Olive trees (Olea africana or europaea) have been cultivated for thousands of years, according to the California Rare Fruit Grower’s Association. Olives are highly valued for their health benefits and taste, which varies widely depending on the variety. The trees themselves are attractive, with distinctive, twisting branches, and can grow to heights of more than 40 feet. In general, olive trees are hardy, although they can suffer from a few insect pests and diseases.
Verticillium wilt, a serious fungal disease, affects all types of deciduous trees, and especially fruit trees. Verticillium wilt is a life-threatening disease to olive trees in California, according to the California Rare Fruit Grower’s Association, especially as there is no cure for the disease. Affected olive trees will show an overall decline in growth. Sometimes, only part of the tree will appear to wither. Infected trees should be removed and destroyed, and no other trees should be planted in the tainted soil.
Olive knot is a bacterial disease that is spread on tainted pruning tools, especially during rainy months, as the bacteria spread on water. Dark, black “knots” or growths appear on the tree. Prune away infected areas of the tree to avoid the spread of the bacteria. Pruning tools should be bleached between each use to avoid spreading this and other diseases. Spraying the tree with chemical deterrents is not usually an option for olives because the valuable oil in the olives retains the scent of the chemicals.
Olive Fruit Fly
Dacus (or Bactrocera) oleae is a serious insect pest, according to the University of California. This small fly poses a severe economic threat to commercial growers of olives. The larvae of the fly feed on the developing olives, creating wounds in the fruit which allow fungi and bacteria to enter and further damage the fruit. Fruitless olive trees, which are grown for ornamental purposes, are not affected by this pest. Traps and sprays can also be effective at controlling the insects in home gardens. Finally, clean up dropped fruit and leaves from under the tree as soon as possible, to prevent the maggots from burrowing into the soil and overwintering under the tree.