Diseases and Insects of Shrubs and Small Trees

Fourth in a series of articles

Cotoneaster - Cotoneaster sp
Dieback/canker - Botryosphaeria dothidea & B. obtusa will invade and kill stressed plant tissue. Under the right conditions, the canker will kill the entire plant. Winter injury makes the plant more prone to these canker fungi. 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 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.

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.

Leaf crumpler, Acrobasis indigenella, is a skeletonizer when it first hatches but with the first molt begins eating most of the leaf. The insect uses silk, grass and dead plant material to protect itself against predators. The insect will over winter as a pupa in this protective covering. Occasionally more than one insect may be found inside this protection. Removal and destruction during the winter of these “crumpled” protected areas on the plant can help disrupt the insect’s life cycle. Moths begin to emerge from the protection about mid June reaching peak emergence in mid July. The insect causes the greatest damage to the plant in the month or so before emerging as an adult.

Dogwood sp. - Cornus spp (shrubs/small trees)
Dieback/canker - Botryosphaeria dothidea & B. obtusa are pathogens that attack stressed (planted too deep, over or under watered, mis-pruned and so on) red and yellow twig dogwoods. Within a few years the diseases can kill out entire plantings. To avoid losing the shrubs to these two fungi, keep plants healthy. Use proper/correct planting, maintenance and pruning practices.

Dogwood borer, Synanthedon scitula, adults are clearwing moths that look like a wasp. This borer not only attacks dogwood trees but also plants in the genera Prunus, Malus and other plant genera. Adults can be present from July to September. The adult female borer lays its eggs on the bark and dies soon after. The larvae eat their way into the bark where they feed on the phloem tissue. As the larvae grow larger, they eventually begin to feed on the sapwood. The adult female borer lays its eggs and dies soon after. The eggs are laid near injuries including pruning wounds. Avoid pruning during summer months.

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.

February - March 2005: Repair Storm Damaged Trees with Care | Diseases and Insects of Shrubs and Small Trees | Think Summer, Buy Summer Bulbs Now | Winter Damage to Home Lawns