California Code, Labor Code - LAB § 4061.5

The treating physician primarily responsible for managing the care of the injured worker or the physician designated by that treating physician shall, in accordance with rules promulgated by the administrative director, render opinions on all medical issues necessary to determine eligibility for compensation.  In the event that there is more than one treating physician, a single report shall be prepared by the physician primarily responsible for managing the injured worker's care that incorporates the findings of the various treating physicians.