Anthracnose of shade trees
- Anthracnose is a common 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 does not require treatment in most cases.
What is anthracnose?
Anthracnose is a common fungal disease of several different shade trees, that results in a wide range of symptoms including leaf spots, blotches or distortion, defoliation, shoot blight, and twig cankers.
In most cases, anthracnose does not cause permanent damage to established trees.
If a tree loses the majority of its leaves multiple years in a row due to anthracnose, this can weaken the tree and make it more susceptible to other pest problems.
Trees affected by anthracnose in Minnesota
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.
- Black walnut, butternut
- Tan to brown irregular shaped spots or blotches on young leaves; often located close to leaf veins.
- Infected leaves may be distorted, cupped or curled.
- Severe infection can result in leaf drop in spring. A second growth of leaves occurs by midsummer.
- Infections on mature leaves are irregular tan spots, often associated with minor wounds like insect feeding. Leaf distortion is rarely seen in these infections.
- Infections on green twigs can be small orange brown blisters to a brown band encircling the young twig resulting in shoot death. These infections are most common on young twigs of oak (Quercus spp.) and ironwood (Ostrya virginiana).
- Disease is often most severe on the lower and inner branches of the tree but may progress up through the canopy.
- In Minnesota, the disease is most common during cool, wet spring weather.
How to manage anthracnose of shade trees
- Reduce stresses on trees by adequate watering throughout the growing season.
- Fertilize only if a soil test shows the need for it.
- Always plant healthy trees on the correct site for the species.
- Wet conditions promote disease so avoid getting water on leaves. Read about watering wisely.
- Rake up and destroy fallen leaves before the first snowfall. This will eliminate locations where the fungus can survive to re-infect the plant the following spring.
- Prune to remove infected twigs, increase light penetration and improve air circulation throughout the canopy.
Species of certain trees may vary in susceptibility to anthracnose. When possible choose the most resistant tree available.
- White oak (Quercus alba) is highly susceptible, while red oak (Q. rubra) is relatively resistant.
- Blue ash (Fraxinus quadrangulata) is highly resistant. Pumpkin (F. tomentosa) and American ash (F. americana) are less susceptible than green ash (F. pennsylvanica) and Chinese ash (F. chinensis).
- Rock elm (Ulmus thomasii) is most resistant, Chinese elm (U. parvifolia) is intermediate, while American elm (U. americana) is susceptible.
- Black walnut (Juglans nigra) is severely susceptible, while heartnut (J. ailanthifolia var. cordiformis) and Japanese walnut (J. ailanthifolia) are less susceptible.
- Fungicides are not necessary unless a tree has been completely defoliated several years in a row.
- 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:
- Thiophanate methyl
- Copper containing fungicides
CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law.
- The anthracnose disease cycle begins in spring when spores are spread short distances by water or spread long distances by air to newly forming leaves.
- Spores are produced within new leaf infections several days to weeks after the initial infection and are further spread to new locations by splashing water.
The disease is most common in spring when new shoot and leaf growth are combined with temperatures ranging from 50-68°F and spring rain.
- Anthracnose can reoccur in the summer when cool, wet weather is paired with succulent leaf growth.
- For ash, maple and oak trees, young leaves and shoots are highly susceptible to infection, but mature fully expanded leaves are largely resistant.
- Mature leaves of these trees only become infected through minor wounds like damage from insect pests.
- Once the weather becomes dry and the leaves mature, disease growth will end, and the tree will replace lost leaves with new growth.
- Anthracnose can continue to progress through summer months on trees like walnut and hornbeam.
- Leaf spotting and leaf distortion have very little effect on the health of the tree.
- If a tree is severely defoliated multiple years in a row, this can weaken the tree.
- In weak trees, pests like boring insects or canker-causing fungi can attack the tree resulting in more significant damage.
- Anthracnose fungi can over-winter in buds, twigs, fruit, fallen leaves or petioles depending on which hosts and pathogens are involved.
Reviewed in 2018