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Extension > Garden > Yard and Garden > Diseases > Anthracnose

Anthracnose

Rebecca Koetter and Michelle Grabowski

M.Grabowski, UMN Extension

Oak Anthracnose

Importance

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.

Tree Fungal Pathogen
Ash Discula Fraxinea
Birch Cryptocline betularum, Discula betulina
Black Walnut, Butternut Marssoniella juglandis
Buckeye Colletotrichum gloeosporioides
Elm Stegophora ulmea
Hornbeam Monostichella robergei
Maple Aureobasidium apocryptum, Discula campestris, Colletotrichum gloeosporioides, Discula umbrinella
Oak Discula quercina

Identification

M.Grabowski, UMN Extension

Ash Anthracnose

M.Grabowski, UMN Extension

Maple Anthracnose

Biology

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.

L.Graney, Bartlett Tree Experts, Bugwood.org

Walnut Anthracnose

Management

Fungicides

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.

*Always completely read and follow all instructions on the fungicide label.

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