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.
Heat stress is common in rose bushes. This results from direct heat during sun exposure or radiant heat from the ground. Radiant heat causes the lower leaves of the rose bush to turn yellow and eventually fall off. The upper leaves become exposed to the radiant heat and the process repeats. Adding pine straw, mulch or any other type of ground protection under the rose bush solves this problem.
If the leaves do not get enough sunlight, they turn yellow and fall off the bush. This happens to leaves at the bottom of the bush when the top leaves grow large and prevent the sun from reaching the lower leaves.
Over-fertilization causes yellow leaves on roses. The fertilizer actually burns the foliage and the leaves change color and fall off the bush. This occurs with both liquid and granular fertilizer. The way to avoid this happening is to only use the recommended amount of fertilizer and to be careful not to spray directly on the leaves.
Too much water also causes rose leaves to turn yellow. If water pools underneath the rose bush rather than the ground absorbing it, it causes yellowing. The sun reflects off the water and causes overheating on the leaves, which then turn yellow. If the ground under the rose bush feels damp, it does not need water.