How to Manage Pests

UC Pest Management Guidelines

Olive

Olive Knot

Pathogen: Pseudomonas savastanoi

(Reviewed 3/14, updated 3/14)

In this Guideline:


Symptoms and Signs

Olive knot appears as rough galls or swellings about 0.5 to 2 inches in diameter on twigs, branches, trunks, roots, leaves, or peduncles (fruit stems). Small shoots may be girdled, defoliated, and killed. Galls also form at trunk or limb wounds.

Comments on the Disease

Olive knot can girdle and kill trees if infections occur on the trunks of young trees. On older trees, it reduces productivity by girdling twigs and branches and causes dieback. Infection is also associated with an off-flavor of the fruit. Olive knot disease incidence is generally correlated with rainfall and is more severe in higher rainfall areas or during years of high rainfall. They are readily spread by water at all times of the year. Bacteria survive in galls and as epiphytes on leaves and twigs. As an epiphyte they may be spread on asymptomatic plant tissue. Infection occurs in fall, winter, and spring. The knots develop in late spring when trees resume growth and continue to develop through summer. Openings are necessary for penetration of bacteria, and these are provided by leaf scars, pruning wounds, or bark cracks made by freezing. Freeze injury can lead to disease epidemics because the resulting defoliation and bark splitting normally occur during the winter when rain occurs and can spread the disease. All cultivars are susceptible, but Manzanillo (table olive) and Koroneiki (oil olive) are the most susceptible of the commonly grown varieties.

The disease is not prevalent along California's coast.

Management

Olive knot is difficult to control and requires preventive bactericide applications., Treatments reduce the pathogen population on the plant surface, decreasing the probability of infection. It is also helpful to carefully prune during the dry season (July to August) to remove galls on twigs and branches. Because the bacteria may be carried on pruning shears, be sure to sanitize them frequently if pruning at other times during the year.

Treatment Decisions

Treatments are preventive and need to be applied before infection. Generally, a minimum of two applications each year are needed in areas of higher disease incidence and for more sensitive varieties (Manzanillo and Koroneiki varieties).

Make the first application in fall after harvest. Apply other applications in spring from March through May. Because leaf scars are susceptible when fresh, time treatment to protect as many leaf scars as possible. If freeze damage results in defoliation, additional sprays may be necessary. Mechanically harvested oil olives should be treated immediately after harvest because harvesting can damage the trees and make openings for the bacteria. Also, harvest occurs in fall or winter when rain is likely. If possible, avoid harvesting when rain is predicted.

Common name Amount to use R.E.I.‡ P.H.I.‡
(example trade name)   (hours) (days)

  Calculate impact of pesticide on air quality
 
When choosing a pesticide, consider its usefulness in an IPM program by reviewing the pesticide's properties, efficacy, application timing, and information relating to resistance management, honey bees (PDF), and environmental impact. Not all registered pesticides are listed. Always read the label of the product being used.
 
CAUTION: Application of methidathion with, or closely following, a bactericide containing lime will negate the insecticide's effectiveness. Apply methidathion before bactericides containing lime are applied. Also, copper may injure trees in areas of low rainfall.
 
A. FIXED COPPER#
  (Various) Label rates See label See label
  MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M1)
  COMMENTS: Allowed with restrictions in organic agriculture. Not all copper compounds are approved for use in organic production; be sure to check individual products.
 
B. BORDEAUX MIXTURE# Label rates See labels See labels
  MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M1)
  COMMENTS: For information on making Bordeaux mixtures, see UC IPM Pest Note: Bordeaux Mixture. Allowed with restrictions in organic agriculture. When used on organically grown produce, all ingredients must be certified organic. Observe the most restrictive label precautions and limitations of all products used.
 
C. COPPER SULFATE#
  (Various) Label rates See label See label
  MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M1)
  COMMENTS: Allowed with restrictions in organic agriculture. Not all copper compounds are approved for use in organic production; be sure to check individual products.
 
D. GALLEX Label rates 24 0

Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.
# Acceptable for use on organically grown produce. Always check with your certifier if products are permissible to be used in your organic production system.
1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions (for more information, see http://www.frac.info/). Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode-of-action group numbers 1, 4, 9, 11, or 17 before rotating to a fungicide with a different mode-of-action group number; for fungicides with other Group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode-of-action group number.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Olive
UC ANR Publication 3452

Diseases

L. Ferguson, Pomology, UC Davis
P. M. Vossen, UC Cooperative Extension, Sonoma County
W. D. Gubler, Plant Pathology, UC Davis
T. J. Michailides, Kearney Agricultural Center
Acknowledgment for contributions to Diseases:
B. L. Teviotdale, Kearney Agricultural Center, Parlier

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