Insect and Disease Control of Rhododendron ZT

The following information is from the American Rhododendron Society http://www.rhododendron.org/disease.htm

Local advice should also be sought for control of insects and diseases, as these vary around the country. A common insect pest in the warmer areas is the lace bug which works on the underside of the leaf. The tiny young nymphs move around on the underside of the leaf and suck the juices causing a yellowish spotting. If there are many insects, the whole leaf will turn yellow, then brown and drop. They may be controlled with an insecticide such as Malathion, Cygon or Orthene, used according to the manufacturer's directions.

Another widespread insect pest is the root weevil. This may be one of several different species. The damage done by the adults shows up as notches chewed in the edges of the leaves.

Greater damage, however, is done after the adult weevil lays eggs on the ground under the plant. The eggs hatch into larvae which feed on the roots and stem, often completely girdling (removing a ring of bark) and killing the plant. Control is difficult because the adults move around freely, often laying eggs under plants on which they have not chewed. By the time root damage is noticed, it is too late to save the plant. Symptoms of a girdled plant are much the same a a dry plant, since the plant can no longer supply moisture from its roots to its top. The insecticide Orthene offers some control. Consult your local Extension Agent for other suggested insecticides.

In certain areas a stem borer may cause damage to azaleas and rhododendrons. The adult beetle, which usually appears in June, makes two girdles at the tip of the growing shoot about 1/2 inch apart and inserts an egg between the girdles. The larva bores downward, expelling frass from holes cut through the stem and pupates in the crown of the plant, just below the soil surface. The weakened stems are easily broken off and die. The girdled tips should be cut or broken off, as soon as observed, and destroyed. Be sure to cut low enough to eliminate the tiny borer whose threadlike brownish tunnel can be seen just below the girdles.

In some areas aphids, mites, scale insects, thrips or leaf eating caterpillars may cause some damage and may be controlled with a spray if they appear. Contact your Extension Agent for recommended chemicals, rates and timing of sprays.

There are several important diseases of azaleas and rhododendrons. They can usually be avoided if the following practices are observed:

Purchase and plant healthy plants. Plant so that the plants have excellent drainage by planting high in well drained soil. Mulch to conserve water. Provide cold protection if needed and moisture when needed as well as nutrients based on a soil test. Prune out dead and dying stems and remove from the vicinity of the plant. Where experience indicates the need, apply fungicides to prevent Ovulinia petal blight, fungus leaf spots and root rot or branch wilt.

Root rots that occur in some landscapes cannot be controlled once active in rhododendrons. Every effort should be made to prevent them from occurring. Fungicides like Subdue, Aliette and Truban are preventative fungicides. The treatment schedule will vary due to the difference in their systemic activity. Follow label instructions or consult your Extension Agent or Extension Plant Pathologist 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.

Die-back of azalea and rhododendron caused by the fungi Phomopsis and Botryosphaeria may be serious diseases. Protection from desiccating cold winter winds and adequate watering of the plants during prolonged dry periods helps to prevent these diseases. Any diseased or dead branches should be pruned out with pruning shears which have been dipped in 70% rubbing alcohol solution or 10% household bleach solution between cuts to avoid the spread of disease.

Powdery mildew caused by two different fungi occurs primarily on the leaves of certain hybrid deciduous azaleas. Usually the disease occurs late in the summer. 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.

Additional information about diseases and pests is available at the following web sites: http://www.ag.ohio-state.edu/~ohioline/hyg-fact/3000/3043.html Maintaining Healthy Rhododendrons and Azaleas in the Landscape (Ohio State University Extension) Discusses common encountered problems and provides useful solutions.

Controlling Insects and Diseases on Azaleas and Rhododendrons (Alabama Extension Service) http://www.aces.edu/department/ipm/Azalea.htm Good information on azalea diseases and pest problems.

The California Oak Mortality Task Force (University of California Cooperative Extension) http://www.suddenoakdeath.org/ Phytophthora disease affecting CA oak trees, huckleberry and some rhododendrons.

How to care for rhododendron http://www.rhododendron.org/plantcare.htm

From "Horticulture" magzine:

"Questiong: I have a number of evergreen azaleas whose leaves are turning yellow and falling off. Do they need to be fertilized? Could it be something more serious?

Answer: It could be normal leaf drop. Evergreen azaleas retain their leaves for three of four years before shedding them from the interior portions of their older branches. Some evergreen azalea varieties are prone to more frequent leaf-shedding than others, especially when one of the parents is semidecidous azalea.

Although azaleas have relatively low nutritional requirements compared to other shrubs, a deficiency will cause leaves to turn yellow. A lack of nitrogen causes leaves to turn yellow; inadequate levels of iron or magnesium cause they youngest leaves to develop interveinal chlorosis, which is characterized by yellow leaves with green veins.

Waterlogged soils or a high soil pH can reduce the availability of iron, as well as hamper roots' absoption of it. Drought or injury from voles, insects, or diseases will cause leaf yellowing and leaf drop. These symptoms can also be caused by certain herbicides accidentally coming into contact with your azaleas.

I recommend that you submit a soil sample from the vicinity of the affected azaleas to your local extension agency and have it tested for pH and fertility levels. The soil test results and a visual inspection of the plants should help you diagnose the exact problem."