There are at least 232 species of the fungus Phomopsis. Several of these produce bark galls on bitternut hickory. The galls are identical in appearance making identification of the associated species in the field impossible.

Spores are produced throughout the growing season and are spread by wind and rain splashes. It is believed that spores infect a host by entering a wound of a young twig. The fungus then spreads to branches and to the trunk. The galls do not kill the host but reduce vigor and girdle small branches causing dieback. Uninfected trees may occur near heavily infected ones.

Galls may occur singly or in clusters on the trunk and branches. They are woody, rough, more or less round swellings. They appear as tight clusters of nodules. They can be very small to 10″ in diameter. If cut open they reveal disorganized woody tissue but no insect chambers or tunnels.

http://minnesotaseasons.com/Fungi/Phomopsis_gall_on_hickory.html