The camellia (C. japonica) is a popular blooming shrub native to eastern and southern Asia. The camellia, which can live for 100 years or more, is known for its large and colourful blooms that appear in cooler months when other plants are dormant, as well as its dark green shiny leaves. However, camellia diseases can damage or kill a camellia, so identification of what is causing uneven growth or discolouration is important for control.
Camellias don't have many insect pests, but the most damaging are the aphid, spider mites and scale. Aphids are tiny insects that live and multiply rapidly under the surface of the leaves and suck the plant's sap, causing stunted leaf growth. Also, the excrement from the aphids can quickly cover underlying leaf surfaces, where it forms a black sooty mould known as honeydew that attracts ants, flies and other undesirable insects and interferes with photosynthesis. Apply neem oil to the underside of the leaves, wait 24 hours, then use a strong spray of water to clean the dead aphids and honeydew off the plant. Insects can spread camellia diseases from one camellia plant to another.
Spider mites also live on the underside of leaves during the summer and cause a bronzing of the leaves with some web development seen with heavy infestation. Use the same method for control as for aphid infestation.
Tea scale shows up as tiny elongated brown spots on the underside of the leaves. Use a soap solution during the summer or light dormant oil in the winter for control. Tea scale is common in nurseries that grow camellias, so look carefully at the top and underside of the foliage of a new plant before purchase.
Nematodes are tiny wormlike creatures that burrow into the roots of the plant, where they multiply and block water and nutrients from reaching the above-ground portion of the plant. The plant will weaken and die, usually during the warmer months. The roots of the dead plant will have numerous bumps or knots. Adding compost around the plant and avoiding chemical fertilisers high in nitrogen will keep the soil healthy so natural microorganisms will destroy the nematode population. Nematodes occur in dry, sandy soil. Nematodes are usually a localised problem, so avoid replanting in the same place.
The most common fungal disease of camellia is known as virus variegation. The leaves have yellow or light green streaks. There is no cure, and the plant can live with the virus.
Dieback is caused when fungus enters a wound in the plant. The plant will suddenly wilt and die back. Prevent this by sterilising cutting tools with bleach. Apply a fungicide designated for camellias.
Camellia flower blight causes the flowers to turn brown and hard and fall off. The fungus lives on the fallen blooms and is carried by the wind. Keep the area around the plant clean of fallen vegetation and mulch well. Apply a fungicide designated for camellias.
Algal leaf spot shows up as circular leaf spots with wavy edges and is caused by too much overhead water and low air circulation. Armillaria root rot causes sudden plant death; the plant's roots are rotted, with slimy fungal growth. The disease is spread from roots of other plants as well as naturally occurring fungi that lives in the soil. Be sure the camellia has good drainage.
Most camellia diseases can be avoided by using good sanitation methods such as raking up and discarding old flowers and leaves and providing good air circulation. Overhead watering should be avoided, especially during the summer months. Camellias flourish in partial shade in acidic soil that drains easily but is not always dry. They will not thrive in alkaline soil. Unlike many southern plants, camellias prefer the north side of buildings where they can warm up slowly on a cold winter day. They are generally hardy to Zone 7, but flowers can freeze if temperatures remain below freezing for an extended period.