Tender leaves can also be harvested from full-grown plants. Store all harvested collards in the refrigerator. Aphids can also be a serious problem, especially during cool weather. Common disease problems include black rot, downy mildew and Alternaria leaf spot. Black rot causes the most serious damage and appears as V-shaped lesions down the leaves and spreads into the water conducting system of the plant.
Black rot is caused by a bacterium that is seed-borne or that can be transmitted by transplants.
Warm, moist weather favors the disease. There is no control for black rot once it is established in a planting. Prevent black rot by purchasing transplants that are marked with a tag indicating that they are certified or plant western-grown chemically treated seed. Leaf diseases include downy mildew and Alternaria leaf spot. Revision of HGL 2, Growing Cole Crops in the Home Garden, 1989.