A previously healthy rose bush that suddenly shows signs of disease is cause for alarm and dismay among rose gardeners. One of the most common causes of yellow leaves in a rose bush is a fungal infection called black spot. When a rose contracts this fungus, black spots will appear on leaves; the leaves will turn yellow and then drop from the rose bush to the soil. Although the prognosis for black spot is bleak, a gardener has several options for minimizing long-term damage.
Clip off the infected leaves and branches from the rose bush with the pruning shears as soon as possible after you find yellowing leaves. Remove every unhealthy, yellowed leaf and branch from the rose bush at the nearest point where the leaf or branch intersects with the next largest stem or branch.
Discard the removed foliage and stems directly into the garbage bag. Do not compost these leaves. Clean up any fallen leaves and foliage that are littering the ground beneath the rose bush and discard these also.
Make a fungicidal spray. Dissolve the 1 tsp. of baking soda in 1 qt. of cool water. Add two to three drops of dishwashing detergent to the baking soda and water and stir the ingredients well to combine them. Pour the mixture into the spray bottle. Alternatively, you can also purchase a fungicidal spray appropriate to use on roses from any garden center.
Spray fungicidal spray onto the remaining foliage of the rose bush (and any surrounding bushes to prevent the fungal infection from spreading). Apply the fungicidal spray two to three times per month to the rose bush. Always reapply the fungicidal spray after any rain showers.
Consider removing the entire rose bush if the fungal infection is so severe that the rose bush does not respond to your pruning and fungicidal spray applications. If one-third or more of the entire rose bush has yellow leaves or is bare because leaves have fallen, the fungal infection is likely too severe to treat. If the yellowing leaves do not decrease after you begin treating the symptoms but rather the infection continues to spread, the infection may be too severe to control. In this case, removing the plant is best, to prevent the infection from spreading.