The olive tree is affected by some pests and diseases, although it has fewer problems than most fruit trees. Around the Mediterranean Sea the major pests is the medfly, the olive fruit fly [Dacus oleae].
In California, verticillium, a fungus disease that causes wilting and death is a serious fungal disease. There is no effective treatment other than to avoid planting in infested soils and removal of damaged trees and branches.
A bacterial disease known as olive knot is spread by pruning olive trees with infected tools during rainy months. Because the olive has fewer natural enemies than other crops, and because the oil in olives retains the odor of chemical treatments, the olive is one of the least sprayed crops.
University of California Cooperative Extension
133 Aviation Blvd, Santa Rosa, CA 95403
Article by: Paul Vossen and Alexandra Kicenik Devarenne . September 1, 2006
The olive fruit fly is a serious pest of olives in California. It was recently introduced from the Mediterranean where it has been a problem for thousands of years. The adult olive fruit fly is rarely seen. It lays its eggs just under the skin of the olive fruit. The larvae feed on the olive flesh, leaving brown tracks and tunnels.
The damaged fruit is susceptible to rot, can drop prematurely, and is useless as a table fruit. Usable olive oil can be made if the damage level is below about 10%, but the risk of off flavors and unacceptably high acidity rises as the damage level increases.