Azalea and Rhododendron Diseases
Dr. Robert C. Lambe
Reprinted from Middle Atlantic Chapter News and Notes
There are several important diseases of azaleas and rhododendrons. They can be avoided if the following practices are observed.
1) Purchase and plant healthy plants.
2) Plant so that the plants have excellent drainage by planting high in well drained soil. Mulch to conserve water.
3) Provide moisture, cold protection and nutrients based on a soil test.
4) Prune out dead and dying stems and remove from the vicinity of the plant.
5) Apply fungicides to prevent Ovulinia petal blight, fungus leaf spots and Phytophthora root rot or wilt caused by Phytophthora cinnamomi.
Root rots that occur in the landscape cannot be controlled. Every effort should be made to prevent them from occurring. Fungicides like Truban, Lesan and Subdue are preventative fungicides. The treatment schedule differs due to the difference in their systemic activity. Follow label directions or consult your State Extension Plant Pathologist or County Extension Agent for assistance. Petal blight occurs in the spring and early summer in the southern, southeastern and middle Atlantic states. The fungicide Bayleton has provided the most satisfactory protection when applied to the flower buds as they begin to show color. The time of application will depend on the flowering period of the particular variety.
Die-back of azalea and rhododendron caused by the fungi Phomopsis and Botryosphaeria may be serious diseases. Apply water to the soil during prolonged dry periods and provide protection from desiccating cold winter winds. Prune out dead branches with pruning shears. Dip the shears in 10% household bleach or 70% alcohol between cuts to avoid spreading the fungi.
Powdery mildew caused by two different fungi occurs primarily on the leaves of certain hybrid deciduous azaleas. Usually, the disease occurs later in the summer. Plants grown in full sun will be less severely affected than those grown in the shade. Rust diseases that attack the leaves of rhododendrons and deciduous azaleas appear as yellow pustules primarily on the lower side of the leaves. Usually, rust appears late in the summer and chemical control is probably not practical.