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Olive knot is caused by a bacterium, which infects wounds and causes galls to form. These galls will appear either singly or close together on any part of the plant.  They are most common on twigs and young branches, but will also form around wounds on the main trunk and on damaged leaves. Galls can damage the stem structure and can deform the scaffold of the tree if infection is severe during the early stages of the tree development.

The bacteria live in the galls and ooze out in wet weather. They also live on the surface of leaves and inside infected wood, so infection may be present without visible galls. The bacteria enter the tree through leaf scars, but will also enter through wounds from pruning, harvesting, hail or frost. Most infection occurs with wet weather in spring, and trees at most risk are those with wounds during periods of rain. Infection can occur at low temperatures and the time from infection to when the galls appear can vary from a week or so in spring to several months in autumn. 

Managing the disease is aimed at preventing infection, as it is difficult to eradicate once it is established. The following information outlines management strategies that should help prevent serious damage by olive knot, and minimise the spread once infection is established in the grove.

General management

• Avoid wounding trees and provide good growing conditions - healthy trees are less susceptible.
• Avoid over fertilising, because excessive vigour produces soft susceptible growth. The nutrient status of trees should be kept balanced to ensure they only produce the leaves necessary for development, and do not have excessive leaf fall that provides scar wounds for entry of the bacteria.
• Possible alternative host plants such as Oleander, Ligustrum (Privet), Forsythia, Fraxinus (Ash), Jasmine, Phillyrea and any other plants of the Oleaceae family should not be planted near your olive grove.
• Clean machinery entering and leaving the grove. The bacterium can survive on plant trash, so it is advisable to ensure all machinery is cleaned and free of trash before moving between blocks or properties. This is also a good preventative measure for minimising the spread of many other diseases.

Pruning

• Do not prune during or before wet weather. Preferably prune in summer. Infection rates are highest in winter and spring, and least in summer. Wounds can remain susceptible for up to 14 days, and in wet weather bacteria will exude from the knots and infect the wounds.
• Practice good hygiene – this is important to minimise the spread of many diseases. Tools should be disinfected after pruning suspect trees and healthy plants should be pruned first. While this may not provide complete control, it will assist in minimising the spread of disease. Removing the pruned branches and burning them has also been recommended, however the bacteria has a limited life in dead branches, so while this is good general practice it is unlikely to assist with control of olive knot.
• Prune to provide the best shape for the tree with uninfected branches as the scaffold. Shoots and twigs damaged during harvesting should be pruned out.

Chemical treatment

• Copper sprays should be applied at least twice. Californian work has shown that additional sprays in spring as well as the usual post harvest application will substantially improve disease control. Most leaves drop during spring, and as the leaf scars are open to infection up to 7 days after leaf fall they should be protected during this time, particularly if wet weather is forecast.  Adding an adjuvant may assist penetration and improve effectiveness of the copper. While routine copper sprays reduce olive knot infections, to be totally effective a continuous cover of copper is required throughout the year. This is impractical, particularly when fruit are on the trees.

Propagation

• Use cuttings from disease free healthy mother stock. The bacterium can be systemic in the wood and not produce galls until the conditions are suitable.
• Plant less susceptible varieties.  While all varieties can be infected, some (eg Carolea, Koroneiki and Pendolino) were relatively tolerant in New Zealand studies.  Frantoio, Barnea, Manzanillo, Picholine and S.A.Verdale are reported as very susceptible, and Picual, Leccino and Manzanilla moderately susceptible.

Harvest

• Olives should not be harvested during rain.
• Harvesting by beating should be minimised where the disease is present, as beating may cause wounds. Protecting the wounds with copper may help, either by spraying immediately after harvest or by wrapping beating sticks with cloth soaked in Kocide.

If any suspect galls are observed in olive trees, please phone the toll free number 1800 084 881. This will put you in touch with the appropriate authority in your state and will enable a follow-up inspection and further information to be provided as appropriate.

Compiled by Barbara Hall, Plant Pathologist, SARDI, 22 July 2004 with assistance from the Olive Knot Management Group. Information was obtained from the following sources:

• UC Davis website http://www.ipm.ucdavis.edu/PMG/selectnewpest.olives.html 
• Olive production manual. (1994). L. Ferguson, G.S. Sbbert and G.C. Martin, University of California, Publication 3353
• Olive Pest and Disease Management (1999). M.C. Lopez-Villalta (IOOC, Madrid)
• Improvements In The Control Of Olive Knot Disease (1999). W.H. Kruger, B.L. Teviotdale and M.N. Schroth. ISHS Acta Horticulturae 474_117.
• Olive Knot Disease and Its Pathogens (2004). J.M. Young, Australasian Plant Pathology 33(2),  J.M. Young (unpublished data).
• Effects of timing of copper sprays, defoliation, rainfall and inoculum concentration on the incidence of olive knot disease (2004).  B.L. Teviotdale and W.H. Kruger, Plant Disease 88(2).

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