Strange, often bizarre growths on the leaves, twigs, or bark of trees are known as galls. These unusual formations can look like small spheres, fuzzy bumps, or woody knots. Although galls are common across many tree species, their nature is often misunderstood, leading to concern among property owners. These growths are a direct result of an intricate biological interaction, representing how a tree can be manipulated by another organism.
Defining Tree Galls: An Abnormal Growth
A gall is a highly specific, abnormal proliferation of plant tissue that develops in response to an external stimulus. This growth is essentially a plant tumor, resulting from the plant’s localized, uncontrolled growth. This occurs through hyperplasia (an increase in cell number) or hypertrophy (an increase in cell size). The plant’s normal growth hormones are hijacked, causing a structure unlike any of the tree’s natural organs.
Galls can take on a remarkable variety of physical forms, ranging from a simple blister on a leaf to a perfectly spherical structure on a twig. Leaf galls are the most frequently observed, appearing as fuzzy growths called erineums, small nipples, or raised spots. Others can be hard, woody spheres, such as the oak apple gall, or irregular, knot-like swellings on stems and branches.
The Biological Triggers of Gall Formation
Gall formation is initiated by an organism that introduces a chemical or causes mechanical damage to actively growing plant tissue. This organism forces the tree to construct a protective shelter and a dedicated food source for its offspring. The chemicals secreted by the invader interfere with the plant’s growth regulators, causing the tree to form specialized, nutrient-rich tissue inside the gall chamber.
Insects are the most common inducers, including wasps, aphids, and midges, responsible for thousands of unique gall types. Gall wasps (cynipids) lay eggs in developing buds or leaves, triggering the formation of hard, distinct galls around the developing larva. Gall midges are the second largest group of gall-makers and use secretions to induce growth.
Tiny eriophyid mites also cause galls, often resulting in smaller, blister-like formations, such as the maple bladder gall. Beyond arthropods, some galls are caused by pathogens, including specific fungi and bacteria. For instance, the bacterium Agrobacterium tumefaciens causes crown gall, which results in large, tumor-like growths near the base of the tree.
Assessing the Damage and Management Options
For most mature trees, the presence of galls is primarily an aesthetic issue and rarely affects the tree’s overall health or longevity. The majority of galls occur on leaves and petioles, which the tree sheds annually, minimizing long-term impact. In these common cases, the tree’s photosynthesis is unaffected, and no intervention is required.
Damage becomes more significant for very young trees or when galls form on the main stem or major branches, potentially leading to branch dieback or girdling. Certain twig galls, like gouty oak gall, can cause serious long-term damage, but these are exceptions. Bacterial crown gall can also weaken the tree and may warrant a more aggressive response.
Management Strategies
Management strategies emphasize prevention and cultural control because, once a gall has fully formed, the causal organism is protected inside, rendering chemical sprays ineffective. For benign galls, natural predators often keep populations in check. For cosmetic concerns, pruning and destroying infested twigs and fallen leaves before the organism emerges can reduce future outbreaks. Chemical treatment is not recommended unless a severe, health-threatening infestation is present, and application must be timed precisely to target the gall-maker before the growth begins.