Diseases
Powdery mildew
Caused by the fungus Microsphaera penicillata, this disease causes azalea and rhododendron leaves to discolour and become coated with a white powdery fungal growth. Control of the disease is difficult, especially in certain weather conditions that favour the disease, i.e. when it is warm and humid. Plants that are young or are growing in the shade are often most susceptible.
For further information please refer to the U.S. Department of Agriculture (USDA) fact sheet.
Petal blight
Caused by the fungus Ovulinia azaleae, this disease destroys the petals of the flowers in spring and summer, thus reducing the aesthetic appeal of the plant. The fungus does not necessarily reduce the vigour of the plant.
Leaf gall
Caused by the fungus Exobasidium azaleae, this disease results in swelling of azalea and rhododendron leaves, and occasionally flower petals. The disease is not generally considered significant enough to warrant control. Usually removal of the leaf will be enough to minimise spread of the disease.
For further information please refer to the Cornell University fact sheet.
Root diseases
Rhododendrons are very susceptible to both Phytophthora and Armillaria diseases. See Information pages on Phytophthora and Armilliaria for more information.
Leaf scorch
It is quite common for rhododendrons to suffer necrosis at the margins of their leaves. This can be a response to adverse weather conditions, to salt, or incorrect soil pH, or it can be an indication that the plant is affected by Phytophthora Root Rot.
Botrytis blight
Azalea and rhododendron flowers can be affected by this disease. The disease is characterised by a grey powdery fungal growth over the flower. The fungus will kill the flower.
For further information please refer to the Cornell University fact sheet.
Pests
Lacebug (Stephanitis pyrioides)
As their name suggests, these small insects have lace-like wings. Lacebugs are sap-suckers and cause azalea and rhododendron leaves to become mottled, and the lower leaf surface to become dotted with brown sticky spots of excreta.
For further information please refer to the University of Minnesota fact sheet.
Thrips (Thrips imaginis)
This insect is a seasonal pest, causing most damage in mid to late summer. Signs of damage include silvering of the foliage and distortion of growing points and flower buds. The underside of leaves may be covered with excreta of the insect, which are like brown tar droplets.
There are no effective specific biological controls and thus control can be difficult to achieve. Thrips are preyed upon by ladybirds and lacewings but these predators may not be able to control thrips in heavy infestations.
Two spotted mite (Tetranychus urticae)
Damage by mites causes silvering and mottling of foliage and distortion of flowers and growing points. The presence of webbing on the underside of the leaf is also a distinctive feature of mite damage. The mites themselves are just barely visible to the naked eye.
For further information please refer to the University of Minnesota fact sheet.
Good control of these arachnids may be achieved by using predatory mites that have been imported into Australia for this purpose. The two main species used are Phytoseiulus persimilis and Typhlodromus occidentalis. Phytoseiulus persimilis is more useful in warmer climates as it cannot survive colder winters - although strains are now being selected for cold tolerance. Typhlodromus occidentalis is able to survive colder winters and is used successfully in apple orchards in colder parts of Australia.
Azalea leaf miner (Caloptilia azalaella)
Caterpillars feeding inside azalea and rhododendron leaves cause angular areas of leaf damage which then go brown. The larvae of the caterpillars then curl the leaf tip over and web it into place. Control can normally be satisfactorily achieved by manual removal of damaged leaves.
For further information please refer to the University of Florida fact sheet. Please note that biocontrol insects recommended in the Fact Sheet may not be available in Australia and biocontrol companies in Australia should be contacted to check availability.