Cankers are often caused by fungus that enters an injury to the bark, and grows between the bark and the wood.These injuries can be caused by damage from weed eaters, lawnmowers, chemicals, insects or environmental conditions. The healthier the tree is, the more likely it is to ward off serious damage from a canker disease. Once a tree has canker, it is essential to remove as much of the canker fungi as possible from the tree to avoid infection and spread. Prune only during dry weather and make cuts with a sterilized cutting tool at least 4 inches below the edge of the canker on trees. It appears that the canker is on the main trunk (stem). Stem cankers can be excised with a sharp, sturdy knife, but the resulting wound allows entrance of wood decay fungi which may pose greater problems than the canker did. This surgery is best done by a professional arborist. There are a number of cankers that this could be. I have listed a few sites to check out to see if any of the symptoms of these cankers match your tree’s injury. Be sure to try to keep your tree healthy with water and nutrients. You may want to consult an arborist to determine the cause. http://www.treesaregood.org/findanarborist/arboristsearch.aspx http://www.gardeningknowhow.com/ornamental/trees/tgen/cankers-on-trees.htm http://www.gardeningknowhow.com/ornamental/trees/maple/maple-tree-bark-disease.htm http://www.ehow.com/facts_7615377_maple-tree-diseases-michigan.html Hope this was helpful. Feel free to contact us again if you have further questions.