Diseases and Insects of Shrubs and Small Trees

Third in a series of articles

Bridal Wreath & Vanhoutte Spirea - Spiraea prunifolia & Spiraea x vanhouttei Dieback/cankers - 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. 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.

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 air borne.

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 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.

Spirea aphid - Aphis citricola is found in every state on spireas. The aphid occurs in large numbers and its feeding often causes the leaves to distort, curl and pucker. The aphid during the warm weather gives birth to live young that are all females. The new aphids can soon produce their own live young. In northern states, as cooling weather and shortening days occur, the aphids are born in a sexual state and lay eggs that over winter on any spirea that is available. Over crowding causes the aphid to give birth to winged aphids so that they can fly to other branches and spireas. Heavy rains can wash the aphids off the plants resulting in their deaths. Occasionally in Illinois (usually during hot muggy weather) a fungus will attack and kill the aphids. In addition, many predators feed on aphids. The aphid has a dark head and thorax while the abdomen is pale green.

Burning bush (also called Winged Euonymus) -Euonymus alatus

Cold injury – 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. 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.

Dieback/canker – See bridal wreath spirea. In addition Botryosphaeria dothidea will infect and kill for similar reasons.

Winged euonymus scale - Lepidosaphes yanagicola occasionally occurs in the southern half of Illinois on burning bush. It is an armored scale. And will attack several trees as well. This scale can cause premature leaf drop, branch die back and cause the plant to become more prone to winter injury. It is found between the “wings” – the bark ridges. It does not move to the plant’s leaves. The scale over winters as an adult and lays its eggs in June. Eggs may be laid for up to a month. Mating occurs before frost.

Euonymus scale - Unaspis euonymi – females are black and males are white. The scale causes the foliage to develop yellowish green spots. Heavy infestation results in early foliage drop and often stems are killed. Eggs survive by over wintering in the female body. The eggs hatch about early June in Northern Illinois. Crawlers emerge and move onto new growth or can be blown by wind to other plants.

December 2004 - January 2005: Choosing a Christmas Tree Variety | Diseases and Insects of Shrubs and Small Trees | Catalogs are Arriving, Plan Your Spring Garden Now | Keeping Holiday Plants | Prevent Ice and Snow Damage to Trees and Shrubs