Fungus balls on the branches of an oak tree are properly known as oak galls and result from an infestation of insects that chew into the bark or leaves. Their saliva is tinged with a chemical that spurs the tissue of the tree to produce the fungal gall, an object that serves as housing for the bug.
Living inside the gall, the insect utilizes the structure as protection from predators and as a food source. The gall is, in fact, a living part of the tree itself, rather than simply a separate pathogen or fungus. The galls typically begin to appear early in the growing season, usually around the time that buds on the tree break. Wasps and flies are the insects that most frequently stimulate the formation of fungus balls on the branches of an oak tree.
The gouty oak gall and the horned oak gall are two types of twig and stem galls that may grow as wide as 2 inches in diameter. These galls are made up of a hard, woody material that is so heavy it can force branches to droop. Horned oak galls are most common on water, pin, black and scrub oak trees, while the gouty oak variety frequently colonizes black, red, pin and scarlet oaks. These galls may require up to two years to form.
While the outer surface of a gouty oak gall is smooth, the horned oak gall, as the name implies, boasts two horn-like projections that grow from the ball. An adult female wasp emerges from each horn when they have reached maturity and are done feeding. Insecticides are usually not effective against galls due to the fact that the wasp and larvae are protected inside the hard shell of the object. The pruning away of infected leaves and branches is the most acceptable control measure.
Oak Leaf Galls
A variety of other types of galls regularly form on the oak, but these tend to exist on the leaves of the tree rather than the branches. Oak apple galls grow to as large as 2 inches in diameter and contain a spongy mass of material that surrounds a single wasp larvae. This fungus ball is usually found on the midribs of leaves. The jumping oak gall is a round, seed-like gall that drops from leaves. The movement of the mature wasp inside causes the object to "jump" up and down from its new home on the earth.