Rose bush leaves can turn yellow due to lack of water, spider mites or fungal diseases, such as blackspot and rust. Other symptoms are necessary to identify the exact cause of the yellowing.
When leaf yellowing is caused by lack of water, it is accompanied by wilting and shedding of leaves. This occurs mostly during hot, dry weather and can be remedied by watering the rose bush more. When a rose bush is infected by blackspot, the leaves develop small red, purple or brown spots with yellow outlines. With rust, some leaves may develop reddish spots on the underside, while others turn yellow and fall off. Spider mites pierce the leaves, leaving behind yellow spots, which eventually turn entire leaves yellow. Mites can be seen on the underside of the leaves.