When you pull a tree out of a pot to work on the roots, or to transplant it into another pot people often see a white fuzz and assume that they have beneficial mycorrhizal fungi.  But without looking closely you are just as likely to have an infestation of Root aphids.
Root aphids are just like aphids that feed on the top of the plant except that they are white and leave a network of fuzzy white stuff in the middle of your roots.  The seem to concentrate around the drainage holes in pots.  You can tell the difference between mycorrhizal fungi and root aphids by looking carefully to see if any of that potential fungus is crawling around on the roots.  If you see anything that looks like an aphid and that moves you are not blessed with beneficial fungus but cursed with sap sucking aphids.

Ridding yourself of them is not particularly difficult, but since you only know they are there when you pull the plant out for repotting it may do you well to use a systemic inseciticide a couple times during the growing season.  Spraying the leaves with systemic insecticide is the long route to killing them but should work eventually, soaking in a dilute solution of insecticide (water based systemic, not oil) will be the fastest and most long-lasting method.

See this article for more information: http://www.gpnmag.com/articles/some.pdf

So far, I have found root aphids on every Japanese Black Pine that I have repotted in my yard.  Good Luck.

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