Brown Bumps on Pin Oak Leaves: Harmless Leaf Galls

Quarter- to half-inch brown bumps on pin oak leaves, with brown undersides, are leaf galls caused by tiny gall wasps. They are cosmetic only—no harm to tree health.

Key Points

• Galls are plant tissue grown in response to wasp chemicals.
• Most leaf galls cause no measurable damage.
• Chemical control is impractical and unnecessary.
• Rake and destroy fallen leaves to reduce next year's population.
• Prune twig galls early if they appear on branches.

Natural enemies usually keep populations in check.