First the foliage becomes thin, with chlorotic or curled leaves at the shoot tips. Twigs then become bare because of lack of sufficient vigor to re-foliate. Then the branches and parts of the crown die, below which a bunching of the foliage develops that tends to be confined to the lower crown. Death usually takes place within three to five years after the onset of symptoms.
There are two common causes which will produce the dieback symptoms on birch. They are: a fungus called Melanconium betulinum and an insect known as the bronze birch borer (Agrilus anxium).
Melanconium betulinum attacks the tree when it has been weakened by drought conditions, winter kill or phenoxy-acetic acid herbicide exposure causing progressive dieback. To control for this you must prune branches back into good wood. Good fertilization and heavy watering (normal watering schedule) will help in revitalizing the tree.
The Bronze Birch Borer is described as being 1/2 to 1 inch in length, flat-headed and light colored. The adult form is a beetle (1/2 an inch long) which feeds on the foliage and deposits its eggs in slits in the bark. The borer damages the tree by making flat, irregular winding galleries just below the bark on the main trunk. This is found to cause a cutting off of the translocation system.
Control for the borer consists of bark applications of Cygon 2E. To apply, paint the Cygon 2E on a smooth area around the trunk of the tree below the branches, in a two inch band (thickness of the band depends on diameter (age) of the tree. Please check instructions on container before you apply). Cygon 2E can and should be applied every spring as a preventative measure to borer infestation. Note Cygon is no longer available from garden centres. Check with your local pesticide applicator to see if they will do this treatment for you. In addition, you may want to maintain the vigor of the tree with regular watering and an application of fertilizer in the spring.
To further control for the Melanconium betulium, one must remove all dead and infected branches back six to eight inches into healthy, green wood. Further, after each cut, your pruning utensils should be sterilized with rubbing alcohol or 1 part bleach to 9 parts water to prevent the spread of the fungal organism.
As a pre-cautionary measure, one can spray with Later’s Copper in the spring, with the first application occurring at bud break. This application is then followed by a second application two weeks later.