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Anthracnose of shade trees

Quick facts

  • Anthracnose is a common fungal disease of shade trees that results in leaf spots, cupping or curling of leaves and early leaf drop.

  • In Minnesota, anthracnose is most common in cool, wet spring weather.

  • Anthracnose is not a significant threat to the health of the tree and doesn’t require treatment in most cases.

How to identify anthracnose

Twisted green oak leaves with brown ends
Oak anthracnose

Leaf symptoms

  • Tan to brown irregular shaped spots or blotches on young leaves.
  • Infected leaves are often distorted, cupped or curled.
  • Severe infection can result in leaf drop in spring. Trees produce a second growth of leaves by midsummer if leaf drop occurs.
  • Anthracnose may cause tan to dark brown spots  on mature leaves but these leaves do not become cupped or distorted. Leaf spots on mature leaves are often found with minor wounds like insect feeding.
  • Leaf symptoms are often most severe on the lower and inner branches of the tree but may progress up through the canopy.
Ash leaves with brown spots that are distorting the leaves.
Ash anthracnose

Branch symptoms

Infections on green twigs are most common on young twigs of oak (Quercus spp.) and ironwood (Ostrya virginiana). These appear as small orange brown blisters or a brown band encircling the young twig resulting in shoot death.

Environmental conditions

In Minnesota, anthracnose is most common during cool (50 to 68 degrees F), wet spring weather.

Anthracnose can occur in the summer if cool, wet weather happens at the same time as leaf growth.

Large brown patches on maple leaves
Maple anthracnose

Trees affected by anthracnose in Minnesota

Anthracnose is caused by several different, but closely related fungi. Most fungi that cause anthracnose can infect only one type of tree. For example, fungi infecting ash trees will not be able to infect maple or oak trees.

  • Ash
  • Birch
  • Black walnut
  • Butternut
  • Buckeye
  • Elm
  • Hornbeam
  • Maple
  • Oak

How does anthracnose survive and spread?

Oak leaves on branch that are turning brown and curling.
Wet oak anthracnose
  • Anthracnose fungi survive winter in buds, twigs, fruit, fallen leaves or petioles (the stem that joins a leaf to a branch) depending on which types of trees and fungi are involved.
  • In spring, spores are splashed short distances by water or carried long distances by wind to newly forming leaves.
  • If weather conditions remain cool and wet, spores will form within the leaf spots and spread throughout the tree canopy. These spores will form new leaf spots. This cycle continues as long as cool, wet weather is present.
  • Once the weather becomes dry and the leaves mature, spread of the disease will end and the tree will replace lost leaves with new growth.
  • For ash, maple and oak trees, young leaves and shoots are most likely to be infected. Mature, fully expanded leaves are largely resistant and only become infected through minor wounds like damage from insect pests.
  • Anthracnose can continue to progress through summer months on trees like walnut and hornbeam.
  • Anthracnose can occasionally occur on any tree in the summer if cool, wet weather occurs when the tree is producing a new flush of young leaves.

How to manage anthracnose of shade trees

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Rebecca Koetter and Michelle Grabowski, Extension educator

Reviewed in 2018

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