- Black spot is very common rose disease, and knockout roses are not fully safe from this fungus. This disease first shows as black spots on young leaves, and progresses by eventually turning an entire rose leaf yellow and then causing it to fall off. Knockout roses are known for being strong, but avoiding wet foliage and not watering in evenings will help keep black spot disease from your knockout rose.
- Mildew is a known rose disease that comes as a result of high humidity and moist conditions. Treat infected knockout roses by removing and disposing of infected plant material.
- Water causes most concern for knockout rose lovers, as it can spread rose diseases. Water daily without over watering or causing soil to become mushy.
- Knockout roses that are planted in containers should have proper water drainage and enough space for roots to grow. Constricting or small containers will cause knockout roses' feeder roots to get hot, dry out and die. This will eventually cause the entire rose to deteriorate and die.
- The appearance of sticky, curled leaves is often mistaken for knockout rose disease; however, this problem is usually caused by aphids. Although aphids decrease aesthetic value of knockout roses, they do not spread diseases.
