Olive Knot Disease

Olive knot is a chronic bacterial disease of olive trees caused by Pseudomonas savastanoi pv. savastanoi. It produces rough, tumor-like galls on twigs, branches, trunks and sometimes fruit stems, especially where pruning cuts, leaf scars, frost injury or other wounds allow infection. Disease pressure rises during rainy periods, and long-term management depends on sanitation, dry-weather pruning and protecting fresh wounds.

Olive knot disease is caused by the wound-infecting bacterium Pseudomonas savastanoi pv. savastanoi. In the United States it is most important in olive-growing regions, particularly areas with winter and spring rainfall such as California. The characteristic sign is a rough, rounded or elongated gall on a twig, branch or trunk. Galls may begin as small swellings and enlarge into hard, irregular, corky growths.

On young branches, a gall can girdle the shoot, causing leaf loss, dieback and reduced fruiting wood. On older trees, repeated infections may weaken limbs and reduce yield. A laboratory diagnosis may be needed when symptoms resemble healed pruning wounds, insect damage, crown gall or other cankers.

Typical symptoms include:

• Rough, greenish, tan, brown or blackish galls on twigs, branches and trunks.
• Knots forming at pruning cuts, frost cracks, hail injuries, harvest wounds, leaf scars or other bark injuries.
• Occasional swellings on fruit stems, leaves or other tissues.
• Girdling of small shoots, followed by yellowing, defoliation and dieback beyond the gall.
• Reduced fruit production and, in severe cases, weakened or dying branches.

Galls are usually firmer and more irregular than normal branch collars or healed cuts. Their appearance alone is not always sufficient for definitive identification.

The bacterium survives in existing galls and can also occur on the surface or inside apparently healthy olive tissues. When galls become wet, bacteria can be released and moved by rain splash, wind-driven rain, runoff, insects and contaminated pruning or harvesting equipment. Infection is favored when bacteria reach fresh wounds or highly susceptible leaf scars.

Common entry sites include pruning cuts, mechanical harvest injuries, frost or hail damage, rubbing wounds and leaf scars shortly after leaves fall. Moving from diseased trees to healthy trees with unclean tools is an important preventable pathway. Soil is not considered the principal long-term reservoir for this pathogen.

Olive knot is a perennial disease cycle rather than a short-lived foliar outbreak. The bacterium persists in knots and on plant surfaces through dry periods. During rainy, mild weather, bacterial populations increase and exude from wet galls. Splashing water carries cells to nearby bark, leaves and wounds.

Fresh wounds are most vulnerable soon after they occur. Leaf scars can be especially susceptible for several days after leaf fall, although infection may occur at other times when moisture and inoculum are present. After entering tissue, the bacterium colonizes the wound and induces abnormal localized woody growth. New knots become additional sources of inoculum, allowing disease to build gradually within a tree or orchard.

There is no dependable curative treatment that removes the pathogen from an established olive tree. On individual trees, cut out knots and visibly infected wood well below the affected tissue when practical, especially where the infection is confined to small branches. Perform this work only in dry conditions, sterilize tools frequently and remove cut material from the site.

Large trunk or scaffold-branch knots may be impractical to excise without creating larger wounds or structural hazards. Severely affected trees may require evaluation by a qualified arborist or plant diagnostician. Protective bactericide applications can reduce infection of new wounds but do not eliminate established galls; timing and coverage should follow current local recommendations and the product label.

For organic and low-input management, emphasize sanitation and wound avoidance: prune in dry weather, remove infected wood, disinfect tools between trees, avoid spreading wet bacterial material and reduce unnecessary mechanical injury. Copper-based products may be permitted in some certified-organic systems, but only a product currently labeled for the crop and use should be considered; check organic-certification requirements and avoid excessive copper accumulation in soil. Biological or plant-extract products should not be assumed effective unless supported by local trials and a legally compliant label.

Prevention is more reliable than attempting to cure established knots:

• Purchase disease-free, pathogen-tested olive trees and inspect nursery stock before planting.
• Prune during a forecasted dry period, not immediately before rain, fog or overhead irrigation.
• Begin pruning in healthy blocks or trees and finish with the most diseased material.
• Disinfect saws, loppers and hedgers between trees or blocks, using a product and concentration appropriate for equipment sanitation.
• Remove and destroy or securely dispose of pruned knots and infected branches; do not leave them beneath trees.
• Minimize unnecessary cuts and prevent mechanical injury during harvesting, transport and routine maintenance.
• Manage irrigation to keep trunks and foliage dry when practical, and maintain good canopy access for inspection.
• Where locally appropriate, select cultivars with lower documented susceptibility, recognizing that susceptibility can vary among studies and bacterial strains.

The disease is commonly called olive knot because its conspicuous woody galls resemble knots in the branch. The pathogen has also been reported from some other woody hosts, but host range varies among strains and pathovars; olive is the principal host documented for pv. savastanoi. Related bacteria, including other Pseudomonas savastanoi pathovars, can cause knot or canker symptoms on oleander, ash and broom and should not automatically be treated as the same pathogen.

Rain is central to epidemics because wet knots release bacterial cells and water transports them to wounds. Research has also shown that pruning-tool sanitation can substantially reduce disease transmission, making clean equipment one of the most practical control measures for home landscapes and orchards.