Contributors: J.R. Baker, Extension Entomology Specialist; D.M. Benson, Plant Pathology, Professor; L.F. Grand, Plant Pathology, Professor; R.K. Jones, Extension Plant Pathology Specialist; M.A. Powell, Extension Horticulture Specialist; D.L. Stephan, Extension Entomology Specialist; J. Scott, North Carolina Department of Agriculture; H. Singletary, North Carolina Department of Agriculture

Azalea Diseases

Generally, azaleas grow and bloom quite well across the state. Occasionally, diseases can be a problem for North Carolina azaleas.

Phytophthora root rot (Phytophthora cinnamomi) is a serious, widespread and difficult-to-control fungus disease affecting a wide range of plants in North Carolina.

STUNTING ASSOCIATED WITH ROOT ROT OF AZALEAS
Photo courtesy of the Dept. of Plant Pathology, North Carolina State University.

The symptoms of Phytophthora root rot vary by cultivar. On Kurume hybrid types, such as Coral Bells, Hinodegiri, and Hino Crimson, new leaves are smaller than normal with yellowing between the veins (interveinal chlorosis), possibly some purple coloration and defoliation. This chlorosis is often confused with a deficiency of iron or other nutrients. At times light applications of iron and complete fertilizer can improve the green color of leaves but only for a short time.

Excessive yellowing and loss of older leaves are the predominant symptoms on Snow azalea. Usually, large plants slowly decline in vigor and die, branch by branch, over a period of several months to years, but sometimes they die rapidly. Roots are reddish-brown, brittle and often limited to the upper portion of the media in a container. The reddish-brown discoloration advances to the larger roots and eventually to the main stem.

Phytophthora root rot is favored by high soil moisture and warm soil temperatures. The disease does not occur as frequently and may not be as severe on well-drained, sandy soils as in heavy clays or poorly drained soils, etc. The disease is common and severe in areas where run-off water, rain water from roofs, etc. collects around plant roots. See Pest Identification: Diseases and Nematodes for a discussion of control measures. The following table lists resistant and susceptible azalea cultivars to Phytophthora root rot.

Resistance of Azalea Cultivars to Root Rot Cause by Phytophthora cinnamomi.

Highly Resistant
Rhododendron poukhanese Fred Cochran Fakir
Formosa Corrine Murrah
Resistant
Merlin New White Higasa
Rachel Cunningham Shin-ki-gen Rose Greeley
Pink Gump Eikan Redwing
Sweetheart Supreme Pink Supreme Alaska
Morning Glow Delaware Valley Hampton Beauty
Glacier White Polar Seas
Chimes
Susceptible
Robinhood Barbara Gail White Gumpo Retschler's Rose
Frothy Gish White Gish Pink Hiawatha Margaret Douglas
Gaiety Gloria Kingfisher White Christmas
Sensation Prince of Orange White Jade Copperman
Hexe Massasoit Martha Hitchcock China Seas
Warbler California Sunset Jane Spaulding Amaghasa
Hinodegiri Hershey Red Herbert Fortune
Catawba Marian Lee Snow Pride of Summerville
Royalty Kow-ko-ku Rosebug Mrs. G.G. Gerbin
Coral Bells Treasure Pat Kraft Saint James
Carror Purple Spendour Pinocchio General MacArthur
Pink Pearl Johga Sunglow Hino Crimson
Elaine Emily Pink Cloud Adelaide Pope

AZALEA LEAF GALL
Photo courtesy of the Dept. of Plant Pathology, North Carolina State University

Azalea leaf gall (Exobasidium vaccinii) is very common and widespread in the early spring. The leaves become thickened, curled, fleshy and pale green to white. In the latter stages of the disease, the leaves are covered with a white powdery substance. The disease is more alarming than damaging. Even highly susceptible cultivars are not seriously damaged. If only a few plants are diseased, it is possible to control the disease by hand picking and destroying diseased plant tissue. Leaf gall seldom causes enough damage to justify spraying. This fungus also attacks rhododendrons. This disease also occurs on camellia but is caused by a different fungus.

Azaleas are susceptible to stunt nematode (Tylenchorhynchus claytoni) damage. Leaves turn yellow and plants are stunted and gradually die. They fail to respond to fertilizer and water. No nematicides are available to control nematodes on azaleas established in the landscape. Good growing conditions are essential. It is important to mulch and fertilize plants on the basis of a soil test report, and to water during dry periods. If plants die, they should be replanted in an area free of stunt nematode.

AZALEA PETAL BLIGHT
Photo courtesy of the Dept. of Plant Pathology, North Carolina State University

Azalea Petal Blight (Ovulinia azaleae) can cause considerable damage to the flowers, especially near the coast. The occurrence and severity of the disease is highly dependent on wet weather conditions during the flowering period. The symptoms are tiny, pale or whitish spots on colored petals and rust-colored spots on white petals. The spots enlarge rapidly and the infected tissue becomes soft and watery. Petal blight occurs more frequently on mid-to-late season varieties than early ones. The disease also occurs on rhododendrons.

Petal blight can be controlled by applying a labeled fungicide to the soil beneath the plants 1 to 2 weeks in advance of flowering. The North Carolina Agricultural Chemicals Manual should be consulted for current recommendations.

POWDERY MILDEW
Photo courtesy of the Department of Plant Pathology, North Carolina State University

The damage from powdery mildew(Oidium sp.) is generally slight, but it can harm highly susceptible varieties such as Hinodegiri, some deciduous azalea varieties, and some forcing varieties. The disease appears as diffuse, white powdery growth on the leaves. It is seen more frequently during fall months. Diseased leaves may drop prematurely. If the infection becomes serious, a labeled fungicide should be sprayed following label directions.

PHOMOPSIS TWIG BLIGHT
Photo courtesy of the Dept. of Plant Pathology, North Carolina State University

Phomopsis twig blight (Phomopsis sp.) usually appears on larger branches of established plants, particularly on Indica varieties. Infected twigs first show wilting and death of leaves on one or more branches. A reddish-brown discoloration can be found under the bark on dying branches, often on one side of the stem. This discoloration may extend from several inches to several feet along the stem. Twig blight is often worse following heat or drought stress. This is one of the most important diseases of azalea.

Twig blight can be controlled by promptly pruning out and disposing of the diseased branches. The plant should be checked carefully to ensure all of the discolored stem tissue has been removed. Fungicides are of little value.

Reaction of Some Azalea Cultivars to Phomopsis sp.

Highly Susceptible
Tradition
Moderately Susceptible
Copperman Fashion
Hinodegiri Treasure
Susceptible
Delaware Valley White Hershey Red
Pink Ruffles Orange Cup
Resistant
Pink Gumpo

Leaf spots may develop on older azalea leaves during fall and winter months causing premature leaf drop, but usually causes very little or no damage to the plants. Control is usually not necessary.

DAMAGE TO AZALEA STEM DUE TO COLD INJURY
Photo courtesy of the Department of Plant Pathology, North Carolina State University

Cold Injury. Azalea cultivars vary widely in cold hardiness. The bark on stems and branches may be killed by low temperatures or sudden drops in temperature. The bark on one or more branches on a plant may be affected. Cold injury appears as peeling, sloughing or cracking bark often near the soil surface. Injured plants or branches may die in early spring, late summer or several years later. Flower buds may also be killed without damage to the stems. Cold-damaged plants should be pruned back below the injured bark as soon as possible.

Fertilizer Injury. Azaleas have fine roots that grow very close to the soil surface. It is easy to burn the roots with fertilizer. Several light applications during spring and summer are better than one heavy application. The fertilizer should be spread around the drip-line of the branches.

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Last Modified: 07/10/96