what causes an 18 yr old white pine turn yellow
Several things can cause a white pine's needles to turn yellow. If this is the first time the needles have turned yellow the probable cause is overly wet soil. Since it has rained 20 out of the last 21 days your tree is most likely suffering from deeply saturated soil.
Another cause of yellow needles is if a tree is too deeply planted. This causes a lack of air reaching the roots. This can result in the roots dying and eventually this will also cause the tree to die
This is a link to the Minnesota Dept. of Natural Resources. This was the most comprehensive resource I could fine in one location.