Diseases and Insects of Shrubs and Small Trees

Fifth in a series of articles

European mountain ash - Sorbus aucuparia (small tree)

Apple scab is a rose family disease. The fungus, Venturia inaequalis f. sp aucupariae, is reportedly a subpopulation and only attacks mountain ash. The fungus will infect both the upper and lower leaf surface, and often invades the fruit too. Infection results in an initial olive green spot that soon turns a velvety brown color due to spore structures forming in the spots. The edge of the spots is not a sharp line but rather a more indistinct (fuzzy) margin. On the more susceptible plants, the pathogen causes the leaves to turn yellow and even drop early. On some plants, premature leaf drop may cause the plant to leaf out again later in the season. This disease seldom kills a plant outright. Instead it weakens the plant so insects and other diseases attack and kill the plant.

The fungus requires wet conditions to disperse spores and for the spores to infect. The colder it is the longer the plants need to stay wet for infection. The warmer it is, the less time a plant needs to stay wet. The fungus can infect during the entire growing season when the weather is suitable for the pathogen. The pathogen will over winter on the dead infected plant material. Remove infected tissue before spring. 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. 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.

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. Leaves curl and drop early. Even though sanitation is not perfect – follow good cultural practices. Follow recommended fungicide treatments.

Shot hole borer Scolytus rugulosus, attack stressed trees. They got their name because all the exit holes look as if the tree had been shot with a shotgun. The shothole adult borer follows the sapwood tissue rather than cutting across it. The female lays its eggs along the gallery it makes.

As the eggs hatch the larvae tunnel out sideways to the parent tunnel. Thus causing even greater damage. Eventually they emerge as adults giving the plant the shothole appearance. There are two generations in Northern Illinois. To minimize the risk of shothole borer infesting the tree – keep the plant healthy by planting in the right location, planting correctly and watering and fertilizing as needed. Remove all infested branches and destroy by chipping, burying or burning to help reduce re-infestation.

Dogwood borer – Synanthedon scitula, adults are clearwing moths that look like a wasp. 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.

Firethorn - Pyracantha coccinea (small tree)
Fire blight - see above under Mountainash

Uglynest caterpillar Archips cerasivorana gets its name from its messy feeding. These caterpillars spin a dense silk web around branches and leaves. This webbing/nest catches pieces of leaves as well as the insects’ frass (fecal matter). Larvae are yellowish green with black heads. They feed and nest in large numbers. Pupation occurs inside the messy web. Adult moths are dull orange and with a wingspan of between 3/4 to1 inch across. Adults emerge from midsummer to early fall depending on weather. Females lay their eggs in masses in plant stems or trunks during this same time period. Eggs hatch the following spring. The damage to the plant is mostly aesthetic.

April - May 2005: Discouraging Canada Geese | Diseases and Insects of Shrubs and Small Trees | Springtime Is Rose Time