This document discusses a study investigating the systemic spread of Pseudomonas savastanoi pv. savastanoi (Pss), the bacterial pathogen that causes olive knot disease, in olive explants. The study inoculated olive explants with Pss and observed the explants over 10 weeks. It found that Pss caused a progressive collapse of the stem from the inoculation site downward. It also caused localized outgrowths on the stem at various distances from the inoculation site. Histological analysis showed the outgrowths resembled knots formed in vivo and that Pss diffused through the xylem vessels. The olive host reacted to the pathogen invasion. Disease was evaluated every 2 weeks by rating stem collapse and out

1. Systemic spread of Pseudomonas savastanoi pv.
savastanoi in olive explants
Riyad Abusamra
To: Dr.Mazen Salman
PhD Program

2. Disease discerption
The manner in which the bacterium
Pseudomonas savastanoi pv. savastanoi
(Pss), the causal agent of knot disease,
infects olive plants is erratic and has not
been fully documented.
The bacterium Pseudomonas savastanoi pv.
savastanoi (Pss) causes knot disease on olive
(Olea europea) and some other Oleaceae
(Ligustrum japonicum, Jasminum spp.,
Forsythia spp. Phyllirea spp.). Other
important hosts of Pss are oleander and ash

3. Introduction (what is the A knot)
A knot: is an outgrowth of plant cell tissue that in its early stages is usually isolated,
irregularly spheroidal, greenish, not very hard and with a smooth surface. With age,
knots tend to become hard, brown to dark, with a cracked surface, and with a size
ranging from a few millimeters to several centimeters in diameter. Knots have been
examined histologically on oleander.
more recently on buckthorn and myrtle
On all these hosts, the pathogen initially colonizes the tissues around the infection
court and disrupts the integrity of the host cells, producing cavities that are filled
with the bacterium. Phytohormones produced by the bacterium

4. Disease discerption

5. investigate the process of Pss
• olive explants were inoculated in vitro and examined visually and by light
microscopy at 2-weekly intervals for 10 weeks.
• In all host genotypes tested, interaction with the pathogen resulted in:
(i) a progressive collapse of the stem, originating at the inoculation site at
the apex of the explant, and proceeding downwards
(ii) localized outgrowths on the stem located at various distances from the
inoculation site.
• Histological analysis revealed that the anatomy of the outgrowths closely
resembled that of knots formed in vivo; they showed that Ps. savastanoi
also diffused within the olive explants through the xylem vessels, and that
the olive host reacted to pathogen invasion, possibly by producing
substances of polysaccharidic and/or phenolic nature.

6. Rooted olive
cuttings showing knot
disease symptoms along the
stem.

7. Disease evaluation
Disease inspections were carried out at 2-weekly intervals for 10 weeks. At each inspection
date, individual plants were rated both for the extent of stem collapse, which started from
the inoculation site and proceeded progressively downwards, and for the occurrence and
location of outgrowths along the stem. The extent of the collapse was rated on the following
scale: 1, limited to the inoculation site; 2, extending to between the first and the second
internode; 3, extending all the way to the base of the explant. As regards the location of
outgrowth along the stem, six classes were defined: A, only at the inoculation site; B, between
the inoculation site and the mid-part of the explant; C, only at the mid-part of the explant;
D, between the mid-part of the explant and the base; E, only at the base of the explant; F, all
along the main axis of the explant.
The test was carried out for 10 weeks, after which time it was decided to halt further
measurements as the control explants were beginning to show irreversible dieback.

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