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Plant NameTypeDescription 
Bacterial wetwood (Slime flux)ElmDiseaseWetwood or slime flux is a bacterial infection common in elm and poplar. The infection causes production of large amounts of moisture in the wood of trunks or large branches. The rancid-smelling, often brownish fluid seeps through the bark and is associated with discolored wood and streaks on the bark. Occasionally other symptoms including wilting, yellowing, and dieback may be associated with wetwood in elms. Typically, however, wetwood is primarily an aesthetic concern.
Dutch elm diseaseElmDiseaseDutch elm disease is a fungal infection transmitted by elm bark beetles. Bark beetles carry disease spores on their bodies and infect trees when they feed. Leaves above the infection site turn yellow and twigs die back, producing a characteristic "flagging" symptom. Premature leaf drop may occur. The infection spreads rapidly throughout the tree, plugging the vascular system and killing the entire tree in one to several years. Infected branches show streaking and discoloration of the outer layers of the wood when cut. The disease can overwinter in infected, dying, or dead trees, dead wood and stumps, and recently cut logs. The disease can also spread from infected to healthy trees through natural root grafts.
Nectria cankerElmDiseaseNectria canker is a fungal disease often found on twigs and branches that have been weakened by drought, frost damage, insect damage, or other diseases. Cankers are initially distinguished by discolored bark with coral or reddish fungal fruiting bodies. The cankers are sunken and often associated with wounds. Older cankers develop concentric, target-like rings of wood. Affected branches may have stunted or wilted leaves, or fail to produce leaves in the spring. Branches may be girdled.
AphidsElmInsectAphids are soft-bodied, pear-shaped insects that feed on the leaves. They are yellowish to green in color and approximately 1/16" long. Aphids often produce honeydew, a sweet, sticky substance which can develop a dark growth of black sooty mold. Deformed leaves are often associated with heavy infestations. Whitish cast skins are often found on leaves, as well. Some aphid species may also cause galls.
Bark beetlesElmInsectElm bark beetles are about 1/8" long and may be shiny brown or black. Adults, larvae, or pupae overwinter under bark. Adult beetles emerge in the spring (around May), leaving tiny "shotholes" in the bark. Adults feed on the young bark of elms, occasionally girdling and killing twigs. After feeding, the adult beetle lays eggs in galleries under the bark of branches, trunks, or in branch crotches of diseased or weakened trees. Adults will also lay eggs in recently cut wood when the bark is intact. The galleries are typically parallel to the grain of the wood, with the larvae making secondary galleries more or less perpendicular to the main gallery. Beetles emerging from trees infected with Dutch elm disease will infect the trees they attack.
CarpenterwormElmInsectCarpenterworms are the larvae of a mottled gray and black moth. The caterpillars are white to pinkish-red with a dark head and 1" to 3" long when mature. They bore in the trunk and main branches of elm, poplar, willow, locust, and other deciduous trees. Discolored or bleeding limbs, branch dieback, and lumpy or gnarled trunks may be indications of carpenterworm infestation. Sawdust-like material (frass) and wood chips mixed with loose webbing on or around infested trees is typical. Adults emerge in early summer. Trees with severe carpenterworm infestations are susceptible to wind breakage and may need to be removed. Infestations may eventually kill trees.
Elm leaf beetleElmInsectElm leaf beetles feed on the leaves both as larvae and as adults. The larvae appear in late spring (typically May-June) from eggs laid by overwintering adults. The 1/4" larvae are greenish or yellow with black stripes and spots. They skeletonize leaves, feeding on the underside and eating all parts except the veins and upper cuticle, which turn brown. Adult elm leaf beetles are approximately 1/4" long and yellow to greenish with two dark stripes. Older adults are darker. Adult feeding causes small holes in the leaves. Severe infestations may result in severe premature leaf loss. Trees may releaf in late summer. Repeated infestations may weaken or kill trees. Weakened trees are also more susceptible to attack by elm bark beetles, which can carry the Dutch elm disease fungus.
Elm leafminerElmInsectElm leafminers feed by removing green tissue from between upper and lower surfaces of the leaves. Symptoms of feeding by the yellowish or green larvae include large brown to gray brown blotches on the leaves. These damaged leaves may remain on the tree throughout the growing season. The adult elm leafminer is a small black sawfly. This pest attacks Scotch, Camperdown, English and American elm.
European elm scaleElmInsectThe European elm scale is a reddish-brown to purplish scale surrounded by a conspicuous fringe of white, waxy material. They sometimes resemble mealybugs. The mature females are about 1/8" in diameter. The crawlers emerge in late spring and summer and are yellowish to dark brown. The scales feed on twigs and branches as well as leaves, and overwinter as immature crawlers in bark crevices of small branches and branch crotches. Feeding scales produce honeydew, a sweet, sticky material which attracts honeydew-feeding ants and may become covered with a growth of black sooty mold. Infested trees may show yellowing leaves and early leaf drop, followed by death of twigs and branches. Severe infestations may cause extensive damage. The European elm scale feeds only on elms.
Spiny elm caterpillarElmInsectThe spiny elm caterpillar is the larval stage of the mourningcloak butterfly. The adult is approximately 2 1/2" across, with purplish-brown to black wings bordered by a creamy yellow stripe and a row of blue spots. The larvae are purplish-black with white specks and have a row of orange to red spots along the back. They also have long forked spines on their bodies. The caterpillars often feed in large groups and eat all the leaves on a branch before moving. Their feeding results in raggedly chewed leaves. Spiny elm caterpillars also feed on willow and poplar trees. They are not a serious pest.