Puffballs on an oak tree come from insects, fungi or bacteria causing round growths on the trees, called galls. Wasp larvae commonly cause oak tree galls. The galls have smooth, hairy or spiny exterior surfaces with puffballs and larvae inside.
Oak Apple Galls
Cynipid wasps cause oak apple galls, one of the most common gall types. These galls have green, apple-coloured exteriors with puffballs inside. The galls grow to 1 or 2 inches in diameter, making them noticeable. A wasp larva grows inside each puffball. Oak apple galls do not seriously damage oak trees, so many people leave them alone and deal with the appearance of the round galls.
Other Gall Types
Roly poly galls, wool sower galls, horned oak galls and gouty oak galls also make oak trees grow round galls that may contain puffballs inside. Wool sower galls appear as hairy masses on white oak tree. Roly poly galls contain structures that usually look more like seeds than puffballs. Gouty oak galls and horned oak galls are spiked or irregular looking on the outside. None of these galls severely damages oak trees, although they may cause smaller leaves or minor foliage drop.
Galls form when insects lay eggs on oak trees. The trees respond to the eggs or feeding larvae by forming a growth around them. The larvae then feed on the galls until ready to hatch as adult insects. Galls usually grow on stems and leaves but also grow from flowers, trunks and roots.
Getting rid of puffball galls with insecticides is difficult because the galls provide a barrier between the insects and the insecticide. Experts at the University of Kentucky recommend physically removing galls whenever possible because removal decreases the insect infestation. Prune heavily infested areas and destroy infested leaves as they drop from trees. Many gardeners leave galls alone because they do not severely damage trees.