Thank you for getting back to me. Oleander bushes and trees are subject to the same diseases. The trees are simply bushes that have been pruned to a single trunk. There is a disease called oleander leaf scorch that began in Orange County in the 1990's, and has since spread throughout southern California. It is caused by the bacterium Xylella fastidiosa
. It is believed to have been introduced in the early 1990's from overseas. It does often begin with leaves becoming yellow, but they turn brown. Many homeowners don't notice the yellowing, but do notice when the leaves turn brown. There is no treatment or cure. If the bushes have undamaged areas on them, pruning can sometimes prolong how long they live, but they will eventually die.
Frost can cause browning leaves, but if you've had no frost, that is not a possibility. With frost damage, new growth appears when the damaged areas are pruned off. If lack of water was the problem, you would probably have seen wilting before the leaves turned brown. The other diseases these plants get cause oozing from the bark, and/or dark, blackened areas on the bark in addition to brown leaves. These factors, along with how common leaf scorch has become make me believe it is the most likely cause of the problem.
It is generally recommended to destroy oleanders that are affected by leaf scorch. If you replace them with more oleanders, they will get the disease, too. It's best to find a different species. Ceanothis is one that grows well in southern California, and many gardeners are replacing oleanders with it. It is sometimes referred to as California lilac. Here is a photo:
You should be able to find these plants in local nurseries and garden centers. If you have more questions, let me know by clicking on REPLY.