Azalea Diseases in the Landscape
D. M. Benson, Plant Pathologist
Tom Creswell, Plant Pathologist
Phytophthora root rot (Phytophthora cinnamomi) is a serious, widespread, and difficult-to-control soilborne disease affecting a wide range of plants in North Carolina besides azaleas and rhododendrons. Other common landscape plants susceptible to root rot include dogwood, Camellia japonica, Pieris, yew (Taxus), Deodar cedar, mountain laurel, juniper, highbush blueberry, Fraser fir, white pine, shortleaf pine, leucothoe; and many other shrubs, annuals and perennials.
The symptoms of Phytophthora root rot vary with the cultivar. On Kurume hybrid types, such as Coral Bells, Sherwood Red, 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 a complete fertilizer can improve the green color of leaves but only for a short time. Older leaves may turn red on these cultivars in late summer. Excessive yellowing and loss of older leaves are the predominant symptoms on Snow azalea. On all cultivars, new shoot growth in the early summer is much less and stems are smaller in diameter than shoot growth of healthy plants. 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 few inches of the soil. The reddish-brown discoloration advances to the larger roots and eventually to the main stem.
High soil moisture and warm soil temperatures favor development of Phytophthora root rot. The disease is more frequent and severe in heavy clays or poorly drained soils than in well drained or sandy soils. The disease is common and severe in areas where run-off water or rainwater from roofs collects around plant roots. Setting woody plants deeper than the soil level in the nursery or container, over-watering plants, or long periods of heavy rain also favor disease development, especially in shallow soils with underlying rock or compacted hard pans.
Phytophthora root rot must be prevented, as chemicals are often ineffective in controlling this disease after aboveground symptoms appear. The following suggestions may aid in the prevention of root rot:
1. Purchase disease-free plants. Choose plants with normal green color and white or light colored roots. Avoid plants that appear wilted in the morning, have reddish-brown discolored roots or evergreen plants that have excessive winter defoliation.
2. When purchasing
new azaleas for the landscape, select disease resistant cultivars such
as those listed in table
4. Do not set the new plant any deeper than the soil level in the nursery container. Firm the soil beneath the soil ball so that the plant will not settle deeper into the soil after watering.
5. Avoid use of azalea in areas where root-rot susceptible plants have died. Instead, replant with plants that are not susceptible to root rot; such as Chinese holly, hybrid hollies (eg. Nellie R. Stevens), ligustrum or others.
6. For azaleas in irrigated landscapes, do not overwater. Established landscape plants need an inch of rainfall or irrigation per week in the growing season. Size irrigation equipment for your soil type so that rate of water application does not waterlog the soil.
for long-term control of root rot, the following control measures should
be utilized. They are listed in order of importance.
Azaleas are susceptible to the stunt nematode (Tylenchorhynchus claytoni). Leaves turn yellow and plants are stunted. Plants with a heavy infestation fail to respond to fertilizer and water and gradually die. There are no chemicals (nematicides) available to control nematodes on azaleas established in the landscape. The useful life of some plants can be extended by mulching, proper fertilization and watering during extended dry periods. If plants die, replant in an area free of stunt nematodes.
Exobasidium leaf gall (caused by the fungus Exobasidum vaccinii) is very common and widespread in the early spring on new leaves as plants leaf out. 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. If left on the shrub the affected leaves eventually turn brown. The disease is morealarming than damaging. Even highly susceptible cultivars are not seriously damaged. If you only have a few plants, control the disease by hand picking and destroying diseased leaves as soon as the swelling starts (April or early May in North Carolina). Leaf gall seldom causes enough damage to justify spraying a fungicide. This fungus also attacks rhododendrons and a related fungus causes leaf gall of Camellia sasanqua, the fall blooming camellia, as new leaves develop in the spring.
Ovulinia petal blight (caused by the fungus Ovulinia azaleae) can cause considerable damage to the flowers, especially on azaleas in the landscape near the coast. The occurrence and severity of the disease is highly dependent on wet, cool 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 infected tissue becomes soft and watery, such that the whole blossom collapses. The petal blight fungus only attacks flowers. Petal blight occurs more frequently on mid- to late-season cultivars than early ones. The disease also occurs on rhododendrons.
If petal blight has been a problem in your azalea planting in previous years, apply a fungicide just as flower buds begin to show color. A second application may be made if favorable weather conditions persist during the blooming period. See the NC Agricultural Chemicals Manual for recommended fungicides.
A serious disease of well-established azaleas in the landscape is Phomopsis twig blight (Phomopsis sp.). Indica cultivars are usually susceptible (see Table 2). Infected twigs first show wilting and death of leaves and defoliation on one or more branches. A reddish-brown discoloration can be found under the bark extending into the woody tissue on dying branches. The discoloration frequently shows up on one side of the stem and may extend from several inches to several feet along the stem. Twig blight is often worse following heat or drought stress.
can be controlled by promptly pruning out and disposing of the diseased
branches. Make sure all of the infected stem tissue has been removed by
making pruning cuts a few inches below where you see any discoloration.
Dip pruning shears in an isopropyl alcohol solution between cuts to prevent
spreading the pathogen. Fungicides are of little value in control of twig
The damage from powdery mildew (Oidium sp.) is generally slight, but the disease can cause damage on highly susceptible cultivars such as Hinodegiri as well as some deciduous azalea cultivars.
The disease appears as a diffuse white powdery growth on the leaves. It is more frequently seen in the fall months particularly after a dry summer. Diseased leaves may drop prematurely.
If the infection becomes serious consider spraying the plants two or three times with an appropriate fungicide.
Leaf rust caused by the fungus Pucciniastrum vaccinii attacks deciduous azaleas including Rhododendron periclymenoides (R. nudiflorum, Pinxterbloom azalea), R. prunifolum (Plumleaf azalea), and R. viscosum (Swamp azalea). The disease may be particularly troublesome on Knap Hill and Exbury Hybrids in the landscape.
Symptoms of leaf rust first appear in mid-summer as circular yellow flecks about an 1/8 inch in diameter on the upper leaf surface. The fungus can be found forming spores in rust-colored pustules on the underside of the leaf below the yellow flecking. The orange-colored spores in the pustules are disseminated by air currents to other azalea leaves. If the leaf remains wet overnight, new infections (pustules) can form repeating the cycle throughout the remainder of the summer and fall. When conditions are favorable for severe disease development with numerous pustules early in the summer, extensive defoliation of infected leaves can occur. In mild climates, the fungus may over winter as spores in pustules of fallen leaves. So removing leaves from deciduous azalea plantings in the spring can help reduce the severity of disease later in the season. However, the fungus also can be re-introduced to azaleas in the landscape each year from nearby hemlock (Tsuga canadensis) that serve as a second (alternate) host of the fungus. Normally, fungicides are not needed to control leaf rust. Several cultivars have been identified with resistance to leaf rust (Table 3).Table 3. Resistance of deciduous azalea cultivars to leaf rust.
Leaf spots may develop on older azalea leaves during fall and winter months causing premature leaf drop, but leaf spot usually causes very little or no damage to the plants. Control is usually not necessary.
There is a great deal of variation in cold tolerance between azalea cultivars. Low temperatures or sudden drops in temperature may kill the bark on stems and branches. The bark on one or more branches on a plant may be affected. Cold injury appears as peeling, sloughing, splitting 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. Consult a knowledgable nursery staff member regarding whether a cultivar is recommended in your area. See also http://www.ces.ncsu.edu/depts/hort/hil/hil-629.html for general information on the hybrids and groups and their adaptation to growing conditions in North Carolina.
fibrous roots of azaleas grow very close to the soil surface and can be
easily damaged by fertilizer. Leaves of azaleas "burned" by
fertilizer dry out and fall off. Severely damaged stems and small plants
with only few stems may be killed. Several light fertilizer applications
during spring and summer are better than one heavy application. Spread
it around the drip-line of the branches. Never apply in one heaping clump.
to Ornamental Disease Notes
with a specific problem, contact your local North
Carolina Cooperative Extension
Outside North Carolina, look for your state extension service partners.
Recommendations of specific chemicals are based upon information on the manufacturer's label and performance in a limited number of trials. Because environmental conditions and methods of application by growers may vary widely, performance of the chemical will not always conform to the safety and pest control standards indicated by experimental data. All recommendations for pesticide use were legal at the time of publication, but the status of registration and use patterns are subject to change by actions of state and federal regulatory agencies.
Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University at Raleigh, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.