The olive tree is subject to attack by a number of pests and diseases. Because the olive tree can grow to a great age and because there are many wild olive trees scattered in olive growing districts, the systematic control of certain pests is very difficult. The main animal pests are: dacus eleae, or olive fruit fly; prays oleae, or olive kernel borer, or olive moth; saissetia oleae, or black scale. The olive tree is also attacked by cryptogamic and bacterial diseases such as olive lenot and olive leaf spot.
The most important pest of olives, especially in Italy, Spain and Turkey, is the olive moth. It occurs in all the Mediterranean olive-growing countries. The caterpillar of the moth feeds on the flower organs, especially the pistil and ovary, and later attacks the young fruit. In some years 50% of the flowers are destroyed, followed by a heavy fall of fruit. The other principal pest is the olive fly which is similar in activity to the notorious Mediterranean fruit fly. The eggs of the olive fly are deposited inside the fruit; the larvae feed on the pulp of the fruit, causing maggoty olives. The olive fly occurs throughout the Mediterranean region, especially in Italy. About 20% of olives are attacked and, where attacks are heavy, they are unsuitable as table olives.
Olive leaf spot is a fungal disease which is found around the Mediterranean and in Eritrea, South Africa, California, Argentine and Chile. The fungus infects the leaves, severe attacks resulting in partial to complete defoliation of the tree, which seriously reduces the crop in the following season. Olive knot is a bacterial disease which occurs in the Mediterranean region, in California and Argentina. The disease causes knots to develop on the young shoots which eventually kill the shoots. The bacteria can also infect the leaves, causing premature leaf drop.