air-borne spores of the fungus are produced in large numbers on old lesions during periods of high humidity. Spores may also be carried on seed. The fungus requires moisture on leaves for infection. A preventative fungicide program should be followed where the disease is normally present, using a spray interval of 10 to 14 days. This disease may occur at any position on the leaf but is most common on margins.
Spots are circular in shape and, with age, they coalesce to form larger spots.
Lesions on the leaves are, sometimes surrounded by a lighter-colored halo. Lesions on the petioles are enlongated with a pale center and dark margin. Spots on the petiole may encircle it causing defoliation. Leaf spot is distinguished from leaf blight by the nearly circular, sharply defined lesions with a yellow halo. The disease usually occurs during the latter part of the growing season and can be controlled with the same fungicide program as used for leaf blight.