Answer: Based upon your description, I suspect the disease called Olive Knot (Pseudomonas syringae). Olive knot appears as rough galls or swellings about 0.5 to 2 inches in diameter on twigs, branches, trunks, roots, leaves, or fruit stems. Small shoots may be defoliated and killed. Galls also form at trunk or limb wounds.
Olive knot does not kill trees, but it does reduce productivity by destroying twigs and branches, and the flavor of fruit from infected trees may be affected. Bacteria survive in the knots and are readily spread by water at all times of the year. Infection occurs at low temperatures, usually in fall or spring. Openings are necessary for penetration of bacteria, and these are provided by leaf scars, pruning wounds, or bark cracks made by freezing. All cultivars are susceptible, and damage can be severe when weather favors disease.
Olive knot is difficult to control and requires preventive fungicide applications to protect leaf scars and other wounds. It is also helpful to carefully prune during the dry season (July to August) to remove galls on twigs and branches. Because the bacteria may be carried on pruning shears, be sure to disinfect them frequently when pruning. Galls on limbs and or trunks can be treated with Fixed Copper, Bordeaux Mixture, or Gallex at label rates. A minimum of two applications each year is needed for control. Make the first application in fall after harvest. Apply other applications in spring from March through May. Because leaf scars are susceptible when fresh, time treatment to protect as many leaf scars as possible.
Best wishes with you olive trees!
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