- The most common bacterial disease affecting roses is known as crown gall, according to the the State of Connecticut Agricultural Experiment Station. Irregular galls appear on the roots, crowns and canes of the rose plant. Avoid crown gall by using care when pruning roses.
- Fungii are the most common cause of disease in rose plants. Common fungal diseases include black spot, powdery mildew, Botrytis blight, rust and cankers. Black spot causes black circles to develop on the leaves of the plants. Prevent black spot by removing infected leaves and canes. Powdery mildew causes a gray, powdery substance to develop on the plant. Treat powdery mildew with a fungicide and remove all infected plant matter. Botrytis blight causes buds to turn brown and decay; prevent it through proper aeration and fungicide treatment. Rust causes orange-brown spores on the underside of leaves. Treat it by pruning infected canes and leaves. Cankers cause sunken lesions to appear on the canes of the rose. In the spring, prune canes back to healthy, new wood.
- Streak and mosaics are the two common viral diseases affecting roses. Mosaic causes discoloration of the leaf veins and spreads through flower grafting, while streak causes brown rings to develop on the plant. Plants infected with mosaic and streak cannot be cured.