Galls on oak trees are irregularly shaped growths, which can be found on the oak’s leaves, twigs, stems, flowers, bark, acorns and roots. Most types of galls are fairly benign and do no lasting harm to the tree. Two varieties, the horned oak gall and the gouty oak gall, can however, do significant damage to an oak tree.
Galls are formed when a tiny wasp or insect species injects a growth hormone into the plant. This hormone causes the plant tissue to grow abnormally and form an enclosure around developing wasp larvae. Early spring bud break is the usual time of year when oak galls form. The young insect then feeds on the gall tissue during its development. Once mature, the insect emerges from the gall, leaving an exit hole where it has left.
Galls have no uniform appearance; they can be any shape or size. Each type of insect species that causes galls creates an enclosure that is unique and indicative of its respective species. Some produce smooth galls; others create hairy or spiny ones. They can be spherical, oblong, dish-shaped or just appear as a large mass of tissue ranging from a fraction of an inch up to several inches in diameter.
Oak galls can be found throughout the United States and southern Canada. Each of the two harmful species, horned and gouty, seek different host oak varieties. The horned oak gall forms on pin oaks, scrub oaks, black oaks, blackjack oaks, water oaks and white oaks. The gouty type of gall has a slightly smaller range and affects pin oaks, red oaks, scarlet oaks, and black oaks. Knowing the species of oak you have helps to identify the species of gall that may be affecting it.
You cannot treat the galls themselves with pesticides, but even treating the pests that create the galls is extremely difficult. Good cultural practices are recommended over the use of chemicals. Infected twigs and branches should be pruned. Infected leaves should be collected and removed when they fall. Oak trees that are in decline are more susceptible to gall damage than healthy ones. A proactive approach including fertilization and monitoring of your trees is the most effective strategy to combat the damage caused by oak galls.