University of Illinois Extension


James Schuster, Extension Educator, Horticulture, Countryside Extension Center

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Diseases and Insects of Shrubs and Small Trees

Second in a series of articles...

Barberry, Japanese - Berberis thunbergi
Verticillium wilt - Verticillium spp. are fungi that live in the soil and attack roots of many woody and herbaceous plants. The fungus invades the root and may travel up the xylem or may release spores that 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.

There are no major insect pests on barberry in Illinois

Boxwood - Buxus spp.
Dieback/canker - Nectria cinnabarina is a saprophytic fungus that will invade and kill stressed plant tissue. Under the right conditions, the canker will kill the entire plant. Winter injury makes boxwood more prone to this canker fungus. This disease also makes the plant more prone to winter injury. In dead areas on the woody tissue during the growing season, spore structures develop. The spore structures vary in color from coral pink to pinkish orange to purplish red. As they age the color changes to tan, brown and almost to a black like appearance. During summer months and into fall, additional spore structures that are round and orange red in color develop among the other spore structures. These can persist into the winter. Wet weather helps disperse the spores in these structures. The spores infect dead buds and other winter injured plant parts. The spores also infect through pruning wounds. Keep plants healthy by growing the plants in the right location and use good cultural practices in maintaining them.

Verticillium wilt – see Barberry

Winter injury may be caused by very low temperatures as well as drought stress. With excessively low temperatures, the moisture in the cells freezes (due to chemical compounds in plants, moisture freezes at various degrees below freezing). Drought stress already has resulted in limited moisture in the plant cells. Dry, freezing winds during the winter reduces the moisture level even farther often resulting in dead plant tissue. Leaves are usually the first to die, followed by buds and then the smaller twigs. Diseases can help magnify or increase susceptibility to winter kill. Nectria canker kills the sapwood tissue thus reducing or even cutting off moisture to tissue further out on the plant. Winterkill also makes plants more prone to infectious diseases and insect problems.

Yellownecked caterpillar, Datana ministra, eats a wide range of trees and shrubs. Found mostly in the southern two thirds of Illinois, they occasionally show up in the northern third of Illinois. They travel and eat in mass (usually thirty to a hundred) for protection. They also raise the front and rear portions of their body in mass when a possible predator (birds & other insects) appears. These insects over winter as pupae and the adult moths emerge in early summer. Eggs are laid on the lower side of leaves. Six to eight weeks later newly emerged caterpillars eat holes in the leaves leaving the veins alone. As the caterpillars enlarge, they eat the entire leaf leaving the just the petiole. There is a yellow to orange band around the body just behind the head. This is how they got their name. The stripes along the side of their bodies also vary some. They may be yellow and black to orangish and brown striped. The head is black.

Boxwood leafminer, Monarthropalpus flavus, over winters as a partially grown larva in the boxwood leaves. Warm weather in the spring helps the larva finish growing and become a pupa. A few days before the adult emerges, the pupa wiggles out of the mine to the surface of the leaf. A fly emerges from the pupa case. The flies are about the size of a gnat. After mating, females lay slightly more than two-dozen eggs inside the upper tissue of new leaves. Several weeks later, the larvae emerge and begin feeding. It is not uncommon to find multiple larvae in the same mine. Larvae grow slowly through the summer. Damage starts to appear about midsummer as yellow spots. Leaves may fall off early. In addition, heavy infestation can cause plants to look thinned out – even killing branches. Heavy infestation make the plants more prone to winter kill and diseases. There are few natural predators but there are boxwoods known to be resistant to this insect.

Oystershell scale, Lepidosaphes ulmi, is an armored scale (its shell is a hardened waxy material). It attacks a wide range of plants. The scale got its name because it resembles an oyster shell. It is found in every state but it is found more often and in heavier numbers in the northern states. The scale often blends in with the color of the plant’s bark. The scale is often “discovered” because branches are dying back from the feeding injury. There are two commonly found races of oystershell scale in Illinois. They are brown oystershell scale and gray ostershell scale. The scale overwinters as eggs under the mother’s shell. Crawlers of the brown race occur on dogwood when Vanhoutte spirea is in full bloom (about mid June in Northern Illinois). The crawlers of the gray race, which feeds on other hosts, emerge later.

October/November 2004: Diseases and Insects of Shrubs and Small Trees | Does Your Ash Tree Have the Emerald Ash Borer? | Harvesting and Storing Pumpkins, Winter Squash, and Gourds


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