Olive Tree with Bumps on the Branches and Stems

Olive Knot Disease

• Olive knot disease caused by the pathogen Pseudomonas syringae pv. savastanoi

  results in the formation of bumps on the branches, stems, leaves, roots, trunk and twigs of an olive tree. These bumps range from 1/2 inch to 2 inches in diameter and are properly referred to as galls -- swellings -- brought on by the disease. They possess the potential to kill an olive tree and often do so when they girdle -- or strangle -- the trunk or roots of the specimen.

Infection Process

• Olive knot disease is most frequently transferred through shears and other gardening devices that carry the bacteria. Mechanical wounds are easily inflicted via pruning and other maintenance techniques, allowing the disease to enter the tree through the point of injury. Frost or hail damage also makes the tree susceptible to the disease. The bacteria is easily spread through water transfer in warm, rainy months, through openings left by natural processes like leaf drop and from infected nursery stock.

Damage

• Girdled twigs are the first to be affected with shoots and buds dying, resulting in a subsequent loss of olive fruit yield. Trees utilized for ornamental purposes will also see their value diminished as twig dieback and leaf defoliation occurs. As a further insult, an olive tree with bumps on the branches and stems will suffer a loss or alteration in the flavor of the olive fruit. This loss of taste also affects people who grow olive trees specifically for the extraction of oil.

Management

• Commercial growers typically utilize copper-based bactericides to manage olive knot disease, but these applications may not be available to home gardeners. The best management technique in a landscape setting is to exercise caution when purchasing nursery stock. Trees should be purchased in the spring when the bacteria comes to life, allowing them to be easily inspected for the formation of galls or bumps on the twigs and stems. Gardeners should also be careful to avoid injuring the bark of the tree and should regularly sterilize their gardening tools.