Olive trees (Olea europaea) are evergreen trees in the Oleaceae family. Olive trees bear grayish-green leaves, white flowers and nutrient-rich black or green olive fruit. These trees grow well in Mediterranean regions as well as places in the United States with warm, sunny climates, such as California, Arizona and Texas. Olive trees can reach up to 30 feet in height and live for several hundred years.
Several fungal diseases commonly attack olive trees. Virticillium wilt causes wilted branches, thin canopies and leaf dieback. Severe virticillium wilt kills entire trees. The peacock spot disease thrives in humid, cold conditions and causes lesions to form on upper leaf surfaces. These small lesions grow into large, blackish-green spots that trigger premature leaf-drop. Phytophthora root rot occurs when excess moisture allows fungus to thrive. Symptoms of Phytophthora root rot include drooping and discolored leaves.
Olive knot is caused by a bacterium (Pseudomonas syringae) that triggers the growth of rough galls, or swellings, on branches and twigs. The pathogens enter the tree through wounds or cracks on the bark. This olive tree disease typically defoliates and kills the affected foliage. Infections usually occur during cooler spring or summer months, and the pathogens are easily spread by water. Bacterial leaf scorch (BLS) is an infectious disease caused by the Xylella fastidiosa bacterium. This disease causes leaves to turn brown.
Olive trees are vulnerable to the olive moth, also called the olive kernel borer. Olive moths are tiny, grayish-silver insects. The life cycle of the olive moth includes several generations in a year's time. The first generation feeds on the flowers, the second feeds on the olive fruit, and the third generation eats the leaves. The flower generation can destroy all of the flowers on an infested olive tree, while the fruit generation causes affected trees to experience premature fruit-drop. The leaf-feeding moths rarely do any serious damage.
Olive tree roots are vulnerable to root knot nematodes, citrus nematodes and root lesion nematodes. All of these microscopic pests cause stunted foliage growth and reduced vigor. Olive lace bugs are sap-sucking insects that feed on the underside of leaves, resulting in the formation of rusty yellow spots. Olive scales, oleander scales and California red scales frequently attack olive leaves, stems and fruit. Rabbits, birds, sheep and cattle all like the taste of olive leaves and new twigs.
Proper care can reduce the risks of olive tree problems. While olive trees are able to grow in nutrient-poor soils, the locations must be well-drained and receive full sun. These trees don't tolerate saturated soils and should be watered only when the top soil is dry. Olive trees need slightly cooler winter conditions for the olive fruit to set. They shouldn't be pruned in the fall because the resulting new growth might not be able to survive a freeze.