Anthracnose of shade trees

How to identify anthracnose

Leaf symptoms

• Tan to brown irregularly 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.

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.

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 cannot infect maple or oak trees.

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

How does anthracnose survive and spread?

• 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, the 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.

Managing anthracnose in shade trees

In most cases, anthracnose does not cause permanent damage to established trees. Leaf spotting and leaf distortion have little effect on the health of the tree. No action needs to be taken to help the tree recover from this minor stress.

Keep your shade trees healthy

• Reduce stresses on trees by adequate watering throughout the growing season. Read about how to water trees and shrubs.
• Wet conditions promote disease so redirect sprinklers that splash water on the tree’s leaves.
• Do not fertilize trees unless a soil test shows the need for it.
• Rake up and destroy fallen leaves before the first snowfall to eliminate locations where the fungus can survive to re-infect the tree the following spring.
• Prune to remove infected twigs, increase light penetration and improve air circulation throughout the tree canopy.

Plant disease-resistant trees

Tree species may vary in how likely they are to be infected by anthracnose. When possible choose the most resistant tree available.

Oak:

• Red oak (Q. rubra) is relatively resistant (not likely to be infected).
• White oak (Quercus alba) is highly susceptible (very likely to be infected).

Ash:

• Blue ash (Fraxinus quadrangulata) is highly resistant.
• Pumpkin (F. tomentosa) and American ash (F. americana) are less susceptible.
• Green ash (F. pennsylvanica) and Chinese ash (F. chinensis) are more susceptible.

Elm:

• Rock elm (Ulmus thomasii) is most resistant.
• Chinese elm (U. parvifolia) is somewhat resistant.
• American elm (U. americana) is susceptible.

Walnut:

• Heartnut (J. ailanthifolia var. cordiformis) is less susceptible.
• Japanese walnut (J. ailanthifolia) is less susceptible.
• Black walnut (Juglans nigra) is highly susceptible.

Using fungicides

• Fungicides are unnecessary unless a tree has lost most or all of its leaves several years in a row because of anthracnose.
• Fungicides are protective and need to be applied before symptoms appear on the leaves.
• Proper timing of fungicide applications can vary widely from growing season to growing season and can be difficult to predict.
• For large trees, high-pressure spraying equipment is needed to get complete coverage. Hire a professional arborist who can safely operate all necessary equipment.
• Chemical treatments include products with the following active ingredients:
  • Propiconazole
  • Thiophanate methyl
  • Copper containing fungicides
  • Mancozeb
  • Chlorothalonil