Posted: 17/02/2015 at 13:07
It may be very innocent. One side of the garden may be more sheltered than the other (less wind burn), one side may be much drier and the other wetter etc etc. There is also a disease spread that affects conifers. Post a pic of the yellowing and we can advise.
Generally speaking anything spread like a herbicide will kill the plant root up and happen quickly.