Yellow leaves on roses can have multiple causes and how you treat them depends on the cause. Not getting enough water, too much wind and pests such as spider mites can cause yellowing. The most infamous cause, however, is black spot disease. This fungus infects the roses and causes black spots on the leaves that eventually turn yellow.
Water the rose bushes two to three times a week, making sure each bush gets 4 to 5 gallons a week and the water goes deep. Water them early in the morning and avoid getting the leaves wet if you know they have black spot.
Cover the area around the rose bushes' roots with mulch, preferably using tree bark. Make the mulch layer 2 to 3 inches thick to help seal in the moisture from watering.
Feed the roses fertiliser specially designed for roses. Look for fertiliser that contains magnesium and other trace minerals. This fertiliser can come in soil or spray form.
Spray the bushes with water two or three times a week in long streams if you see to prevent spider mites from nesting. Avoid this if you know the leaves have black spot, as the water spray transmits the fungus.
Trim off all leaves that are confirmed to have black spot and clear away all leaves in the vicinity, preferably by raking.
Treat black spot-infected roses by dusting them with sulphur powder to prevent further fungus from developing. In the morning watering, spray a quart of warm water mixed with 1 tsp of baking soda.
Knockout roses do not suffer from black spot disease, so avoid treating them for this condition; the cause of yellow leaves lies elsewhere if you have these roses.
Do not add too much fertiliser, this can lead to too many leaves with not enough blooms and start causing the yellowing condition.