You didn't say which house is yours but looks like the house on the right may have some new walkway. If they were doing foundation/walkway work several years ago when the tree began dying back, I would say the roots were injured on that side of the tree. The roots for the dead side supply moisture for the same side of the tree. If the tree began dying PRIOR to when the foundation/sidewalk work was done, then you may need to look for a different cause.

What John says is quite reasonable and the probable cause.

Other thoughts:

it got spray/drifted with RoundUp;

It was grafted with two separate pieces and one has died.

I’d suggest cutting away all that is dead and hope the rest doesn’t follow.

Can you upload a closer picture of the trunk? it looks like there's a pretty substantial injury (phytophthora?) to the base on the side that's dead, but it's pretty dark in that picture.

Ill try and upload a pic tomorrow.

Thanks to all who replied. Yeah, the tree got plenty of water this season, what with the "atmospheric river and all." I'm finally getting around to putting a photo of the trunk. Also even the dead part had some fruit this year, so it's not totally dead. The walkway at the corner of the house(garage) was extended almost ten years ago. The tree didn't start dying off until a couple of years ago.

 Time to check the roots for problems  and to check  for fungus or  diseases  the they should be living for many more years  but what do I know  I'm no expert   

 Howardt

  Time to get the  chainsaw out and cut  off the dead section sence It Is a major section of the tree