It could be, but not all fungi on trees are honey fungus, there are some that grow as a natural part of breaking down of dead wood. Dig a little way down in the soil where the tree is, and if you find some long black boot-lace type strands, this is an indication of honey fungus, but, as I have just found out there are a few different types, some more dreaded than others. I have just moved and, though not 100% sure yet, think there may be some in my garden, but even though there are no chemical controls, its not all doom and gloom, there are ways of dealing with it and plenty of resistant plants, so chin up. Anyway if you google it theres plenty of info (too much sometimes..confusing) on there that will help.