Olive trees (Olea europaea) are evergreen trees prized for their billowing, graceful shape, grayish-green foliage, a long lifespan and low maintenance requirements. Fruitless culitvars like "Swan Hill" produce little or no fruit, making them more suitable for planting in yards or near driveways than their fruiting relatives, which can leave messy stains. It can be alarming when a prized fruitless olive tree develops brown leaves. Properly identifying the cause of this browning will determine the best course of action.
If the fruitless olive tree is container-grown or has been transplaned recently, suspect a water-related issue. Olive trees can develop brown leaves or suffer from leaf drop if they do not receive adequate water or if the site has poor drainage and water-logged soil. Olive trees are generally well-adapted to withstand dry conditions but container-grown trees dry out quickly and require regular watering. Trees grown in the ground require less-frequent waterings but should receive supplemental irrigation during periods of drought. Poor drainage will prevent air from reaching the roots and will make trees more likely to develop root rot. Ensure that container-grown fruitless olive trees have adequate drainage. For in-ground trees, decrease irrigation or increase the amount of air reaching roots using vertical mulching or other means.
Verticillium wilt of olive trees, caused by the fungus Verticillium dahliae, first appears on trees as a sudden wilt on one or multiple branches before progressing to additional branches throughout the growing season. Trees infected with verticillium wilt can die, and there is no treatment once trees have been planted in soil that harbors the pathogen. Preventative measures are the only means to avoid this disease. Avoid planting olive trees where highly susceptible plants have been grown recently.
Olive trees are hardy plants able to withstand low soil fertility and a wide range of soil pH. Olive trees actually tend to be excessively vigorous and grow too tall in fertile soils. Although nutritional deficiencies in olive trees are relatively rare, they can occur and symptoms often appear first in foliage. A nitrogen deficiency, most common in heavy soils during winter months, causes leaves to develop a yellow tinge. A severe potassium deficiency appears as a brown tip burn on leaves and portions of the canopy die. Olive trees that have insufficient boron have small leaves with tip burn as well as short branch growth and limb dieback. If a nutrient deficiency is suspected, have a soil test conducted and fertilize or amend the soil as recommended.
Additional Possible Problems
A handful of additional problems could be responsible for the olive tree's brown leaves. Most pests of concern with olive cultivation are notable because they damage olive fruit. These pests do not primarily feed on leaves, causing them to turn brown, but can stress a tree enough that it suffers from foliage discoloration or drop. Damage to the root system ranging from heavy traffic to grade changes or nearby construction can also damage trees.