Dark Clusters on Oak Branches: Insect Galls, Not Fungus

Identification

The large dark clumps are woody twig galls (gouty or horned oak gall) caused by cynipid wasps. The black tar on the deck is likely honeydew/sooty mold from other insects, not the galls.

Gall Facts

• Two-year life cycle; adults emerge spring.
• Protected inside woody tissue—sprays ineffective.
• Heavy infestations can girdle branches.

Management

• Prune/destroy galled twigs when small (winter).
• Rake leaves.
• Fertilize/water to maintain vigor—healthy trees tolerate galls better.

Bottom Line

Galls rarely kill trees. No chemical control needed. Monitor; prune only if dieback occurs.