AZALEAS: INSECTS & Disease

The following list gives a description of several of the most damaging insects that attack azaleas.

Pesticides are under constant review by the Environmental Protection Agency (EPA), so use only those that bear the EPA registration number and carry directions for home and garden use. County Extension agents and garden center operators have up-to-date information on pesticides for home use.

Peony scales infest the twigs, branches, and stems of plants. They are circular or oval with a tough covering. A waxy covering forms over their bodies; then, flakes of the tree bark seem to grow over the outer edges of the insects so that they become somewhat depressed in the bark pits.

Azalea bark scales are white, oval-shaped insects that are covered with fine threads of a wax-like secretion. Unlike most covered scales, they can move from one plant to another. They are usually found in the axils of branches and stems. These scales give off large amounts of a sweet, sticky substance called honeydew. A sooty mold fungus grows on the honeydew. This causes the branches and stems of heavily infested plants to look black and unattractive.

Spider mites are serious pests of many ornamentals, including roses, boxwoods, azaleas, carnations, hollies, pyracanthas, junipers, and several shade trees. Adult spider mites are oval and have eight legs. They vary in size and may be green, orange, red, brown, black, or a combination of these. As red is one of the most common, they are often called red spiders. Young mites (nymphs) resemble adults except they are smaller and have only six legs.

Spider mites puncture the tissues of leaves and flowers with needle-like mouthparts and suck juices from the plant. This destroys the chlorophyll around the puncture, giving the leaves and flowers a speckled appearance. As mites multiply, entire leaves become discolored and distorted, and they may drop off. These pests are very small and feed mainly on the underside of leaves. They often go unnoticed until plant damage is obvious.

Azalea lace bugs are small insects with black bodies and colored or variegated, lace-like wings. They feed on the underside of leaves. The upper leaf surface opposite the feeding areas becomes speckled, and the leaf looks light or bleached and eventually turns brown. Lace bugs give off large amounts of a dark, sticky substance on the underside of leaves. The substance may drop from the plants. Black globules on the lower surface of the leaves is an obvious symptom of this insect.

Azalea whiteflies are tiny, moth-like insects about 116 inch long and are covered with a snow-white, waxy powder. The immature forms are flat and oval, light yellow to green, and resemble miniature scales. Both adults and nymphs have sucking mouthparts and feed on the underside of leaves. The leaves become covered with honeydew, which causes a black sooty mold fungus to grow.

Azalea caterpillars are colorful, hairy insects capable of completely defoliating azalea plants. The full-grown caterpillars have a red head and a black body with rows of yellow spots and white hairs. The partly grown larvae are red to brownish black with white to yellow stripes. Most damage occurs in August and September; apparently there is only one generation each year.

DISEASE CONTROL

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Nematodes-microscopic eel-like worms-cause severe root damage. The root-knot nematode causes swollen knots or galls on the roots. Other kinds of nematodes feed on the outside of the roots and prevent normal root development. Affected plants grow slowly and quickly wilt in dry weather.

Damage from nematodes can usually be avoided by buying noninfested plants and planting them in sterilized soil. If nematodes are a problem around established plants, they can be treated.

Leaf gall is caused by the fungus, Exobasidium azaliae. Leaves or stems of the first flush of growth thicken and enlarge greatly if affected. Only a few leaves or twigs will be affected on each plant. Remove and destroy diseased parts as soon as they are detected to prevent spore formation. Spraying with recommended fungicides when new growth begins in spring helps prevent leaf gall infection.

Lichens are green to gray-green mossy growths on the stems of old, neglected plants. The lichen itself does not actually damage the plant. The lichen is usually a sign that the plant needs to be fertilized, cultivated, mulched, or treated for nematodes.

Phytophthora root rot is the most damaging disease of many container- and field-grown woody ornamentals. Disease outbreaks may occur during favorable conditions at almost any time in the production cycle as well as in landscape plantings to this devastating disease. Complete losses of azalea and rhododendron crops to Phytophthora root rot have occasionally occurred. Serious losses have also been seen on a range of Kurume azalea and juniper cultivars.

Symptoms. Phytophthora root rot greatly reduces the volume of the roots, which are needed by the plant to absorb and transport water and nutrients. The roots on diseased plants are brittle and brown to reddish brown in color. A network of fine, discolored feeder roots may be confined to one area, or it may include the entire root system. The causal fungi usually colonize the crown of the plants, often girdling the stem at or just above the soil line. A brown to reddish-brown discoloration of the tissues occurs just below the bark and may extend up the stem above the soil line. On some trees and large shrubs, brown, water-soaked cankers oozing a dark-colored fluid or gum may develop at the soil line.

Symptoms vary according to the degree of root colonization by Phytophthora root rot fungi, plant age, root-rot susceptibility, and environmental stress. Typically, symptoms first appear on one plant and later on surrounding plants. Some yellowing of the foliage, particularly at the shoot tips, leaf shed, slowed plant growth, and possibly limb dieback may occur in early stages of the disease.

These symptoms can easily be confused with those of a nutritional disorder, overwatering, drought stress, and a number of other factors. Slight yellowing of the leaves followed quickly by permanent wilting and plant death are the symptoms usually associated with Phytophthora root rot on Kurume azaleas and rhododendrons in Alabama. In landscapes, established plants may show symptoms of general decline for one or more years before succumbing to root rot, while newly planted azaleas will quickly die.

Table 1. Reaction Of Hybrid Azalea Cultivars To Phytophthora Root Rot.

Resistant Formosa Fakir Corinne Murrah Merlin Hampton Beauty Higasa Glacier Rose Greeley Polar Sea Redwing Chimes Alaska New White Shinkigen Rachel Cunningham Pink Gumpo Eikan Sweetheart Supreme General MacArthur Pink Supreme Morning Glow

Moderately Resistant Barbara Gail Rentschler's Rose Dorothy Gish Margaret Douglas Gaiety Gloria Kingfisher White Christmas Sensation Prince of Orange White Jade Copperman Hexe Massasoit Martha Hitchcock China Seas Warbler Amagasa Price of Summerville Hinode Giri Flander's Field

Susceptible Robinhood Hershey's Red Herbert Fortune Catawba Marian Lee Snow Royalty Rosebud Mrs. G. G. Gerbing Coral Bells Treasure Pat Kraft Saint James Carror Purple Splendor Pinocchio California Sunset Pink Pearl Johga Sunglow Hino Crimson Elaine Emily Pink Cloud Adelaide Pope Jane Spalding Sherwood Red Delaware Valley White

The foliage of azaleas and rhododendrons may also be invaded by some Phytophthora root rot fungi. Irregular blotches, which are first olive-colored and later brown, sometimes with a red margin, develop on the leaves. Diseased leaves are usually shed by the plant. Damage usually appears on limbs near the base of the plant.

Overwatering in the landscape will contribute to losses from disease. Phytophthora root rot is most commonly seen in landscape plantings on poorly drained, waterlogged soils. Disease development is usually slow or absent on well-drained sites.

Low soil pH (3.5 to 4.5) will suppress spore release, thereby reducing disease. However, Phytophthora activity is not slowed at soil pH's most conducive to plant growth. Overfertilization with nitrogen can greatly increase the disease. Soft, succulent tissues produced in response to excess nitrogen are readily colonized by root rot fungi.

Reaction Of Woody Ornamentals To Root Rot.

Azaleas and rhododendrons vary in their susceptibility to Phytophthora root rot. Among hardy azalea hybrid groups, the Indian, Glenn Dale, and Satsuki hybrids are more root-rot resistant than the more popular Kurume azaleas. In container nurseries, Kurume hybrids such as Hershey's Red, Sherwood Red, Snow, Coral Bells, and Hino Crimson have suffered heavy Phytophthora root rot losses, while nearby blocks of Indian and Satsuki (Gumpo) hybrids were largely untouched by the disease. The reaction of many popular hybrid azaleas to Phytophthora root rot is listed in Table 1.

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