Rosemary leaves can turn yellow because of cultural issues, whiteflies, botrytis blight and spider mites. Cultural issues are a common cause of yellowing rosemary leaves and occur when the plant's root system is too large for the container that it is planted in. This problem can be corrected by replanting the rosemary in a larger container or trimming the roots.
Spider mites feed on rosemary leaves, causing the leaves to turn or spot yellow. To remove spider mites from the plant, spray it with pressurized water for a few days to dislodge the mites, or use a natural insecticide. Whiteflies can cluster on the underside of rosemary leaves to feed on the plant. Shaking the plant can remove the whiteflies from the leaves, or plant owners may use a natural insecticide. A mature rosemary plant may not be harmed by a small whitefly infestation; however, a younger plant may die or become subject to mold due to residue left behind by the whiteflies. Botrytis blight affects the older leaves on a rosemary plant by infecting them with a fungus that turns the leaves yellow. The Botrytis blight fungus thrives in areas with poor air circulation, high humidity and cloudy weather. Infected rosemary plants should be destroyed to help prevent infection of other nearby plants.