In article .com, motor man wrote:

Even without a picture, I strongly guess that the tree was attacked by
brown snails (Helix aspersa). This is a serious problem in the
commercial citrus orchards here just south of you in Ventura County.
They climb lemon trees and eat the bark, leaves, and fruit. They can
kill a tree by girdling the trunk (eating the bark off completely around).

There are several ways to control brown snails. See
http://axp.ipm.ucdavis.edu/PMG/r107500111.html.

Hmm.. I wonder if this is related to what I was seeing earlier in the year on
my plum tree.. The slugs on it were really small (perhaps no larger than
1/4 inch -- more like 1/8 inch) and all over the leaves on the entire tree
-- eating away at the foliage.. I didn't notice any eating either the bark,
branches or the fruit which was never touched.. Is this a related pest perhaps?

If it matters, I'm in Torrance not too far from Ventura or Santa Barbara..