Tree Care 101

Common Tree Pests and Diseases

Learn about the most common problems for trees.
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Hemlock Woolly Adelgids

Adults lay eggs in cottony white masses on young twigs of hemlock trees in early spring. • Feeding causes needles on infested branches to desiccate, turn grayish-green and drop from the tree, preventing new apical buds. Dieback of major limbs can occur within two years and progresses from the bottom of the tree upward. Heavy infestations can kill trees in as little as four years. Related: Adelgids, Aphids
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Aphids

Small, typically less than 1/8 inch long, range in color from bright orange or red to dull gray, may or may not have wings. • Attack almost all types of trees and shrubs, sucking sap from twigs, stems or from leaves, causing them to curl. Large infestations reduce plant growth and vigor. • Most aphids excrete large quantities of honeydew, a sweet, sticky substance that attracts wasps, ants and bees and can serve as a medium on which black fungus can grow.
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Scales

Divided into three groups: armored, soft and mealybugs. • Armored scales are less than 1/8 in diameter and have a waxy covering that is separate from the insect body. Soft scales are less than 1/4 inch in diameter and have a waxy layer that is part of the insect body. Both types suck the juices from trees and shrubs. • Feeding may reduce growth and cause yellow spots to appear on leaves prior to premature leaf drop. • Soft scales excrete honeydew, a sweet, sticky substance that attracts wasps, ants and bees and can serve as a medium on which black fungus can grow. Related: Aphids,
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Cottony Cushion Scales

About 1/8 inch long with hard, tan shell covered in white, cottony secretion. • Suck vital plant fluids from host, causing leaf and needle stunting and yellowing, twig and branch dieback and potentially death. • Can weaken plants, making them susceptible to borers or environmental extremes. Scales also excrete honeydew, a sweet, sticky substance that attracts wasps, ants and bees and can serve as a medium on which black fungus can grow. Related: Scale, Aphids, Whiteflies, Mealybugs
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Gypsy Moths

Larvae, or caterpillars, are about 1/4 inch long and are dark gray or blue. Have black hairs and colored spots. Can be up to 2 1/2 inches long. Egg masses are tan, teardrop-shaped and about 1 1/2 inches long. • Larvae feed on foliage of hundreds of plants but common hosts are oaks and aspen. • Feeding creates holes in leaves and leads to defoliation. Related: Caterpillars
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Japanese Beetles, Adult

Oval, 1/2 inch long, metallic green with copper-colored wings. • Feed on leaves of hundreds of trees and shrubs, including cherry, birch and elm trees. Feeding results in skeletonizing of the leaves, making them appear lacelike. Related: Beetles
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Azalea Lace Bugs

About 1/8 inch long with light-brown bodies. Adults have lacy wings with dark markings; nymphs are black and spiny. • Suck sap, causing spotting on upper leaves. Tarry excrement buildup on undersides of leaves. Heavy infestations result in white leaves and leaf drop. Related: Lace Bugs
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Elm Leaf Beetles

Olive-green with black stripes on back. Yellow to gray eggs appear on undersides of leaves in double rows of 5 to 25. • Larvae skeletonize leaves; adults chew entire leaf, causing defoliation, leaf drop and elm tree weakening. Related: Beetles
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Birch Leafminers

Small (up to 1/4 inch long) larva; adults are black. • Typically attack gray birch, paper birch, white birch and cut leaf birch. Larvae feed between upper and lower surfaces of the leaves, causing blisters, leaf browning and leaf drop. Related: Sawflies
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Mealybugs

Have white waxy protective coverings that make plants appear to be covered in cotton. • Suck sap from tree, causing yellowing, stunted growth, galls and dieback and excrete honeydew that attracts wasps, ants and bees and can serve as a medium on which black fungus can grow. Related: Scale, Aphids, Whiteflies
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Black Vine Weevils

Adults are about 1/4 inch long and black. • Feed on outer edges of leaves of a wide variety of broadleaf evergreen and deciduous trees, creating a notched margin. Larvae feed on roots, increasing risk of root diseases such as Phytophthora. Related: Beetles
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Thrips

One-eighth inch long or smaller, slender with fringed wings • Suck sap from tree. Adult feeding on leaves causes discoloration, wilting, stunted growth and black spots. Some thrips also carry pathogens that can lead to fungal growth and damage. Related: None
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Whiteflies

Tiny, white, fly-like insects that usually congregate on undersides of leaves. Often swarm in clouds when disturbed. • Suck juices from plants, leading to wilting and stunted growth.
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Foliar Fungal Diseases

Includes leaf spot and anthracnose, which cause brown, black or tan spots on leaves. Angular spots around veins often indicate anthracnose. Lead to yellowing and leaf drop. • Caused by fungi. • Often appear on trees that also have cankers.
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Gall

Plant tissue growths, often on leaves, that vary in appearance based on the plant. • Caused by insects such as aphids, wasps, beetles and psyllids or bacteria. Treatment rarely necessary.
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Powdery Mildew

Fungal growth on surface of leaves and buds. • White and powdery. • May cause leaf deformity or curling.
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Rust

Fungi. • Causes leaf drop and brown, orange or yellow leaf-spotting.
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Fusarium Wilt

Fungi. • Infects roots and spreads through vascular system, causing dieback of young twigs and branches. Black or brown ring visible in tissue of dead twigs.
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Sooty Mold

Fungi. • This black-colored, threadlike fungus grows in honeydew secretions. • Coats the top sides of leaves, interfering with photosynthesis by blocking sunlight. Can lead to stunted growth and premature leaf drop.
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Borers

Borers are often the larvae of beetles and moths. • Larvae and adults make tunnels in a tree's branches, trunk or roots, leaving behind exit holes and excrement that looks like sawdust. • Damage can weaken structure, interrupt transport of nutrients and lower value of tree. Related: Beetles, Moths
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Asian Longhorned Beetles

Larvae of Asian longhorned beetles are roundheaded borers. Adults have long black-and-white antennae • Attack hardwood trees such as maples, willows, birches and elms. Emerging adults create perfectly rounded exit holes in bark. Larval feeding on outer sapwood creates tunnels, weakening tree structure. Related: Beetles
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Emerald Ash Borers

Larvae are flatheaded borers. Adults are bright green. • Larval feeding on the inner bark of ash trees disrupts the tree's ability to transport water and nutrients, resulting in dieback and bark splitting. Emerging adults create D-shaped emergence holes. Related: Beetles
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Bark Beetles

Vary in appearance. • Attack trees that are weakened, stressed or in decline. Attacks can be recognized by presence of brown boring dust, pitch tubes on the outside of the bark, galleries under the bark, beetle adults and larvae in the inner bark, tiny holes in branches and trunk, and yellowing or reddening tree crowns • Can transmit fungi that cause disease, such as Dutch elm disease.
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Cankers

Dead areas on branches or trunks of trees that are a secondary effect from diseases. May ooze or appear sunken or discolored. • Most cankers are caused by fungi, but can also result from insect infestation or environmental or mechanical damage. • Branches with cankers should be pruned back to healthy wood.
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Root Rot

Root decay usually caused by fungi favored by overwatering, root damage, drought stress or poor soil conditions. • Affected trees often wilt, even with ample soil moisture, and display dieback and leaf discoloration.

Photos

Image 5367245: William Jacobi, Colorado State University, Bugwood.org Image 1193003: Dean Morewood, Health Canada, Bugwood.org Image 5334046: Charles Hoysa, Virginia Cooperative Extension, Bugwood.org Image 5379740: Andrej Kunca, National Forest Centre - Slovakia, Bugwood.org Image 1455016: Whitney Cranshaw, Colorado State University, Bugwood.org Image 5383247: John H. Ghent, USDA Forest Service, Bugwood.org Image 2187069: Joseph LaForest, University of Georgia, Bugwood.org Image 5357472: Howard F. Schwartz, Colorado State University, Bugwood.org Image 5179083: GB Edwards,

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