Abiotic Disorders of Trees: Abnormal Growths and Biological Defects
Many plant species exhibit unique characteristics that are considered abnormal. These characteristics often form as the result of an abiotic disorder. The following examines several common abiotic disorders that are biological or botanical in origin.
Graft incompatibility can occur when two sections of a plant create a weak union. Failure of the union disrupts translocation in both the xylem and phloem, impeding the plant’s development. Translocation is an essential process that is responsible for distributing water and nutrients throughout the plant. When translocation is hampered by a weak union, sugars build up in the plant during leaf senescence. This causes the fall foliage to become more intense in color.
Weak graft unions are not easily detected. They generally surface when trees are mature, and have been weakened by a mechanical injury, or an environmental stressor. Graft incompatibility may also occur when there is variation in the growth rates of the scion and the understock. These differences can lead to disproportional growth at the junction of the two tissues.
Galls and Burls
Galls and burls are abnormal growths that can develop on woody plants. Galls may be caused by an insect or disease. They appear as swelling growths on the branches, twigs, trunks, flowers, or roots of susceptible plants. Galls vary in shape, resembling balls, knobs, lumps, or warts. There are more than 1,500 species of gall producers, the majority of which are comprised of insects. Galls that enlarge on smaller plants can disrupt the vascular system, impeding the flow of water and nutrients.
Burls form when a plant’s vascular system is disrupted by a virus, fungus, or bacterium. Crown gall bacterium is responsible for the formation of many burls. The bacterium contains a plasmid that alters the plant’s genetic composition, causing it to produce amino acids and chemical substances that form the burl. Burls are the preferred habitat for the bacterium. However, the bacterium will often disappear before the burl has fully developed. Removing a burl can cause extensive damage to plants, leaving a gaping wound that renders the plant susceptible to invasion from disease pathogens and insects.
The genetic composition of a plant contributes to its branch structure, or branch architecture. It also influences the angle of attachment that each branch establishes. Angle attachment is an important factor in plant development. It determines the strength and flexibility of the wood, while also affecting the plant’s ability to withstand environmental challenges. Plants that are predisposed to forming weak attachments are more susceptible to heavy winds, and loading from ice or snow. Vigorous upright branches with narrow branch unions are prone to storm damage. U-shaped branch angles are generally more stable, providing the plant with greater strength and flexibility.
Some plant species develop co-dominant stems. Co-dominant stems are formed when two or more branches emerge from the same junction, or tree fork. Many co-dominant stems contain included bark, a type of branch defect that often develops when two or more stems grow closely together. Included bark prevents branches from establishing strong attachments to the trunk, rendering them prone to failure during storms. To ensure the stability and longevity of plants that develop narrow branch unions, or co-dominant stems, corrective pruning should be performed on a regular basis. Cabling or bracing should be considered to enhance the branch architecture of plants that would not benefit from corrective pruning.
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