Potomac Valley Chapter
of the
American Rhododendron Society
Rhododendron Pests and Diseases

Common Problems

• Phytophthera Root Rot
  The root systems dies and the entire plant turns yellow and appears to wilt as though it needs more water. This is usually the result of poor drainage or when the soil is too wet.
• Botryosphaeria Dieback
  Tips of branches and full stems turn dark brown, wilt, and die. If not cut out, the disease can work its way down the stem and kill the plant. The disease is usually more prevalent in hot dry weather when plants are under stress.
• Azalea Lace Bug
  Leaves become speckled and lose green color because tiny insects are sucking the chlorophylll out from the underside. Lace bug is more severe when plants are grown in too much sun.
• Petal Blight
  Sections of flowers or even entire blossoms become soggy and collapse. This disease is more common when hot, humid weather or rainy conditions develop, especially during midseason.
• Borers
  Eggs laid on a branch hatch into borers that chew their way down the center of the branch, eventually reaching the bas. Branches turn yellow and die, and the girdling at the base can occur. Some varieties are more prone to borers.
• Root Weevils
  Circular notches at the edge of the leaves may indicate that adult weevils are feeding on the plant during the night. The larva that hatch from eggs laid at the base can cause serious damage to the root system or girdle the crown at the soil line.

Insects and Diseases by Dr. Sandra McDonald

Azaleas and rhododendrons are not troubled by many insects. Southern red spider mites and two spotted mites are sometimes found on the underside of azalea leaves and very occasionally on rhododendron leaves. Plant grown in hot sunny dry locations tend to be troubled by these mites more frequently than plants grown in more ideal conditions. The mites feed by sucking juices from the undersides of leaves producing a stippled or sometimes burnt or rusty appearance on the leaves. Spider mites can be controlled by spraying with a miticide.

Azalea leaf miner is the larval stage of a small moth which lays eggs on the leaves. The eggs hatch into small caterpillars which mine the leaf tissue leaving tunnels and blotches in the leaves. The caterpillar then comes out, folds down the tip or edge of a leaf and does further feeding. Check with your local extension agent for insecticides currently labeled for control of azalea leaf miner.

Other insects which may occasionally trouble azaleas are aphids, lacebug, defoliators (caterpillars), leaf tier, scale insects, borers, weevils, thrips and whiteflies. Rhododendrons are occasionally troubles by aphids, borers, budworm, giant hornet, Japanese beetle, lacebug, scale insects, thrips, weevils and whiteflies. For identification of these insects and control methods, contact your state or local extension agent.

Twigs and branches of azaleas and rhododendrons can be killed by dieback diseases caused by fungi or by borers. Dead twigs and branches should be pruned back to healthy wood and removed from the vicinity of the healthy branches or burned to avoid reinfection or reinfestation. Pruning shears should be sterilized between cuts by dipping in dilute household bleach (1 part bleach and 8 parts water) or 70% alcohol to prevent infection of healthy branches. Plants that have been stressed by drought tend to be more susceptible to dieback diseases than unstressed plants.

Phytophthora root rot of rhododendrons and azaleas should be prevented rather than cured by selecting healthy plants and planting them in locations with good drainage. Root rot should be suspected if the plant is wilted despite adequate water. This is generally noticed in summer when warm soil temperatures favor root rot fungi. The diseased plant will quickly die from the roots upward and should be immediately removed along with the soil around the roots and disposed of where it cannot contaminate other azaleas, rhododendrons or any other plants susceptible to this root rot. A different kind of plant not susceptible to root rot can be planted in this spot. Presently, there are only temporarily preventative chemical drenches for root rot; no cures are yet available.

Petal blight can be an annoyance when azaleas and rhododendrons are blooming in a rainy spring. It is caused by a fungus which causes brown spots on the flowers which soon turns the flowers to a slimy mush. The flowers will not stay fresh and pretty as long as they normally would. Petal blight is more of a cosmetic problem than a health problem. A fungicide has currently been registered against ovulinia petal blight which goes by the trade name of Bayleton.

A few other diseases may trouble rhododendrons and azaleas, but they are not normally very serious.

Sandra McDonald, Ph.D.