Corylus is a plant genus that is part of the Betulaceae family. It contains about 15 species of shrubs and trees that are native to the northern temperate regions of Europe, Asia, and North America. Commonly known as hazels, most of these plants feature clusters of yellow flowers and edible nuts. Landscapers plant the trees and shrubs in this genus as individual specimens or border shrubs. A common problem associated with growing these plants is pest infestation.
The name of this genus is derived from the Greek word korus, which means "helmet." It describes the shape of the nut shells. Most of the species in this genus have variations of the common name, hazel. For example, Corylus colurna is called Turkish hazel, while the contorta variety of Corylus avellana is commonly referred to as corkscrew hazel.
Hazel shrubs and trees are located in deciduous forests across three continents. Corylus yunnanensis is native to central and southern China, while Corylus maxima is distributed from the Balkans to Turkey. Corylus americana populates the eastern half of Canada and the United States.
Typically, the hazel tree grows about 26 feet (8 m) in height and spreads a similar length. The leaves are green and become yellow during the fall. In the summer, clusters of sweet nuts develop. They are oval in shape and have green husks.
Most hazel trees develop clusters of flowers called catkins before the leaves sprout. Catkins are shaped like a tube or cylinder, with flowers bunched together along a single drooping stem. There are male and female catkins. On hazel trees, the male catkin is usually yellow, while the female is red and shorter.
Hazel trees are typically planted in well-draining, fertile soil that has an alkaline pH. It tolerates loamy or sandy soil. The tree can be placed in an area that is exposed to direct sunlight or partially shaded.
Several insects feed on the leaves, flowers, and nuts of the hazel tree including aphids, caterpillars, gall mites, and sawflies. The adult sawflies feed on pollen, while the larva feeds on the leaves. Gall mites produce round, brown balls of organic matter on the branches. Each gall contains colonies of tiny insects. Aphids also leave behind a noticeable sign of infestation—honeydew.
Honeydew is a sticky residue that attracts mold spores. If left untreated, entire branches become enveloped in black sooty mold. Treating affected branches with insecticidal soap and fungicide reduces the chance of insect infestation and fungal growth.