Native to the Mediterranean region, olive trees are popular in parts of the U. S. with a suitable climate. Olive trees thrive in areas with a long, hot growing seasons. They are susceptible to some diseases affecting the foliage, particularly in humid conditions.
Common olive tree leaf diseases include peacock spot, a fungal disease caused by Spilocae oleagineas; ceropsora leaf spot, caused by Mycocentrospora cladosporioides; verticillium wilt, a serious fungal disease caused by Vertillium dahliae; and Phytophthora crown and root rot, which affects the entire olive tree and may cause thinning of the olive tree's canopy.
Fungi are the most common diseases of olive tree leaves. Some fungi are soilborne, such as the one causing verticillium wilt and phytophthora, while the fungi that cause peacock spot and cercospora leaf spot are airborne and spread by rain. Humid, warm conditions increase the incidence of olive leaf disease overall, presenting opportunities for the fungi to grow and spread.
Damage ranges from cosmetic to fatal. Cerospora leaf spot causes sooty mould to form on the underside of the leaves, as well as yellowing and leaf drop. Peacock spot causes sooty blotches on the leaves. Both diseases reduce production. Phytophthora can weaken a tree and kill it. Verticillium wilt causes leaves to suddenly wilt and drop. It eventually cause death to limbs and entire trees.
Control includes fungicides for peacock spot and cerospora, such as a fixed copper fungicide, which has been shown in various trials to be effective, according to the University of California. There is no cure for verticillium wilt, and control consists mainly of avoiding planting on infested soil and removing damaged limbs from the tree. Phytophthora is controlled by managing poorly drained soil to prevent prolonged periods of high moisture.