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'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 

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