Olive fruit tends to absorb the smell of chemical insecticides and fungicides. As a result, these chemicals will usually not be applied to the commercial olive tree until after its crop is harvested. By that time it is possible that an infestation has already occurred in the tree. These trees are always watched carefully to keep track of their general well-being. Most olive tree diseases leave obvious symptoms on the tree so they are easy to identify.
Olive Fruit Fly and Olive Lace Bug
The fruit fly chews a hole into the outer skin of the olive and lays its eggs inside the skin. The larvae feed on the fruit, making brown tunnels throughout the olive. This results in fruit rot and the olive prematurely dropping from the tree.
Olive lace bug is a flat insect with fragile, lacy wings. It feeds on the sap of the tree, typically through the leaf and this is noticed as a yellow spot on the top side of the leaf, contrasting strongly with the normal dark green color.
Brown Olive Scale
The scale is an insect that lives a parasitic life on the olive tree. The female adult scale lays thousands of eggs. Upon hatching, the new scales crawl to the leaves where they puncture the veins and drink from the sap. When they reach maturity they move to the stems of the leaves to lay more eggs. They mostly cause aesthetic problems as they secrete a gooey substance that coats the tree and its fruit.
Verticillium Wilt and Olive Knot
Verticillium wilt is a fungal infection of the roots of the olive tree. It is often caused by overwatering of the tree as olive trees are drought friendly, preferring drier soils. Prevention of this disease involves checking the soil prior to planting the tree. If no fungus is present, the area is safe to plant but it should not be in a high rainfall area.
Olive knot is a disease caused by bacterial infection through cuts in the bark, broken branches or injury to the tree during the harvesting procedure. The bacteria grows large knots around infected areas of the tree. This can eventually kill the tree if the infection is girdling the trunk and cutting off the circulation of the tree.
Caused by the fungus Spilocaea oleaginea, the leaves of the olive tree develop mottled, color changing spots, causing the leaves to drop. Defoliation of the entire tree is possible as is the death of the tree. The defoliation process is usually a lengthy one, as not all of the leaves will fall during the same growing season. Cooler weather causes the fungus to spread more rapidly, but it is thwarted by heat and will lie dormant during hot months of the year.