Olive Tree

This could be caused by more than one issue but the most likely cause (or aggravating factor) could be too much water. We recommend you inspect the soil around the roots closest to the tree and at the feeder roots (at the dripline). If this tree is getting "indirect water" as you decribe it, on a regular basis then the soil is probably too wet for this tree. Mediterranean plants (like Olive trees) don't do well with regular water and become much more susceptible to root rot and crown rot diseases. If this damage is occurring on just one side of the trees, that could be a clue especially if the irrigation is watering that same side regularly; could this be the cause of the "random" damage you see. We've included a link from the UC Davis website that has info about how to look for/test for some of the common disease of these treess
http://www.ipm.ucdavis.edu/PMG/GARDEN/PLANTS/olive.html

I've been thinking more about your Olive trees, because honestly I wasn't satisfied that the reponse I gave you would be helpful... something else was on the tip of my tongue and I finally got it! Unfortunately, this may not be good news. In the recent past few years, Xylella fastidiosa (Xf), a different strain (but related) of the bacterium that causes leaf scorch on grapes is infecting Olive trees; they're calling it OLSD, or Olive Leaf Scorch Disease. It is also vectored from plant to plant by leaf hoppers (pretty sure its the glassy winged sharp shooter just like with grapes). You may want to get an opinon from the Ag Dept Pathologist by taking a sealed bag with leaf & stem samples to their office in San Marcos, CA (off of Twin Oaks Valley Rd). They will check out your specimen for free. You could also call and talk to the Farm Advisor at the University of CA Cooperative Extension in S.D. for more info. If you do an internet search you'll find more info about OLSD. http://www.sdcounty.ca.gov/fha/programs.html