The Camellia genus (Camellia spp.) is primarily celebrated for its generous display of winter- and spring-flowering rose-like blooms, though its glossy green leaves are worthy of equal praise. Yellow foliage can severely downgrade the beauty of these evergreen shrubs and trees. Providing the right cultural conditions is key to preventing yellow leaves.
Scale insects are sap-sucking insects that generally appear in colonies on leaves, though some may also attack stems. A severe infestation may yellow leaves and cause stunted, shrunken blooms and twig dieback. You can combat scale insects by spraying the shrub with a refined horticultural oil in spring after the shrub has finished flowering. The Clemson Cooperative Extension recommends spraying twice, 10 days apart, when temperatures are between 40 and 85 degrees Fahrenheit.
If leaves are streaked or mottled with yellow, your camellia may be suffering from a virus such as Camellia yellow mottle virus or Camellia variegation virus. Though the growth of the plant may be impaired, the virus will generally not cause serious harm and some gardeners may appreciated the new "variegated" pattern on their plant's foliage. Viruses cannot be treated. Watering regularly will help keep your shrub healthy, though you can also replace it with a disease-free plant.
Camellias generally prefer partially shaded conditions and can develop leaf sunscald if planted in full sunlight or against a south- or west-facing building. Leaves may turn bronze or brown, eventually attracting leaf-spotting fungi. Prevent sunscald by planting camellias in a location with more shade. Only plant camellias that are suitable for your climate zone. Japonica camellias (Camellia japonica) may be grown in U.S. Department of Agriculture plant hardiness zones 6 to 9.
If your camellia's leaves are yellow, but the veins are still green, then your plant is probably suffering from a nutrient deficiency. Camellias require an acidic soil and generally do best with a pH between 5.5 and 6.8, according to the American Camellia Society. You can help make up for a lack of nutrients by fertilizing with a 10-10-10 or 12-4-8 fertilizer, following the rates recommended on the label. Camellias prefer a moist, well-draining soil generously covered with organic mulch.