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,
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
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
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.
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.