Bunt or stinking smut disease of wheat is widely distributed in all the wheat growing areas of the world. In India, the disease is confined to cooler regions such as Kashmir, Himachal Pradesh, parts of Punjab, and western U.P. It is either rare or completely lacking in other regions.
The disease is also known as ‘hill bunt’ due to its occurrence in the hilly areas and also as ‘covered smut’ due to sori being covered. This disease causes loss in yield, poses problem during threshing and reduces the grain quality. Bunt, moreover, results in explosions in combines and elevators during threshing or handling of smutted wheat because of the extreme combustibility of the oily smut spores in the presence of sparks from machinery.
It has been estimated that bunt disease results in 25-50% reduction in yield. Bunt possesses poisonous properties and, therefore, the flour of contaminated seeds is harmful for human consumption. The straw is harmful and dangerous to cattle.
Symptoms of Stinking Smut Disease:
Although the stinking smut is a systemic disease resulting from seedling infection, the symptoms of the disease are not usually evident until the heads are formed. It is because this is the inflorescence which shows differences between normal and smutted florets a few days prior to the emergence of head from the boot.
Their colour becomes usually bluish-green rather than the normal yellowish-green, they are slimmer than healthy ones, and the glumes seem to spread apart and form a greater angle with the main axis of the head than they do in healthy plants.
The infected kernels become shorter and thicker than healthy ones and impart greyish-brown colour than the normal golden-yellow or red colour. When these kernels get broken following their maturation, they are found to be full of a shooty, black, powdery mass of fungal spores, which give off a distinctive odour resembling that of the decaying fish.
It is this smell of rotten fish, caused by the presence of a volatile compound, the trimethylamine, in the spore mass, that gives the characteristic name ‘stinking smut’ to this disease. One can see, however, large clouds of spores released in the air during harvesting of the crop in an infected field.
Causal Organism of Stinking Smut Disease:
The disease is caused individually or jointly by Tilletia tritici (= carries) and Tilletia leavis (=foetida). The life histories of the two species of the same fungus and the disease development are almost similar. One of the main differences between the two is in the external morphology of their epispore. In T tritici (= carries) the wall has reticulations ranging from shallow meshes to deep indentations, while in T. leavis (= foetida) the wall is smooth.
The mycelium of the fungal pathogen is septate and each of its cells are binucleate and hyaline. During sporulation most mycelial cells get transformed into almost spherical, brownish, binucleate teleutospores (formed in the manner of chlamydospores, hence also called ‘chlamydospores’), which are thick-walled and measure 15-23 µm. The remaining mycelial cells, however, remain hyaline, thin-walled, and sterile. When matured, the teleutospores become diploid as a result of karyogamy and each develops a basidium.
Each basidium gives rise to 8-16, long, hyaline, haploid, and uninucleate basidiospores at its tip after passing through meiotic division in its diploid nucleus. The basidiospores are usually called ‘primary sporidia’, which generally fuse in pairs by means of lateral branches developed between compatible mating types. After being fused, the two primary sporidia appear as “H-shaped” structure. The nucleus of each primary sporidium divides
mitotically and both partners of the H-shaped structure become dikaryotic as a result of exchange of one of their daughter nuclei.
When these primary sporidia germinate, they give rise to ‘secondary sporidia’, also called conidia. These are the secondary sporidia which germinate giving rise to dikaryotic germ tubes which penetrate and infect the wheat seedlings. The mycelia develop systemically inside the growing wheat plant and the symptoms appear at the time when heads are about to form.
Stinking Smut Disease Cycle:
(i) Perennation
The pathogen survives by means of its teleutospores carried on the seed or present in the soil. However, under the climatic conditions of our country the disease mainly perennates through infected seeds, i.e., it is seed-borne. The spores are sticky and get easily adhered to the seeds during threshing and to the soil particles during harvesting.
(ii) Primary Infection:
When contaminated seeds are sown or healthy seeds are sown in bunt-infested fields, approximately the same conditions that favour germination of seeds also favour the germination of teleutospores. As the young seedlings emerge from the kernel, the teleutospores adhering the kernel or present near the seedling in soil also germinate giving rise to basidia, primary sporidia, and, finally, secondary sporidia.
The secondary sporidia germinate and the dikaryotic germ tube produced by them penetrates the young seedling directly. The hyphae so developed grow intercellularly along with the growing wheat plant.
When plants so infected form heads, the mycelium invades all parts of it even before the head emerges out of the ‘boot’. Finally, most of the mycelial cells produce teleutospores. This is the stage when symptoms appear and the disease becomes apparent. However, the teleutospores are carried to soil through seeds or dissemination, perennate therein, and serve as the source of primary infection for the next growing season.
(iii) Secondary Infection:
There is nothing like secondary infection reported during the course of the disease in one growing season and the disease incidence and its severity is considered solely dependent on the primary infection of the crop.
Predisposing Factors:
Temperatures ranging from 5-15°C have been considered highly conducive to infection. Similarly, high soil moisture and sandy and humus rich soils favour severe infection. It has been reported that potassium and phosphetic fertilizers increase the severity of infection.
Management of Stinking Smut Disease:
(i) Bunt disease can be controlled effectively by using inoculum (teleutospore) free seeds. For this, the seeds chosen should be properly treated with an appropriate fungicide before sowing. Copper sulphate (2% solution), formalin (1 lb/40 gallons of water), copper carbonate dust (2-3 oz/60 lbs of seeds), Agrason GN (4 oz/80 lbs of seed) have been reported quite effective. Singh et. al. (1977) have reported Bavistin, when employed as a seed dressing fungicide at 0.1% concentration, renders almost complete control of the disease.
(ii) In order to avoid infection from soil-borne inoculum, sanitary precautions such as burning of straw, rouging, and crop rotation should be followed.
(iii) Use of resistant varieties should be followed. Among varieties resistant to this disease are – Kalyansona, S 227, PV 18, HD 2012, HD 4513, and HD 4519.
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