In this section:
Olive Knot Information
Olive knot is a potentially serious disease of olives caused by the bacteria Pseudomonas savastanoi. Symptoms include rough galls or swellings of variable size that occur on twigs, branches, trunks, roots, leaves or fruit stems. Galls similar to olive knot have been found in South Australia and bacteria have been recovered from these galls. Similar galling can be produced on olives by a related bacterium, which also causes Oleander knot (common in Australia). Therefore tests are underway to determine which bacterium is causing the galling found on olives, however these are likely to take some time to complete.
As a precaution, this article aims to provide growers and others in the industry with information on the disease and protective management procedures that can be taken. 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.
Symptoms
Galls will appear either singly or close together on any organ of the plant. They are most common on twigs and young branches (Figs 1 and 2), but will also form around wounds on the main trunk (Fig 3) and on damaged leaves. Starting as small swellings, they grow rapidly into smooth spherical green knots 3–5mm across, increasing in size as they mature (up to 2.5cm) and becoming darker and more furrowed (Fig 4).
Infection and Spread
The bacteria live in the galls and ooze out in wet weather. The bacteria enter the tree through wounds, including leaf scars, damage by hail and frost, pruning wounds or wounds caused during harvesting. Infection can occur at low temperatures and time from infection to galling can vary from a week or so in spring to several months in autumn. Trees at most risk are those with wounds during periods of rain.
Management
Managing the disease is aimed at preventing infection, as it is difficult to eradicate established infections. This includes a summer pruning to remove galls to reduce the inoculum load and applying copper based products to protect wounds.
The following practices are recommended:
- Provide good growing conditions - healthy trees are less susceptible.
- Avoid excessive fertiliser application – trees should be kept balanced to ensure they only produce the leaves necessary for development.
- Avoid wounding trees.
- Prune healthy plants before infected ones.
- Practice good hygiene - tools should always be disinfected after pruning suspect trees.
- Do not prune in wet weather or just before wet weather. Wounds remain susceptible for up to 10 days.
- Shoots and twigs damaged during harvesting should be pruned out.
- Pruned branches should be burned immediately on site.
- 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 olives as they are possible hosts of the bacteria.
- Copper sprays should be applied in autumn and spring, before rain and after pruning and leaf fall to protect wound sites. Use label rates and apply at least once in each season.
For more information, the following web sites may also be of assistance.
http://www.ipm.ucdavis.edu/PMG/selectnewpest.olives.html
http://cetulare.ucdavis.edu/pub/olilst.htm
Compiled by Barbara Hall, Plant Pathologist, SARDI, 16 June 2003, using information from the UC Davis website, UC Olive production manual, IOOC Olive Pest and Disease Management handbook and NZOA Information Bulletin.
This article is also available as a pdf version (.PDF 708.0KB).
