Fungus on Knock Out Roses

Knock Out Facts

• Knock Out varieties include double-flowering forms, and they come in a variety of colors. In addition to its disease-resistance, it has low-maintenance requirements compared to other varieties. Success with Knock Out roses depends on suitable growing conditions and a regular watering and feeding schedule. If fungus does form on the Knock Out, it needs to be treated immediately.

Detection

• An untreated Knock Out rosebush affected with black spot is subject to defoliation and dying canes. First, circular black dots appear on leaves. The foliage eventually yellows and drops, and damaged canes are susceptible to winter kill. Powdery mildew affects new growth, distorting young canes. The fuzzy white spore-containing growth covers shoots, leaves and buds during warm, humid weather. Botrytis blight causes a gray fungus to attack flowers and buds, and stems appear rotted just below the buds.

Prevention

• Crowded, unkempt Knock Out roses are most susceptible to fungus. They should be planted in a spot that gets at least six hours of sun exposure a day. There must be enough space in the planting zone so each mature bush gets unblocked air circulation. Avoid overhead watering, and remove dead and diseased canes anytime during the season. Remove any leaves near the ground around the bushes. Destroy any infected foliage. Apply surfactants, such as fungicidal soap, as a preventive throughout the season in susceptible climates.

Controls

• Biological controls or natural enemies do not exist for fungal diseases. Depending on what the fungus is, you should need to perform minimal fungal control on the Knock Outs. Studies at Cornell University demonstrated that products containing baking soda control powdery mildew. Sulfur products control black spot and powdery mildew, and products containing natural neem oil have fungicidal properties that should help the roses. Avoid using potently toxic conventional fungicides or products combining fertilizers, fungicides and pesticides.