Red Spots on Oak Leaves: Leaf Spot or Gall Insects?

Diagnosing Reddish-White Spots on New Hampshire Oak Foliage

An oak in New Hampshire shows reddish spots on the upper leaf surface and a whitish underside with red lines. Leaves are not misshapen or blistered. The owner wonders if it's a disease.

Two Likely Causes

1. Fungal Leaf Spot

• Caused by various fungi (e.g., Tubakia, Actinopelte) or bacteria.
• Spots vary: brown, tan, red, with concentric rings or dark margins.
• Favored by wet springs, overhead watering, poor air flow.
• Primarily aesthetic; rarely kills established oaks.

2. Leaf Gall Insects

• Tiny wasps or midges lay eggs on leaves, secreting chemicals that induce gall tissue.
• Galls can be red, white, fuzzy, or spiny—colors and shapes vary by species.
• Insects develop inside and emerge; galls remain.
• No control needed or practical—timing is impossible for homeowners.

Management: Live With It

• Rake and destroy fallen leaves to reduce fungal spores.
• Avoid overhead irrigation; water at base early in day.
• Prune for air circulation.
• Fertilize based on soil test to maintain vigor.
• Fungicides only justified for high-value specimen trees with repeated severe defoliation—must start at bud break.

Neither issue seriously harms the tree. Next year's weather will determine recurrence.