Collards (Brassica oleraca) are mild-flavored leafy green vegetables with tough, fibrous leaves and stems. They grow best in the spring or fall when the temperatures are between 60 and 65 degrees, although they are more tolerant of temperature extremes than other plants in the cabbage family. They are very frost tolerant and taste sweeter after a frost. Most diseases and pests do not cause significant damage to collards, but cabbageworms are very destructive.
A bacterium causes black rot, which begins as yellow lesions on the edges of the leaves. The lesions dry out and turn brown or tan, and may have black veins. In severe infections, the leaves wither and fall off the stalk. Black rot occurs in warm, humid weather. Avoid overhead irrigation to reduce the chance of infection. Remove and destroy all infected plant material and weeds that can host the bacterium to stop the spread of infection.
Alternaria leafspot, a fungal disease, can occur on collards during cool, moist weather. It begins as small dark specks on the leaves that expand to tan circular spots. Concentric rings of dark green spores may form in the spots. The center of the leafspots often becomes thin and falls out of the leaves. Downy mildew is another fungal disease that occurs in cool, moist weather. It begins as irregular yellow patches that turn tan or light brown. A white, fluffy mass of fungal spores grows on the undersides of the leaves. If the leaves of seedlings are severely infected, the seedlings may die. Thoroughly spray the upper and lower sides of the leaves with a ready-to-use fungicide to control fungal diseases.
All plants in the cabbage family, including collards, are prone to damage by cabbageworms. Cabbageworms are small and green-colored, so they blend into the collard leaves. Cabbage loopers, also called measuring worms, move in a distinctive way by pulling up the back end of their body towards the center to form a loop or hump, and then sliding the front end of the body forward. Imported cabbageworms are hairy or velvety, while diamondback worms are thicker in the middle than at the ends. Cabbageworms chew on the leaves to eat and can cause significant damage. After identifying cabbageworms on collards, spray infested plants with a ready-to-use pesticide that is effective on cabbageworms.
Several types of aphids attack collards, especially new growth. They are small, pear-shaped, and come in a variety of colors, including green, brown, and black. Aphids suck the sap from plant tissue, which causes the leaves and stems to become distorted. In addition, they secrete a sweet substance called honeydew that attracts ants. Use a strong spray of water to wash off aphids and the black fungus that grows on the honeydew. Harlequin bugs are shaped like a shield and colored black with markings of red, orange, or yellow. They suck the fluid from plant tissue, which leaves white or yellow blotches. A severe infestation can cause collard plants to wilt and die. Pesticides are not effective against harlequin bugs; handpick them off the plants.