Anthracnose is a general term used to describe diseases that result in a wide range of symptoms including leaf spots, blotches or distortion, defoliation, shoot blight, twig cankers and dieback on many different deciduous trees and shrubs. In most cases, anthracnose does not cause permanent damage to established trees. However, consecutive years of defoliation can decrease the tree's vigor, weakening the tree and thereby predisposing the plant to opportunistic pests that may further harm or damage the tree.
Pathogen and susceptible plants
A number of related fungi are responsible for anthracnose. Most anthracnose causing fungi are fairly host specific; they will infect many species of ash for example but will not infect maple or oak trees. Green ash, elm, sugar maple, Norway maple, white oak and black walnut are examples of frequently infected tree species in Minnesota.
|Birch||Cryptocline betularum, Discula betulina|
|Black Walnut, Butternut||Marssoniella juglandis|
|Maple||Aureobasidium apocryptum, Discula campestris, Colletotrichum gloeosporioides, Discula umbrinella|
- Tan to brown irregular shaped spots or blotches on 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.
Anthracnose fungi can over-winter in buds, twigs, fruit, fallen leaves or petioles depending on which hosts and pathogens are involved. The disease cycle begins in spring when spores are dispersed 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 also 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 from the anthracnose fungi, but mature fully expanded leaves are largely resistant. Mature leaves of these trees only become infected through minor wounds like damage from insect pests. As a result, once the weather becomes dry and the leaves mature, disease growth will end, and the tree will replace lost leaves with a new flush of growth. In contrast, anthracnose can continue to progress through summer months on trees like walnut and hornbeam.
Leaf spotting and leaf distortion have very little affect on the health of the tree. However if a tree is severely defoliated multiple years in a row, this can weaken the tree. In such cases, opportunistic pests like boring insects or canker causing fungi can attack the tree resulting in more significant damage.
- If a tree is suffering from anthracnose, reduce other stresses on the tree by maintaining them properly with adequate watering throughout the growing season and fertilizing only if determined necessary by a soil test.
- Always plant healthy trees on the correct site for the species.
- 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.
- Wet conditions promote disease so avoid or redirect sprinklers that wet the lower canopy of the tree.
- Rake up and destroy fallen leaves before the first snowfall to eliminate locations where the fungus can survive to re-infect the plant the following growing season.
- Prune the tree or shrub to remove infected twigs, increase light penetration, and improve air circulation throughout the canopy.
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 therefore can be difficult to predict. For large trees, high-pressure spraying equipment is needed in order to get complete coverage. Therefore 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
*Always completely read and follow all instructions on the fungicide label.