This could be caused by a few things, including Botryosphaeria, wood decay, or a type of root rot such as Phytophthora. Are there any other symptoms on the trunk or leaves? Do you see white mycelium, shelf fungi, or sunken lesions on the trunk? Is the bark falling off? Did you observe any leaf spots? Did you observe any wilting of leaves? When did you first notice the symptoms? A little more information would allow me to dig a bit deeper.