|
'Karnal bunt' disease
in wheat
Dr S.A. Jamil
Khan
In 2004, Pakistan imported 0.5 million tons of wheat from
Australia to meet its deficit. When the consignment reached
Karachi, seed samples were examined and found to be infested
with a fungus (Neovossia indica) causing 'karnal' or partial
bunt disease.
Subsequently the wheat was declared unfit for human
consumption. Wheat generally suffers from two major groups of
diseases: rusts and smuts. Of the smut diseases, 'karnal bunt'
is one of the important diseases of wheat.
Below are given the symptoms and mode of the infection of
'karnal bunt' for the benefit of progressive farmers and
importers of wheat.
SYMPTOMS:
Karnal or partial bunt is a soil-borne infecting disease which
eventually affects flood parts of plants and the seed becomes
bunted. The diseased spikes can be recognized in mature crop
as the glumes of such spikelets spread apart exposing the
bunted seeds.
The diagnostic symptom of the disease is blackening of the
grains at germinal end which becomes slightly swollen and
gives a silvery and shiny appearance. Usually, the infection
spreads to the seed tissues along the grooved portion and the
dorsal side of the seeds remain unaffected.
Normally, seeds are partially infected but in a severe
infection, the whole endospermic material may be converted
into a mass of bunt spores which comes out and seeds become
hollow. In a diseased plant, a few spikes and within a spike
only few seeds are infected, thus the disease is also known as
partial bunt affecting about 0.5 per cent of the total yield.
DETECTION TECHNIQUE:
(visual test) In preliminary studies, wheat samples are
examined by the naked eye or under stereo-binocular microscope
for detecting bunted seed. They appear different from healthy
ones because of being partially affected on the groove side
(embyo) blackened with mass of bunt spores.
WASHING TEST:
This is a simple laboratory method to find out fungal spores,
mostly present on the surface or loosely attached, with the
tissues of seeds. One hundred seeds are taken in 250 ml flask
containing 25 ml of sterilized distilled water and shaken by
hand or by a mechanical shaker for 10 minutes.
The sediment is placed an a slide under microscope. The fungal
spores including 'kernal bunt' is identified on the basis of
morphology of spores.
SOAKING METHOD:
It is a quantitative assessment for the identification of
karnal/partial bunt spore. Two thousand seeds in 2
replications of 1000 seeds each are soaked in a flask/beaker
containing 250 ml of 0.2 per cent NaOH solution for 24 hours
at 25xC).
After 24 hours, water is decanted and the seeds are thoroughly
washed in tap water. Seeds are examined visually with the help
of a magnifying glass. The seeds giving black appearance are
distinguished with seeds without blackness. The seeds on
rupturing in a drop of water release a stream of bunt spores,
which can be seen under a microscope.
CONTROL:
Resistant varieties are recommended for cultivation.
Continuous cropping of wheat in the same field should be
avoided. Crop rotation programme and green practice should
encourage spore germination and its self-destruction.
Only disease-free seeds should be used for sowing. Seeds
should be treated with any of the fungicides which limit the
incidence of the disease before sowing.
Courtesy:The
DAWN |
Pakissan.com;
|
