Why do birch trees have black streaks on their trunk?

The black streaks on birch tree trunks are primarily caused by a fungus known as birch canker or by a form of sooty mold. The sooty mold typically develops on honeydew, a sugary substance excreted by insects like aphids that feed on the tree. Additionally, birch cankers can result from physical damage or stress, leading to dark discoloration as the tree responds to the injury. These streaks are usually not harmful to the overall health of the tree but can indicate underlying issues.