Losing Leaves on a Large Black Olive Tree

Armillaria Root Rot

• Armillaria-infected olive trees drop their leaves slowly, leaving a thinning canopy and discolored bark. The tree will appear weak. Armillaria root rot is sometimes referred to as oak root fungus because trees planted on soil where oaks have grown are particularly susceptible to the disease. Leaf drop and yellowish discoloration often appear on one side of the tree, progressing to the other side over the span of several years. Roots infected with Armillaria mellea fungus have white to yellowish fan-shaped mycelial mats between the bark and the wood. It persists in soil for decades and is more prevalent after wet winters. Allowing the upper root and infected crown areas to dry out thoroughly might control the fungus.

Cercospora Leaf Spot

• Cercospora leaf spot is caused by the Mycocentrospora cladosporioides fungal pathogen. Leaves turn slightly yellow and discoloration appears on the underside as the disease develops. The leaves appear to be covered with black dust. Fruit on the tree may also develop brown lesions and not mature uniformly. Leaves covered with the fungus may fall. Not all diseased leaves fall, and the fungus survives on those that remain. They develop a white crusty appearance as a new crop of fungal spores grows. Preventative treatments are applied after fall harvest but before winter rains begin.

Peacock Spot

• The fungal pathogen that causes peacock spot on olive trees is the Spilocaea oleaginea. It appears on leaves as circular sooty blotches that develop into muddy green color and then black. Fruit and fruit stems are also affected but the disease most often shows in the lower canopy leaves. Twig death may occur and leaves fall prematurely. The fungus survives on leaves that do not fall and a new crop of disease spores develops there. High temperatures restrict fungal growth and thus slow the disease progress. Treatments are applied in late fall and early spring.

Verticillium Wilt

• Another fungal disease that causes damage to olive tree leaves is Verticillium wilt, caused by the Verticillium dahlae pathogen. The tree canopy thins and leaves wilt suddenly early in the growing season.There is no cure for this disease, so preventative measures are encouraged: Do not plant olives where cotton, cucumbers, eggplant, peppers, potato or tomatoes have been grown, as there are likely to be Verticillium spores present. Plant several seasons of cover crops, and practice solarization of the soil before olive trees are planted.