Rhododendron scale disease is actually caused by an insect, Eriococcus azaleae, commonly called azalea bark scale. It affects rhododendron family plants, including azaleas. Rhododendron scales feed on the sap of the plant. Unhealthy and weak plants sustain more injury from scale insects than strong plants. Prevention and control of azalea bark scale requires proper plant maintenance and identifying early signs of scale attack.
Scales suck the juices from rhododendrons through stylets--tiny mouthparts that function as straws. They insert the stylet into the leaves and twigs of rhododendron, causing damage and depleting the plant's health. The insect releases a liquid and sugar mixture called honeydew. A large infestation can cause severe damage or kill the plant.
The life cycle of scales always includes an egg, immature and adult phases. The female azalea bark scale lays her eggs in the spring in a white, waxy coating she produces. This wax covers the entire body. The female dies as the eggs grow. The immature crawlers emerge over the course of 2 weeks. The immature scales feed on the newer growth of the rhododendron. The adult phase females typically stay in one place on woody plant parts, while the winged males suck on the leaves. The azalea bark scale survives the winter in the immature stage.
Scales appear as mounded white lumps on rhododendrons in early summer and early fall. Female scales are dark red, but remain hidden under the white mass. Watching the white mounds during hatching season will reveal the small crawlers moving along the plant searching for a spot to suck the sap. The azalea bark scale also feeds on ornamental cherry, poplar, willow and hawthorn trees.
The symptoms of rhododendron scale disease include a change in color, poor growth, damaged foliage and a decline in health. A black, sooty mold covers the plant where it grows on the honeydew. Masses of the white, waxy covering will be apparent in an infestation. Black ants, attracted by the honeydew, appear near scales and attack the natural predators of the scales.
Identifying azalea bark scale early gives an advantage in treatment. Scraping the scales off of the rhododendron effectively eliminates them. Beneficial insects, such as wasps, control scale population. The use of insecticide requires careful timing and close adherence to the manufacturer's instructions. Scales are only vulnerable to insecticides during the immature phase before they have created the white coating.