Olive knot disease is one of the most common and serious problems affecting olive groves. Its main characteristic is the formation of knots and swellings on branches and the trunk of the trees. A bacterium causes this disease, and its spread mainly depends on agricultural practices during winter and spring.
The problem becomes more severe in olive varieties that are harvested by beating. In addition, pruning, winter frost periods, and the area’s relative humidity all play significant roles. Dense planting of olive trees can also lead to a more widespread problem.
What causes the olive knot disease?
Olive knot disease is caused by the pathogenic bacterium Pseudomonas savastanoi pv. savastanoi. This pathogen is external, meaning it cannot enter plant tissues on its own and requires a wound for penetration. The knots that appear on the bark are caused by the bacterium’s ability to insert a gene into the plant’s genetic material.
This gene activates the synthesis of growth hormone (IAA) in cells, leading to their division and uncontrolled multiplication, which can form knots. Olive knot disease affects, multiplies, and damages the bark of the trees, including trunks, branches, shoots, and twigs.
How does olive knot disease spread?
The pathogenic bacterium is transmitted through bacterial exudates released from the knots after rainfall and spreads via air, rain, and contaminated tools, infecting nearby trees.
When pruning tools or harvesting rods are used without proper disinfection, the infection can spread over long distances. When the bacterium comes into contact with cuts and open wounds on healthy bark, it infects the trees.
How are wounds and cuts created on tree bark?
Wounds and cuts may be caused by pruning tools and harvesting rods, as mentioned above, but also naturally by the scars left from fallen leaves, flowers, and fruits. Additionally, natural phenomena such as hail and frost can damage the bark.
After pruning or mechanical damage, trees remain vulnerable to bacterial infection for at least 14 days. In the case of naturally occurring scars (like leaf fall), trees show increased susceptibility for about 7 days.
Visible symptoms of olive knot disease
The primary and characteristic symptom of olive knot disease is the presence of intricate knots, known as galls. These are round bumps that are 1 to 3 cm in diameter and can be found on the bark of branches, twigs, trunks, and near old pruning cuts. In rare cases, such knots may also appear on the roots or the central veins of leaves and fruit stalks.
Small twigs surrounded by knots may lose their leaves and dry out. Larger branches remain shortened and suffer severe damage. Such damage leads to weakened trees and reduced yields.
In addition, olive knot disease affects fruit quality, reducing size and altering organoleptic properties (chemical and nutritional composition, taste, etc.). The economic consequences are significant for both olive oil producers and table olive growers.
Factors favoring olive knot disease
The risk of infection from olive knot disease is higher in winter and spring. In winter, frost can cause cracks in the bark, while hail can injure plant tissues. Spring rains can carry bacteria from the exudates of existing knots to nearby trees—especially those recently pruned.
Later in spring, during heavy leaf fall, trees become more vulnerable to infection. This risk increases if trees are affected by other diseases, such as olive leaf spot (Cycloconium oleaginum) or olive verticillium wilt, since the scars left by fallen leaves can serve as entry points for bacteria. These bacteria can move through the vessels a few centimeters above and below the infection site, causing secondary infections on the same branch.
Winter infections do not appear immediately; symptoms are visible later, in spring, during vegetative growth. This phenomenon explains why nursery trees may appear healthy when purchased but later develop knots. Choosing resistant varieties and healthy planting material is essential.
Measures to control olive knot disease
Olive knot disease requires systematic care and prevention for adequate control. Below are four primary measures to manage the disease.
Removal of knots
If olive trees are infected with olive knot disease, it is necessary to remove the knots carefully using a saw or pruning shears. Always disinfect the tools before each cut to reduce the risk of spreading the infection.
It is essential to remove tumors from already infected trees, even if they are few in number, as this prevents the spread of the disease through bacterial exudates.
You should carry out this work during the summer months (July–August), preferably in dry conditions with low humidity. Avoid removing knots in winter or spring, as unexpected rain may hinder your efforts.
Copper spraying
Combine the above process with the application of a copper-based fungicide to protect the pruning wounds. Spraying with copper preparations is also recommended after pruning, after frost, and following intense leaf fall in the spring or after knot removal in the summer.
It is also advisable to apply copper sprays immediately after olive harvest in late autumn and after winter frosts. The wounds can also be coated with Bordeaux mixture (which also contains copper). Avoid harvesting olives right before rainfall in autumn.
Tool disinfection
Before and after any work on the trees, disinfect all tools. Clean blades and surfaces with a 10% bleach solution to eliminate bacteria and prevent the spread of disease. Ethanol can also be used as an alternative disinfectant.
Choosing resistant varieties
According to research, table olive varieties such as Kalamon and Megaritiki appear to be slightly resistant to the disease, while Koroneiki and Conservolea are more susceptible.