Knock Out roses are landscape roses with brightly colored petals and deep green leaves. They are self-cleaning--requiring no deadheading--with roses usually blooming in 5- to 6-week cycles. The roses, which generally grow 3 to 4 feet tall and wide, were bred to be low maintenance and highly disease resistant. According to the Missouri Botanical Garden, Knock Outs have excellent resistance to the common diseases of roses and usually will not need regular fungicide applications if properly cared for.
Resembling an orange powder, rust is a common rose disease that appears under leaves and soon spreads to the top. This fungal disease spreads in moist air. Avoid late-evening watering to keep your Knock Out rose healthy. Water the plant at soil level to prevent the foliage from getting wet.
Black Spot Disease
Black spot is a fungal disease that can appear on the leaves or stems of Knock Out roses. It appears as circular black spots, 1/16 to 1/4 inch in diameter, and can destroy the entire plant if left untreated. Although Knock Out roses are hardier than regular roses, again, avoid wetting the foliage later in the evening. Remove and dispose of infected parts of the plant to prevent the disease from spreading. Sanitize your clippers in diluted bleach after pruning to prevent transferring the disease to other plants.
Also known as gray mold, Botrytis Blight is a fungus disease that prevents buds from opening and covers them in a grayish-brown mold. Open rose blooms are covered with brown or yellow markings while their lower petals are brown and drooping. It spreads in moist air and cool temperatures. Cut and remove affected roses immediately to prevent the disease from spreading to other parts of the plants.
Powdery mildew is a fungal disease that causes white powder along the top and bottom of the foliage and stem, and the base of the flower bloom in extreme conditions. This disease prevents a rose from reaching to its maximum height, and causes affected leaves to dry out and eventually fall off. Prevent this disease from spreading by trimming off effected parts and disposing them properly, and clearing up fallen leaves or plant debris. Knock Outs are resistant to mildew, but should be monitored for the condition. Water the plant at soil level.
Use chemical cures, such as a fungicide, as a last resort if your Knock Out rose is afflicted with a disease. As the plants are disease resistant, prevention is your best path to a healthy rose. Never water the plant itself, but the soil around it. Also make sure you water in the morning so the foliage is dry by the time temperatures drop at night.
According to the Conrad-Pyle, Co., which markets Knock Out roses, the plants should be cut back to a height of 12 to 18 inches once a year in early spring, and receive five to six hours of full sun per day for best performance.