Rhododendrons and Azaleas | Insect and Disease Management

If rhododendrons and azaleas are not properly planted and maintained, insects and diseases can be a problem. 

Listed below are some common problems that may be encountered when the correct environment is not maintained.

Die-back of entire branches is commonly caused by the fungi Phomopsis or Botryosphaeria. These fungi thrive during dry periods when the plants are stressed.  If you notice that the leaves are wilted in the morning and the ground is dry, it is best to water to keep these fungi at bay. Infection takes place through wounds, such as new leaf scars, pruning damage, bark cracks, etc. 

Root rot, which can kill entire plants, is usually caused by the fungi Phytopthora.  If the leaves are wilted in the morning and the ground is moist, this is a symptom of root rot.  It is usually fatal.  Hence, it is best to avoid planting rhododendrons and azaleas in areas with poor drainage.  In fact, good drainage is one of the most important considerations.  Once symptoms are visible it is usually futile to try to stop Phytopthora from progressing in a plant.

Powdery mildew sometimes exhibits the typical white powdery or fuzzy growth, but often takes on a completely different appearance.  The white powder form is particularly prevalent on deciduous azaleas.  It tends to be  more severe on shaded plants or where plantings are particularly crowded and the location is also damp. Management of the mildew includes increasing air flow within and around affected plants. It is also very important to remove dead leaves from the ground to reduce the number of infected leaves in the area. It can be controlled with fungicides which should be applied during periods of new growth on the new leaves. Fungicides won't get rid of existing infections on old leaves.

On evergreen rhododendrons, light green or yellowish patches on the top of leaves sometimes accompanied by purple-brown areas on the backside of leaves are signs of powdery mildew.

Common insect pests found on rhododendrons and azaleas include:

 Weevils: (Curculionoidea)

If leaves have notches in the outside edges, these are caused by weevils. Weevils spend the daytime in the ground and come out at night to feed. Weevils not only eat the edges of leaves, but their larvae feed on the plants roots and stem, often completely girdling (removing a ring of bark) the stem and killing the plant. If a plant is in general decline and weevil feeding is evident, you may need to use a systemic insecticide recommended by your garden centre.

Lacebug (Stephanitis pyrioides)

As there name suggests, these small insects have lace-like wings. 

If you notice leaves that look diseased, always look at the under side.  A common pest is the lace bug.  Both adults and nymphs feed on the backside of the leaf resulting in formation of yellow spots.  Heavy infestations can cause leaves to turn brown and drop.  Lace bugs can be controlled by frequent spraying. Lace bugs thrive in sunny locations since their natural enemies avoid these locations. 

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 only barely visible to the naked eye.

The information above had been provided from the following sources:

https://www.rbgsyd.nsw.gov.au/science/plants/pests-diseases/azaleas-rhododendrons

https://www.rhododendron.org/disease.htm

Reference articles:

 Diseases Of Rhododendrons And Azaleas by Robert D. Raabe
 Rhododendron Diseases by R. K. Jones and D. M. Benson
 Azalea & Rhododendron Diseases by Clemson University Cooperative Extension
 Common Problems of Rhododendron and Azalea by Dr. Sharon M. Douglas
 How To Identify Rhododendron and Azalea Problems by A.I. Antonelli, et al.
 More on Mildew by Fred Minch
 Pests Of Rhododendrons by Arthur L. Antonelli
 Insecticidal Soap As An Azalea Lace Bug Control by Stanton A. Gill & Michael J. Raupp
 Root Weevils: Troublesome Rhododendron Pests by Hank Helm
 Thrip Information by UC Davis IPM