Question

depth of rotten area  

general area of rotten   I have a large oak (8ft in circumference) and about I noticed that there is a section of decayed bark on the trunk.  The area of decay is about 2-3 in deep x 3ft wide x 4ft tall near the base of the trunk.  The rotten wood is spongy but is intact and looks like normal bark.  I opened a small area, the wood is moist and about 3in deep but didn抰 see any ants (have seen carpenter ants on patio near tree). The tree seems healthy with a good canopy of green leaves.
What should I do? Should I remove the rotten bark and spray pruning tar?  If I leave it alone will more damage result?  Generally speaking at what point would the damage be a danger?

Answer
You are dealing with a very large area of basal rot. The trouble with this issue, is that it affects the structure of the tree. While it is possible to maintain top growth and root growth with a rot like this, it is impossible to reinforce the strength loss that the tree has suffered at the base of the tree. In fact, any improvement in health would result in improved growth and vigor and would cause more stress on the comprimised tree.

The rot was likely caused by some damage to the trunk and then continued to decay over time. Keep the area as dry as possible. Redirect any irrigation heads. Do not dig into the tree with any tools because this will cause the decay to reach further into the wood of the tree. Adding anything like tar or patch material will also cause the decay to move further inward. Unfortunately, this type of woodrot generally comes with a recommendation for removal of the tree at some point.