Yellow Leaves on a Knockout Rose Bush

Excessive Moisture

• Because the Knock Out cultivar is naturally disease resistant, the most common cause of yellowing in the leaves is over watering. Roses require regular watering, but slow-draining soil can lead to excessive moisture in the soil. Do not water the shrub until the ground around it feels dry to the touch. Then soak the shrub with water, or install a drip irrigation system to provide a small amount of constant moisture for the rose.

Insects

• Spider mites and thrips feed on the leaves, stems and blooms of roses and other plants. Spider mites are small, nearly invisible creatures that create webs between the leaves and stems of the rose bush. Thrips are also small insects that are difficult to see. Both spider mites and thrips suck the sap from the shrub, which leaves yellow spots on the leaves and can cause the blooms to brown and wilt. Control spider mites with a strong blast of water to knock them off the plant, or try insecticidal soaps or horticultural oils. Remove open blooms with thrips and apply an insecticide for thrip infestations on the rest of the shrub before the remaining blooms open.

Viral Infections

• Rose mosaic virus causes yellow spots or streaks to appear on rose leaves. As the disease spreads, the yellow spots become larger and will eventually cover the entire leaf. This virus does not spread through pruning like some other viruses can, but an infected plant can pass the virus on to other nearby plants. There is no cure or treatment for rose mosaic virus, and extended infections will eventually lead to the plant's death. You must dig up infected plants and destroy or dispose of them far from other vegetation to avoid spreading the disease.

Fungal Infections

• Black spot and rust are the most common fungal infections that cause yellowing of the leaves of Knock Out roses. These roses are naturally resistant but not immune to these diseases and can become infected. Black spot causes small black spots on the leaves that later develop a yellow halo. As the disease progresses, the infected leaves fall from the plant. Rust is a fungus that causes an orange substance on the leaves and stems of the plant. The leaves eventually turn yellow before falling in severe infections. Treatment for black spot and rust involves the application of a fungicide and removal of diseased plant parts. Prune and thin dense bushes to improve air flow between the branches and avoid crowding to help limit fungal infections in rose bushes.

Nutrients

• Nitrogen and magnesium deficiencies cause the leaves of Knock Out roses to yellow. A nitrogen deficiency causes yellow leaves, slow growth, spindly branches with few leaves and fewer flower buds. A magnesium deficiency causes yellowing along the edges of the older leaves on the shrub. To combat a nitrogen deficiency, apply a nitrogen-containing fertilizer until the symptoms improve and the shrub begins to grow again. Add Epsom salt to the soil around the rose bush to correct a magnesium deficiency.