Growing Green

By Robin Trott, Extension Educator

Anthracnose is a common disease affecting many shade trees in Minnesota, including ash, maple, birch, buckeye, elm, hornbeam, oak, walnut, linden, hickory and willows. This disease is caused by fungi that are specific to each type of tree, meaning the fungi infecting ash trees won’t infect maple or oak trees.

Symptoms and Identification

Anthracnose usually causes minimal damage to mature trees, though it can look severe. Symptoms differ based on leaf maturity:

• Young leaves: Small, irregular tan, deep brown, or black spots that can cause leaves to distort or curl. Severe infections may lead to early leaf drop, but trees often grow a new set of leaves by midsummer.

• Mature leaves: Tan to deep brown spots without distortion, often appearing with minor wounds like insect bites.

• Branches: Green twigs, especially in oak and ironwood, may show orange-brown blisters or bands, leading to shoot death.

Symptoms are usually worse on the lower and inner branches but can spread throughout the tree.

Environmental Conditions

Anthracnose thrives in cool (50 to 68 degrees F), wet spring weather but can also occur in summer if similar conditions are present during leaf growth.

Management

Strategies

To manage anthracnose effectively:

• Sanitation: Clean up and destroy fallen leaves to reduce overwintering fungi.

• Pruning: Remove diseased twigs and branches, especially in sycamores, to improve air circulation and light penetration.

• Tree care: Support tree health with proper watering, fertilization, and mulching.

• Resistant species: Choose tree species that are less susceptible to anthracnose.

• Fungicides: Use fungicides only when necessary, as they rarely provide significant control and are seldom needed.

These practices can help keep your trees healthy and resilient against anthracnose.

For more information, visit University of Minnesota Extension.

Until next time, happy gardening!

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“This oak tree and me, we’re made of the same stuff.” ~ Carl Sagan