University of Illinois Extension


James Schuster, Extension Educator, Horticulture, Countryside Extension Center

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Diseases and Insects of Shrubs and Small Trees

Sixth in a series of articles.

Forsythia - Forsythia spp.

Phomopsis gall - Phomopsis spp. fungi cause forsythia to develop abnormal growths (galls). The Phomopsis fungus can cause similar galls on azaleas. Galls can be as small as a pea to as big as an inch or more. The size of the gall varies with plant infected, the age of the plant tissue and where on the plant infection occurs. In general the galls are “round”. And are found singly or in small clusters. On forsythia the galls are often clustered. The fungus lives for several years in the galls before dying. Little is known about this disease’s life cycle. Prune out and destroyed infected plant tissue.

No major insect pest in Illinois

Honeysuckle - Lonicerasp.

Powdery mildew - Podosphaera clandestine is a fungus that has subgroups that are host specific. Powdery mildew fungi are white and look like powdered sugar. Most of the fungus grow on the leaf surface, but some of it may penetrate the inner cells of leaves and buds and even small twigs. In Illinois, the weather conditions are the same for almost all the powdery mildew fungi. The fungi need three consecutive days and nights that are warm & dry during the day and cool and humid at night. Freestanding water on the leaves actually inhibits powdery mildews from growing. However, freestanding water encourages many other foliar diseases to grow. If infection starts in the spring or early summer, consider treatment with a fungicide to prevent further infection and severe symptoms. If infection starts in late summer or fall, do not treat. Some powdery mildews can overwinter on dead leaves as well as live tissue. The disease is also air borne.

Honeysuckle aphid – Hyadaphis tataricae became a problem in Illinois in the early 80’s in the Chicago area. It has since spread throughout Illinois. The toxins released during feeding cause growth changes in the honeysuckle. Leaves fold over and protect the aphid from predators. The folded leaves also makes it more difficult to control these insects. In addition there is a shortening of branch tips and growth of dormant buds causing a witches-broom appearance. Eventually the affected tissue dies due to frost injury. About 90% of the aphid eggs are laid on the damaged tissue. Removing the dead witches-broom slightly delays severe infestation the following spring. As with most other aphids, only live females are born to the adult female till cooling fall temperatures and shortening days causes sexual males and females to be born. The sexual adults mate and the female then lays the over wintering eggs.

Hydrangea, Hills-of-snow - Hydrangea arborescens

Botrytis/gray mold – Botrytis cinerea is a fungus that attacks a wide range of woody and herbaceous plants. The fungus can attack leaf buds, flower buds, leaves, flowers, stems and fruit. On most woody plants, Botrytis is most often noticed in the open flowers. It also causes buds to die (bud blast). Plants growing in stressful conditions such as damp moist areas are most prone to infection. The disease can over winter in dead as well as live plant material. The more petals a flower has, the more likely it will get Botrytis following rainfall or overhead watering. The fungus can infect and grow between 32 degrees F. and 80 degrees F. Good air circulation and other good cultural practices can help reduce Botrytis infection.

Powdery mildew – see Honeysuckle

No major insect pests in Illinois

Japanese Flowering Quince – Chaenomeles japonica

Fire blight – Erwinia amylovora attacks plants in the rose family. The bacterium needs prolonged cool and wet weather to cause infection and death. The longer it stays cool and wet the more dieback that will occur. The disease most often causes the branch tip to bend back on itself (shepherds crook). The dead tissue turns a very dark brown. However on infected pear trees the dead tips turn black. The dead shepherds crook tissue is often described as looking like it was in a fire; hence the name fireblight. The bacterium overwinter in along the edges of cankered tissue from the previous year and oozes out in the spring. In the spring, bees can transfer the bacteria from these cankers to flowers during pollination. Hot or dry weather and even better – hot dry weather shuts the disease down till additional prolonged cold, wet weather occurs again the following year. Buy resistant plants when possible. Plant in the right location where there is adequate air circulation. Prune out infected tissue during very dry, cold (subzero if possible) weather or during very hot, dry (very low humidity) weather.

Cedar-quince rust - Gymnosporangium clavipes occurs on a wide range of rose family plants, including mountain ash, hawthorn, quince, serviceberry, crabapple, and apple (apples are resistant). In addition, eastern red cedars, common, prostrate, Rocky Mountain and savin junipers are possible evergreen hosts. In order to survive, the fungus must "move" from one type of host to another (e.g., from juniper to quince). On deciduous hosts, leaves, petioles, young branches and fruit are may be infected and symptoms vary widely among the various hosts. On quince the disease tends to infect the veins, causing the veins to swell to twice their normal size. Leaves curl and drop early. Even though sanitation is not perfect – follow good cultural practices. Follow recommended fungicide treatments. On the Internet (, see cedar apple, hawthorn and quince rust for additional information.

Eastern tent caterpillar Malacosoma americanum predominantly feeds on fruit type trees both edible as well as the ornamental forms. The insect over winters in an egg mass that is wrapped (not glued on) around twigs no thicker than a pencil. Finding and removing these egg masses while the tree/shrub is dormant and defoliated during the winter, eliminates damage to the tree the following spring. Eggs begin to hatch as new growth begins to emerge. The small caterpillars crawl down the branch to a branch crotch. Working as a group, each egg mass spins their own silk tent in the branch crotch. Waiting till the caterpillars return to the tent after it gets dark or it is cloudy and then physically crushing the nest with the caterpillars in the nest, greatly helps reduce feeding damage. There is one tent for each egg mass. Six to eight weeks after hatching the caterpillars leave the tree to pupate. Cocoons are white. In late June/early July the adult moths appear. Eggs are wrapped around twigs in Illinois soon after. Larvae are usually black with a white or yellowish stripe down their back. In addition there are blue spots between two yellow stripes that run down both sides of the caterpillar. The caterpillars are also “hairy”. Adults are reddish brown with two white stripes across each forewing.

Lilac, Common - Syringe vulgaris

Bacterial blight/canker - Pseudomonas syringa pv. syringae is a bacterium. Bacteria are weak pathogens; therefore, they need an injury (another disease, insect feeding & so on) or a natural opening (stomata, lenticels, pollen tube) to get in. The pathogen causes leaf spots, blossom blast, shoot blight and canker diseases. The more susceptible lilacs are Chinese, Japanese, Persian and common lilacs. If the lilacs have white flowers, they are the most susceptible. On leaves, the bacterial spots will be brown with a yellowish halo. Spots may merge and cause a blighted dark brown appearance. Entire leaves may die. Flower buds may be infected before they open. Flower buds may then die (bud blast). Succulent new growth may be girdled, causing the tips to droop and die. The infected area is usually black while the tissue above turns brown as it withers and dies. The pathogen tends to attack the new succulent growth. More mature growth is more resistant. Infected plants tend to be more prone to winter injury.

Botrytis/gray mold – see Hydrangea

Powdery mildew - see Honeysuckle

Verticillium wilt – Verticillium spp. are fungi that live in the soil that attack roots of many woody and herbaceous plants. The fungus invades the root and may travel up the xylem or may release spores which move upwards more quickly. In either case, this infection results in the development of brown streaks in the sapwood. In some plants, the color will vary from brown to green to yellow to purple and various combinations of all these colors. All gymnosperms (e.g., pine, spruce, juniper) and monocots (grasses) are immune to the two Verticillium species found in North America. Although not immune, there are several dozen types of woody dicots that are considered resistant to infection. Verticillium wilt can cause a rapid death in plants or a slow death. When the plants die quickly, removal and destruction of the plants is the best recourse. For plants dying slowly, prune out the dead and dying, fertilize correctly, and water properly. Make sure plants have adequate drainage. When putting new plants in, make sure they are the right plant for the growing site and plant and maintain using optimum cultural practices.

Lilac borer – Podosesia syringe is also called ash borer when it is in ash. The adults are clearwing moths that look like a wasp. Eggs are laid on the bark. The larvae eat their way into the bark where they feed on the phloem tissue. Eventually the larvae tunnel deeper and feed on the sapwood. This is a borer who makes two holes that can be seen with the naked eye. On lilac, the entrance hole is irregular and just above the ground. There is usually frass around the hole or just below the hole. The exit hole is larger and round. The adult female borer lays almost 400 eggs in less than a week and dies soon after laying the eggs. The eggs are laid near injuries including pruning wounds. Stressed lilacs and tree lilacs and ash trees are also susceptible. Keep lilac shrubs properly pruned, however, avoid pruning lilacs and ash during summer months.

June - July 2005: Last Issue | Use Mosquito Repellents Appropriately | Diseases and Insects of Shrubs and Small Trees | Garlic Mustard

Past Issues

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