Any chance of a picture? and where is the fungus sited? e.g just above ground level? main branches? lateral branches?

If anything like the usual question about white fungus on Apple is is Woolly aphid.

Agree with Teegee, a picture would help, although wooly aphids are one of the common pests. You can treat now with Methylated Spirit, dab it on with an artists brush or you can wait till winter and spray with a winter wash if it is a large tree.

Sorry Owdboggy, posted at the same time.