When rhododendrons and azaleas are properly planted and maintained, insects and diseases are less of a problem.  Usually, when problems do occur, they can be traced back to the plant's environment.  Two diseases, branch die-back and Phytophthora root rot in particular, are opportunistic this way.

Die-back of entire branches is usually 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.

Armillaria Root Rot is a common soil fungi on the U.S. west coast and elsewhere. Infected plants grow slower than normal plants, affected leaves may yellow, wilt and drop off. Death may occur after several years. Armillaria infection can be identified by the presence of honey-colored mushrooms at or near the base of the plant, and by a layer of white tissue between the bark and wood on the plant's trunk or on large diameter roots.

Powdery mildew sometimes exhibits the typical white powdery or fuzzy growth, but often takes on a completely different appearance.  The white powder form is prevalent on deciduous azaleas.  It is more severe on shaded plants and is worse in crowded plantings in damp locations. It is more severe in periods of cool, moist weather.  Management includes increasing air flow within and around affected plants, removing dead leaves from the ground and reducing the number of infected leaves.  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.  Consider growing mildew-resistant deciduous azalea cultivars.  When buying plants look for those specifically mentioning mildew resistance.

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.  A puzzling aspect of this fungal disease is its varying appearance on different cultivars.  For instance, the rhododendron cultivar 'Unique' shows almost no upper leaf change other than occasional very faint lighter yellowish areas, while the underside of the leaves is completely covered in brown spots.  Another cultivar, 'Virginia Richards,' gets brownish purple spots on both tops and bottoms of leaves.  Vulnerable cultivars include 'Elizabeth', 'Lady Chamberlain', 'Unique', 'Virginia Richards', 'Seta,' and 'Mrs. G.W. Leak'.  Many rhododendrons, if basically healthy, will coexist with the disease and seem to outgrow or at least survive the symptoms.  If you notice the symptoms on last year's leaves, consider protecting the new growth with a fungicide. Apply it to the new growth as it expands, before the symptoms appear.  Thorough leaf coverage is necessary for effective prevention.  The fungicide, 'Remedy,' which is a potassium bicarbonate (made by Bonide Company), is registered for the problem.  Fungicides containing sulfur (such as Safer Garden Fungicide RTU) are also registered, but should not be used when the temperature is over 85°F or within a few weeks of an oil spray application.

Gall is fruit-like growth in a leaf or flower petal caused by spores of the fungus Exobasidium.  Fungicide control of the disease generally has not been successful.  Removal and disposal of galls before they become white-colored is the most effective means of controlling the disease.

Petal blight causes spots in a flower petal to look like they are wet.  Eventually the entire flower becomes slimy and sticks to the leaves.  Spores of the fungus Ovulinia cause petal blight.  There are sprays for this disease, but the best control method is to remove all diseased material as soon as it is found, and to remove old plant material under the plants to prevent future infections.  Also avoid overhead watering and prune plants to keep them open so air can circulate through the plants since prolonged moisture promotes this disease.

Common insect pests on rhododendrons and azaleas include:

Lace bug: 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.  There are lace-bug resistant varieties, that are usually labeled as suitable for planting in sunny locations.  Lace bugs thrive in sunny locations since their natural enemies avoid these locations.  Rhododendron varieties that are susceptible to damage should be grown in more shaded locations.

Weevils: 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.  Since they cannot fly, they can be easily controlled with sticky substances like Tanglefoot that catch the adults when they climb the stem at night.  If you use this technique always make sure that no branches touch the ground or the weevils can bypass your trap.  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 extension service or garden center.  Biological control using entomopathogenic nematodes is also effective.

Thrips: Thrip damage is characterized by a bleached, silvery white discoloration on the leaf's upper surface and small black specks of excrement on the leaf's underside.  Thrips are a problem in warm and dry climates like California and New Zealand.  In cooler climates it's a common pest in greenhouses.  Foliar spraying with an insecticides or insecticidal soaps is used to control trips.  Insecticide application on the both the top and bottom of leaves is important for best results. Two sprayings one-to two weeks apart may be required for effective control.  It is best to remove infected flowers.

Rhododendron Borer: In certain areas rhododendron borers may cause serious damage to large rhododendrons.  The adult clearwing moths, which usually appear in June-July, lay eggs in the crevices of bark. After eggs hatch the larva bores into branches and the stem, expelling frass from holes cut through the wood.  Weakened stems or branches may break or die.  As soon as observed, infected branches should be cut and destroyed.  Be sure to cut low enough to eliminate the borer.

Other pests that may cause problems are spider mites that feed on the underside of leaves, scales that feed on stems, aphids that feed on new growth, the red-headed azalea caterpillar that feeds on leaves, and leafminers that tunnel in leaves.  If these problems are sufficiently troubling, seek advice from your local extension service or a garden center.

Insecticides and Other Chemicals: Always check with your local extension service or garden center when looking for chemical controls for insects and diseases.  These chemicals are controlled substances, and must be labeled for the problem and plant in question.  When using chemicals always carefully follow the instructions on the label, and use appropriate personal protection to insure your own safety.

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

Extension service websites with advice on dealing with insects and diseases:
Consult these sites for photos of insect and disease problems.
Alabama, California, Georgia, Maryland, Nebraska, New Jersey, North Carolina,
Ohio, Oregon, Pennsylvania, South Carolina, Texas, Virginia, Washington