Olive Knot

Olive knot appears as rough galls or swellings about 0.5 to 2 inches in diameter on twigs, branches, trunks, roots, leaves, or peduncles (fruit stems). Small shoots may be girdled, defoliated, and killed. Galls also form at trunk or limb wounds.

Olive knot can girdle and kill trees if infections occur on the trunks of young trees. On older trees, it reduces productivity by girdling twigs and branches and causes dieback. Infection is also associated with an off-flavor of the fruit.

Olive knot disease incidence is generally correlated with rainfall and is more severe in higher rainfall areas or during years of high rainfall. They are readily spread by water at all times of the year. Bacteria survive in galls and as epiphytes on leaves and twigs. As an epiphyte they may be spread on asymptomatic plant tissue. Infection occurs in fall, winter, and spring. The knots develop in late spring when trees resume growth and continue to develop through summer. Openings are necessary for penetration of bacteria, and these are provided by leaf scars, pruning wounds, or bark cracks made by freezing. Freeze injury can lead to disease epidemics because the resulting defoliation and bark splitting normally occur during the winter when rain occurs and can spread the disease.

All cultivars are susceptible, but Manzanillo (table olive) and Koroneiki (oil olive) are the most susceptible of the commonly grown varieties.

The disease is not prevalent along California's coast.

Olive knot is difficult to control and requires preventive bactericide applications. Treatments reduce the pathogen population on the plant surface, decreasing the probability of infection. 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 sanitize them frequently if pruning at other times during the year.

Treatments are preventive and need to be applied before infection. Generally, a minimum of two applications each year are needed in areas of higher disease incidence and for more sensitive varieties (Manzanillo and Koroneiki varieties).

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. If freeze damage results in defoliation, additional sprays may be necessary. Mechanically harvested oil olives should be treated immediately after harvest because harvesting can damage the trees and make openings for the bacteria. Also, harvest occurs in fall or winter when rain is likely. If possible, avoid harvesting when rain is predicted.