Answer: It's really difficult to diagnose a plant problem without actually seeing it! Olives tend to form thick canopies of tightly bunched shoots, which limits productivity and encourages pests. Pruning cuts are designed to open the canopies to light, increasing the depth of canopy involved in fruiting and discouraging black scale infestation since these insects congregate in shaded areas. Spray penetration is also enhanced on trees with open canopies. Thinning cuts are made in dense areas, removing shoots growing downward, and long, willowy shoots that do not lend themselves to shake harvest. Time of pruning is critical; the bacterium causing olive knot disease is spread by rain and can only enter trees through wounds. Thus, unlike most fruit trees, olives are pruned in spring or summer when rain does not occur to minimize this disease.
I don't know if your trees have a disease or not, but I will highly recommend you contact the University of California Cooperative Extension Office in Costa Mesa for guidance. (714-708-1606; fax 714-708-2754) They will have a list of Certified Arborists to share with you and some printed information on the general care of olive trees.
Best wishes with your trees!
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