Thank you for your question.  However, I cannot begin any research until I know what kind of pine tree you have.  Where are the needles being lost - top, bottom, middle, individual branches, only on one side, etc.  If you could send a picture of the tree that would be helpful.  Have you looked for the presence of insects? 

Here is a picture of the tree and what the needles look like.  It is not just one tree, but many of them.  They are clear of any pine beetle problem.

thanks

tom d.

Thank you for the add'l details.  Based on the picture you sent, I believe the tree is a Ponderosa Pine. 

I have been unable to find an information about what might cause (disease or disease) the loss of green needles.  I recommend you take a sample of the needles to the El Paso County Extension Office and ask a Master Gardener to examine them under the microscope.  Perhaps he/she will see some evidence of disease.  The link to the office is below.  The Master Gardener Help Desk is not staffed during all business hours so you should call first and ask when the desk will be staffed.
http://elpaso.extension.colostate.edu/