by Ken Gohring
Following is a discussion of some of the known pests and diseases and descriptions of their harmful practices. This is not a complete listing but does include many of the more common pests and diseases
Rhododendrons and azaleas usually do not suffer greatly from insect damage but this is not always the case. Good culture practices are a key element of keeping plants resistant to bugs and germs.
There are 17 species of lace bugs that damage ornamentals in the US. Four of these damage plants in the heath family to which rhododendrons and azaleas belong. The most damaging of these is the azalea lace bug, which damages azaleas and some rhododendrons. This bug, about 1/8 inch in length, has lacy growth on its body and lacy like wings. These pests attack the underside of a plant's leaves causing white or yellow flecking on the top of the leaf. The primary damage is unsightly leaves. There are several insecticides, such as Sevin, that can be used to treat this insect. The pest is usually more harmful to plants growing in full sun and less harmful to those in shade. Another lace bug is the rhododendron lace bug, which feeds on rhododendrons and some azaleas. It is somewhat larger than the azalea lace bug but causes similar problems. The same chemicals can be used on both of these bugs.
Spider mites are not insects but are related to spiders. However, they can be serious pests. Two mites, the azalea mite and the southern red mite are problems for azaleas and rhododendrons. They are very small and are hardly visible to the human eye. They inhabit the underside of leaves but if the infestation is large, they will also appear on the leave's upper side. They suck the leaves causing them to change in color to dull green or in some cases gray or bronze green and usually resulting in the leaves dropping to the ground. One insecticide that can be used for mites is Ortho Systematic Insect Killer.
Plant injury caused by scales is usually slight to moderate in nature. Heavily infected plants become ragged and sometimes lose branches. Both rhododendrons and azaleas are attacked by scales. They are unusual insects. The most common scale is the azalea bark scale. These insects have protective surfaces, which make it difficult to control them with sprays. They vary in form by age and sex and adults are limited in mobility. Usually scales are found in leaf axils, branch forks and main stems. Both nymphs and mature forms suck sap for the plants. They can be scraped off and discarded. Insecticides such as Sevin will treat the nymph form of the insect. There are at least 3 other types of scales that can affect azaleas.
The larva of a moth, called redheaded azalea caterpillar, is a pest that can cause significant damage to azaleas. They are black with white stripes and a red head. They grow up to 2 inches in length and can eat large portions of azaleas before being detected. They can be easily recognized by their habit of raising their body in an upright position when disturbed. Sevin and Bayer Advanced Garden Multi Insect Killer are recommended insecticides.
The rhododendron borer attacks rhododendrons and azaleas. It is a caterpillar of the rhododendron borer moth, an insect that resembles a wasp. The adult lays eggs on the bark of the plant. The resulting borer is pale yellow with a dark head, about1/2 inch long. It chews a hole in the inner bark and digs tunnels in the plant's branches. In the fall, it enters the sapwood where it lives through the winter. The leaves of the branches inhabited by the borers change
color and wilt much like a plant stressed for water. The borer larvae can be detected by pruning wilted branches and looking closely to see if the borers are present. There is no known method of dealing with the borers other than pruning and destroying the larvae. The adult moths can be treated with Bonide Borer/Miner Killer or Spectracide Eight.
The azalea white fly is a small winged insect about 1/16 of an inch in length. In the adult form, white flies have a whitish coloration caused by a powder on their wings. The adults lay their eggs on the leaves of plants. The larvae hatch out over winter and appear in the spring, usually in large numbers. Both forms of the insect suck sap from leaves, causing them to turn yellow. A plant's lower leaves can become coated with a black mold that grows from secretions of the insects. The rhododendron whitefly is very similar to the azalea white fly. Both are controlled by insecticides such as Malathion applied at 7-day intervals at least 3 times.
Weevils are insects characterized by extended snouts or heads. When feeding on azaleas and rhododendrons, they cut notches in the outer edges of leaves and sometimes girdle a plant's stem. Their larvae feed on the plant's roots. The adults feed at night causing detection to be mostly by the damage done by the insects. Some weevils can reproduce without fertilization by another weevil, so a single weevil can infect a plant. Control measures include treatment of a plant's surrounding soil and spraying the plant with an insecticide that contains pyrethrum.
Root and Crown Rot
A fungus named phytophithora is a common disease problem for rhododendrons and azaleas. It is a watery mold that thrives in wet conditions. Symptoms include leaf curling and droop. The lower portions of the stem near the soil line turn brown. Plants that are set too deep are frequently affected by the disease. A commercial product, Subdue can be used to treat the spread of the fungus but it mhy not kill it. Good planting practices in good soil are helpful in preventing the disease from appearing.
The fungus affects mostly azaleas and sometimes rhododendron flower petals. Southern Indica and Kurume azaleas are readily susceptible. It starts with tiny spots giving the bloom a speckled appearance. These enlarge and the flowers rot. The infection spreads from bloom to bloom. It lives in the soil during the winter. Picking and destroying affected flowers and avoidance of watering the plants leaves and flowers help. If it is known that the fungus is in the ground at wintertime, new mulching will help. Fungicides with captan, maneb and other chemicals can be used to control the blight.
This disease causes rhododendron and azaleas to first have white powder-like spots on young leaves, which can spread to the entire upper and lower surfaces of the leaf. Cloudy, overcast weather conditions cause favorable conditions for the disease to occur. It is spread mostly by winds. Fungicides are used to treat infected plants. The commercial products, PlantShield and SoilGard contain a chemical that is recommended for this disease.
Leaf gall affects both leaves and flower buds. Under certain conditions, it can become quite severe causing considerable damage to mostly azaleas but can infect rhododendrons. The gall causes a growth on the plant that first appears as a light grayish green growth that enlarges in a contorted shape. Later the galls turn brown and become hard. It usually does not harm plants but does cause significant unsightliness. The recommended treatment is to pick the galls by hand and discard them. At times chemicals are used. These include the fungicides Mancozeb and Triadimefon.
Dieback is characterized by dying branches. The leaves of the branch die but remain on attached until late summer. When the infected branch is scraped away, a red-brown colorization is present. It is difficult to control. Best approaches include pruning the diseased branches, using sterile practices with equipment and avoiding any wounding of the rest of the plant. Plants in partial shade that are well; watered are more resistive to dieback.
Leaf rot first results in small round spots forming on upper leaf surfaces. Then yellow or orange spores form on the leaf's lower surface. These usually occur in late summer and fall. Some azalea cultivars are so susceptible that the entire lower leaf surfaces are covered with these spores, causing the leaves to fall to the ground. It can be controlled by fungicides that contain sulfur and copper sulfate (Bordeaux mix).
In the east, white tailed deer can cause significant problems for azaleas and rhododendrons. The best control measure is fencing, either fairly high woven wire or electrified fences. In recent years, several deterrent sprays have been formulated and have proven to be effective to some extent. Many of these deterrents replicate the smell of predators, like coyotes. The repellent, Dear Scram, has been used effectively by Azaleas Chapter member, John Kohli.
Squirrels and Chipmunks
The animals can cause problems by digging into mulched plants. Sometimes chipmunks dig a hole near or under the plant for habitation purposes. These animals can be humanely trapped and relocated to other areas. Pet dogs and cats are sometimes effective in controlling these rodents.
These animals will eat parts of azaleas. They can be trapped and relocated. Relocation will not completely solve the problem because new animals will move in replacing those relocated.
There are multiple sources on the Internet that can provide more detailed information in dealing with diseases and pests. A particularly detailed reference is available at http://pubs.cas.psu.edu/FreePubs/PDFs/agrs025.pdf. This publication by Penn State University is 114 pages long and includes information on most trees and ornamentals.
Clemson University offers the following publications, which are oriented toward azalea and rhododendron disease and pests: /hgic/pests/plant_pests/shrubs/hgic2050.html and http://www.clemson.edu/extension/hgic/pests/plant_pests/shrubs/hgic2051.html
Probably the best source for disease and pests may be found on the Culture and Problem pages of Steve Henning's excellent website.