OAK ANTHRACNOSE

Dan Gillman

Oak anthracnose is a common leaf spot disease caused by the fungus Apiognomonia quercinia (Discula quercinia). White oak (Quercus alba) is most susceptible, although all species of oak grown in Minnesota can be infected. Plant tissues infected the previous year produce spores that are rain splashed to new growth in the spring.

Oak leaf with anthracnose
Photo: Chad Behrendt

Anthracnose of oak
Photo: Robert Blanchette

Infection of young, immature leaves early in the season often results in necrotic (brown/dead), deformed margins of the leaf tissue as well as necrotic, irregularly shaped spots. These spots tend to form along the veins or be confined by them. Usually, a distinct margin develops between the dead and healthy leaf tissue. Heavily infected leaves may appear misshapen and curled. Mature leaves tend to be more resistant to infection than immature leaves. Older leaves develop small, brown spots in the summer during wet weather. Heaviest infections tend to be located in the lower portion of the trees where relative humidity is highest and leaves remain wet. Severe infections may result in defoliation and dieback of infected branches. When severe leaf loss occurs in the spring, trees usually try to produce a new flush of leaves to help compensate for leaf loss. Twigs infected during the growing season may often die before their buds are able to open the following spring, causing branch dieback.

Oak anthracnose may be confused with oak wilt, a deadly systemic disease of oak trees. The following features help distinguish the two diseases.

Oak anthracnose is most severe on white oak, whereas oak wilt is most severe on oaks in the red oak family.

Oak anthracnose produces irregular spots/blotches randomly over the leaf. Spots may coalesce forming large brown areas. In addition, leaves may be curled or deformed along the outer margins of new growth. When leaves are dying as a result of oak wilt, browning generally begins at the margins of leaves, resembling symptoms of drought. This is due to blockage of water and nutrients in the vascular tissue preventing these nutrients and water from reaching the leaves.

Symptoms of oak anthracnose are most severe on the lower, inside branches where humidity levels are higher. Symptoms of oak wilt begin on branch extremities, usually in upper portions of the tree.

Oak anthracnose may be aesthetically displeasing, but does not cause permanent damage to well established, vigorously growing trees. Cultural control measures should include proper watering, mulching, and sanitation procedures. Raking leaves in the fall and pruning dead or dying branches helps reduce the number of new infections the following year. Trees should be pruned only during dry weather. Do not prune oaks in April, May, or June. Pruning wounds attract Nitidulid beetles which are responsible for the overland spread of the oak wilt fungus! Spring fertilization of severely stressed trees can improve tree vigor.

Trees that have been severely stressed from defoliation, defoliated for several years in a row, or are newly transplanted, may benefit from chemical control. Fungicide application should begin at bud break to protect new growth. Repeat applications according to the fungicide label during cool wet weather. Read the label carefully and apply only as directed. Application to large trees requires special equipment to ensure adequate coverage. Currently labeled fungicides include thiophanate-methyl (Cleary's 3336), Mancozeb, and copper containing fungicides. In addition, chlorothalonil (Daconil 2787) can be used on oaks in the red oak family.

Representative trade names may be included along with generic names. This information is supplied with the understanding that no discrimination is intended and no endorsement is implied.

P432O
Revised 2/99
Chad Behrendt, Crystal Floyd

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