What does the fungus look like? Do the leaves turn yellow with black spots or is there powdery white mildew on the leaves? The Knock Out rose is usually very disease resistant so it is also important to figure out why the roses have such low defense - are they in too much shade, poor soil or poor drainage or is their crowding and a lack of air circulation? A fungicide will combat the disease but keeping your rose plants healthy is the best way to keep any fungus among us from coming back. Perhaps you can send a photo of the damage and I can be more accurate with my recommendation. Keep growing, Marianne Binetti