Question
Trunk of tree is spliting and a white substance(similar to a mushroom)is spreading up the trunk.  Trunk is about six  inches in diameter.  What is it and can the tree be saved

Answer
This is the fruiting body of a fungi. More than likely a decay fungi has entered a wound on the trunk and the wood inside the trunk has started to decay. The way a fungi spreads is by spores--this fruiting body (mushroom type) has spores on the underside and these will be carried by the wind to other wounds on trees where they can grow. Over many many years the trunk may become hollow. The living part of the tree is just under the bark  and  the decay fungi is not infecting these cells so the health of the tree is not effected. The tree may be come hollow but can live this way. The strength of the trunk may be lessened but his will take many may years to show up as a problem IF ever. There is no way to treat the fungi in the wood so the best thing is to keep the tree healthy by fertilizing with 10-10-10 fertilizer at the rate of 1 lb per inch of trunk diameter scattered around the tree and watered in good--do this in the Fall.