NUCIS number 10. December 2001. 48 pages (full ... - IAMZ - ciheam

experiments partially fulfil Koch’s postulates,
a protocol used by pathologists to determine
the cause of a “new disease”.
This protocol requires the association of
the pathogen with disease, the isolation
of the pathogen in pure culture from diseased
trees, the observation of
symptoms in the inoculated trees and the
isolation of the same pathogen from those
inoculated trees. At present, foliar
symptoms and trunk and limb lesions
have not yet been reproduced in inoculated
trees. More time may be required as
foliar and trunk symptoms are not seen in
the orchard until the trees are at least five
years old.
Figure 4. Internal staining (indicated with arrows) in a section of a branch of a diseased tree
Mapping of symptoms in the orchard
In 1997 and 1998 a mapping kit was developed
as a tool for growers, to allow
them to map the incidence of dieback and
trunk lesions in their own orchards. The
mapping kit consisted of photographs of
disease symptoms along with a method
for assessing the damage based on recording
the number of trunk lesions, the
presence or absence of dieback, shoot
death and resinous exudate on each tree.
In case abiotic factors were involved in
the disease, information was also gathered
on the main management factors in
each orchard, including irrigation, fertiliser
regime and rootstock.
Figure 5. Resinous exudate
Role of the bacteria in disease
A suspension of the bacteria was injected
into healthy young pistachio trees in the
glasshouse and the development of
symptoms monitored over 24 months.
Destructive sampling showed that staining
of the xylem occurred in 87% of the
Figure 6. Lesion on the trunk of a diseased tree
inoculated trees and Xanthomonas species
were re-isolated from the stained tissues
of 73% of those trees. Control trees
injected with sterile distilled water developed
limited staining around the injection
point only and Xanthomonas species
were not detected in the tissues. These
The statistical package, Patchy®, used to
study the pattern of spread of disease did
not provide conclusive results about the
pattern of spread of this disease. Mapping
of the symptoms will continue concentrating
on foliar symptoms as well as
on the ones mentioned before, and the
use of other programmes for spatial
analysis will be evaluated.
Control of the disease
A field trial was initiated in 1998 to test
the effectiveness of phosphorous acid
(mono-di potassium phosphite) and Bion
(acibenzolar-S-methyl) in controlling the
disease. Neither of the treatments alleviated
disease symptoms.
CURRENT RESEARCH:
The aims of the current project are:
• Determine the bacterial pathovars associated
with pistachio dieback.
The bacteria consistently associated with
the disease have been identified as Xanthomonas
translucens by various means
and have been sent to taxonomists with
expertise in Xanthomonas species (University
of Gent, Belgium) for confirmation.
Once results are available, inoculation of
various hosts to identify pathovar(s) and
confirm pathogenicity will be conducted.
• Develop protocols for detection and
recognition of pistachio dieback.
Isolation from twigs and culture on SPA
has proved successful. Isolation from
30 FAO-CIHEAM - Nucis-Newsletter, Number 10 December 2001