Sarah Denkler

Horticulture Educator

Daily Dunklin Democrat – At Your Service

April 12, 2009

Dogwood

            I was asked many questions last year about dogwood problems.  Most of these questions came after the problem occurred.  In most cases, the solution must be implemented before the problem is noticed.  Following are some common problems found on dogwoods and possible solutions that can be used to restrain or thwart those problems.  If these solutions are put into place now, problems may be thwarted in the future. 

The first sign of a problem with a dogwood tree may be canker or ‘lawnmower blight’.  This can be caused from wounds to the trunk by weed eaters or mowers hitting the trunk.  This damage is often followed by insect larvae or borers tunneling into the trunk which causes a crusty look on the bark and the visible appearance of holes in the trunk.  If the weather is cool and wet enough, and it usually is, spot anthracnose may be present.  This fungus causes small purple spots on the leaves and flowers in spring.  In the summer you may find powdery mildew on the leaves.  This is a white or grayish coating that will cover the leaves in areas of full shade. 

Several cultural practices can help with some of these problems.  Use mulch around dogwoods to help reduce the amount of water loss from roots as well as protect the trunk from damage by yard machinery.  Do not let dogwoods dry out during periods of hot weather.  This will further stress the tree and attract other problems to it.  Likewise, do not plant trees in wet or poorly drained soil.  When planting your tree, place the dogwood in shade but not in shade that is too dense.  Leaves will remain wet instead of drying off when they are shaded completely.  If leaves are able to dry it will reduce the time period in which disease is able to spread.  Remove diseased limbs and twigs below the infection as they occur.  Use sanitized pruners when pruning and remove and destroy any limbs that have been pruned from the tree.  Remove and burn any leaves that fall from the tree to minimize the spread of any fungus present.

If you have had a problem with leaf spot in the past and the tree is valuable to you, fungal disease can be limited through the use of chemicals.  These include: chlorothalonil (Daconil), benomyl, thiophanate-methyl (Cleary’s), or propiconazole (Banner and Banner Maxx). Spraying should begin when buds begin to open and be repeated when bracts have fallen, four weeks after bracts fall and in late summer after flower buds have formed.

Borers can often be a pest in dogwood trees.  Newly planted trees are in stress already so any attack will weaken them further.  Control of borers usually centers on the flight of the adult in summer, controlling the population before it has a chance to lay new eggs.  Adults are similar in appearance to small wasps, the male is dark and the female is yellow and black.  As the adult emerges, usually between May and June, spray the trunks thoroughly, from the limbs to ground level, making sure to cover anywhere that damage is visible in the wood.  Permethrin is the active ingredient that should be present for control to occur.  Several products including Hi-Yield Lawn, Garden, Pet and Livestock Insect Control as well as Bonide Borer Miner Killer contain permethrin.  Look for the active ingredient and the control of borer on the label.  The label may recommend a monthly application in order to control borer.  Be aware that Permethrin is also toxic to bees.

Once the larvae, or borer, have entered the trunk, they are protected from control strategies.  Imidacloprid is sometimes used as a systemic in the tree to control borer.  Control with a systemic is dependent on what part of the trunk the borer feeds on.  If the larvae feed on the vascular tissue then you may be more successful with this method.  It is for this reason that results have not been consistent with this type of application and why control is usually focused on the adults.  Bayer Tree and Shrub Insect Control is an example of a systemic insect control.

Helpful Publications: Hanson, Mary Ann.  “Foliar Diseases of Dogwood.”  Department of Plant Pathology, Physiology and Weed Science, Virginia Tech.  Publication 450-611.  May 2000.;  Barrett, Bruce A.  Publication G7190, “Insect Borers of Fruit Trees”.  University of Missouri. Mu Extension.  Rev. August 2007

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Sarah Denkler, Horticulture Educator University of Missouri Extension P.O. Box 160, Kennett, MO 63857 573-888-4722   Denklers@missouri.edu