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Date: October 8, 2007
Source: British Society for Plant Pathology, New Disease Reports
(NDR) vol. 16 [edited]
<http://www.bspp.org.uk/ndr/jan2008/2007-85.asp>
Natural occurrence of phytoplasma associated with chickpea
phyllody disease in Pakistan -- a new record
During spring 2005-06 chickpea plants (cultivars Desi and
Kabuli) were found to be affected by a previously undescribed
disease in Pakistan. Symptoms consisted of proliferation of
branches with smaller leaflets, giving a bushy appearance to the
plants (Fig. 1). Affected plants were scattered in the field and
were more easily spotted at flowering and podding time. The
flowers developed abnormal green structures (phyllody) instead
of normal flowers (Figs. 2 and 3). At the time of crop maturity
when the healthy plants were drying, the diseased plants in the
field were conspicuously green (Fig. 1).
Tissue samples from infected and uninfected plants were examined
using a transmission electron microscope (TEM) to ascertain if
the disease was associated with a phytoplasma. Typical
pleomorphic bodies (phytoplasma) mostly spherical to oval of a
size ranging from 200-600 nanometers were observed only in the
sieve elements of affected samples. The bodies had opaque, low
electron density cytoplasm that contained ribosome-like
granules, DNA-strand-like structures, and lacked nuclear
membranes (Fig. 4), similar to previously reported phytoplasmas.
Such bodies were absent from healthy samples as well as from
xylem cells, phloem parenchyma cells, and companion cells of
affected plants.
In addition, DNA was analysed by polymerase chain reaction (PCR)
and products were characterized. All samples from infected
plants gave a profile associated with 16SrII phytoplasmas, and
partial sequencing confirmed that the phytoplasma had the
greatest homology to 16SrII phytoplasmas. Koch's postulates were
fulfilled in a greenhouse. 5-week-old chickpea plants grown in
pots were inoculated by grafting. A set of control plants was
kept without grafting. Phyllody symptoms similar to those
observed in the field started to develop after 25-35 days in
graft-inoculated plants while no disease symptoms were observed
on control plants.
Chickpea has been grown in Pakistan for many years without any
reported incidence of phyllody. The presence of phytoplasma
disease in chickpea presents a new threat to the chickpea in
Pakistan. Previously the disease was found in Ethiopia, India,
Myanmar, Australia, and Oman. To the best of our knowledge this
is the 1st report of chickpea phyllody disease in Pakistan.
Figures
(1) Conspicuously green diseased chickpea plant
<http://www.bspp.org.uk/ndr/jan2008/2007-85-1.jpg>
(2) Healthy flower on left and abnormal green structure
(phyllody) on right
<http://www.bspp.org.uk/ndr/jan2008/2007-85-2.jpg>
(3) Phytoplasma infected branch on right and healthy branch on
left side
<http://www.bspp.org.uk/ndr/jan2008/2007-85-3.jpg>
(4) TEM of phytoplasma colonizing the phloem of an infected
stem.
<http://www.bspp.org.uk/ndr/jan2008/2007-85-4.jpg>
[Khalid P. Akhtar et al]
--
Communicated by:
ProMED-mail
<promed@promedmail.org>
[Chickpea is an important crop in many developing countries
because of the high protein content (more than 25 percent) of
its seeds. The Kabuli type is grown in temperate regions while
the Desi type is grown in the semi-arid tropics. Seeds and straw
are also used as animal feed.
Phytoplasmas are a unique taxonomic group of small,
unculturable, intracellular plant-parasitic bacteria within the
group of the Mollicutes. 16S ribosomal RNA sequences were
adopted as a framework for their formal taxonomic
classification. There are still uncertainties in phytoplasma
taxonomy reflected in the genus name _Candidatus_ Phytoplasma.
At least 20 groups and numerous subgroups have been resolved.
The taxonomic rank of species has been proposed to describe each
of the primary phylogenetically coherent phytoplasma groups.
The 16SrII phytoplasma group is called the faba bean phyllody or
peanut witches broom group with type member _Candidatus_
Phytoplasma aurantifolia (lime witches broom phytoplasma -
'witches broom' is descriptive of abnormal proliferations of
bundles of small twigs).
Phytoplasmas are located in the phloem and can spread within
their host via the sieve tubes. They are usually transmitted by
insect vectors (for example phloem-feeding planthoppers) and
grafting. They have been found to cause important diseases in a
range of crops including grapevine, papaya, rapeseed, rice,
potato, citrus, pome,
and stone fruit. Yellowing, phyllody, and witches broom are
symptoms frequently associated with these diseases. Disease
management relies on the use of clean planting material and
vector control. Tetracyclines can inhibit phytoplasma growth but
will not normally eliminate the pathogens from the host.
Maps of Pakistan:
<http://www.lib.utexas.edu/maps/middle_east_and_asia/pakistan_pol_2002.jpg>
and
<http://healthmap.org/promed?v=30,69.4,5>
Links
16SrII group taxonomy and species list:
<http://beta.uniprot.org/taxonomy/85621>
Phytoplasma taxonomy:
<http://sipav.org/main/jpp/volumes/0405/040506.pdf>
and
<http://ijsb.sgmjournals.org/cgi/content/abstract/57/8/1855>
Genus _Candidatus_ taxonomy:
<http://www.bacterio.cict.fr/candidatus.html>
Phytoplasma resource centre:
<http://www.ba.ars.usda.gov/data/mppl/phytoplasma.html>
List of chickpea diseases and pathogens:
<http://www.apsnet.org/online/common/names/chickpea.asp>
Information on chickpea:
<http://www.hort.purdue.edu/newcrop/cropfactsheets/Chickpea.html>.
- Mod.DHA]
[see also in
the
archive:
2000
----
Lime witches' broom phytoplasma - India 20000125.0129]
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