Maple trees are susceptible to numerous fungal diseases that cause cankers--areas of dead bark--on tree trunks and branches. Although, cankers are unsightly, most will not kill a tree. Limbs and branches with cankers should be pruned back to the closest main joint. There is no chemical preventative or treatment for most types of cankers.
Several species of phytophthora fungi infect the base of maple trees, causing basal cankers. Dead bark in the affected area peels off easily to reveal dead wood underneath. The fungi kill the tissue that transports water and nutrients for the tree, sometimes resulting in the tree's death.
The fungus Phytophthora cactorum causes cankers that ooze a reddish substance. The dead wood behind the wet areas is stained red.
When narrow-winged tree crickets make holes in the bark of maple trees for their eggs, they introduce the fungus cryptosporiopsis. An elongated sunken canker forms around the single hole, which oozes sap in the spring. A callus of healthy tissue forms over the edges of the canker within a year and contains the fungus to the canker.
Eutypella parasitica fungus causes large cankers on the trunk or branches of maple trees. The cankers are long with raised edges of callus tissue. In some cases, the cankers may girdle the trunk of the tree and cause the tree to decline and die.
Elongated shallow cankers caused by Valsa ambiens usually occur on smaller limbs of maple trees. The centers of the cankers are covered with gray to white small bumps of spores. An affected tree may die after a severe summer drought or an extremely cold winter.