Flag smut disease has been reported from almost all the wheat growing areas of the world. It was first found in Australia in 1868. In India, Butler was the first person who reported flag smut disease of wheat from Lyallpur in West Bengal in 1918. Since then, this disease has been observed in different part of our country especially Punjab, Haryana, Himachal Pradesh, M.P., Delhi, U.P., Bihar, and Rajasthan.
Although this disease does not show its widespread occurrence, it causes considerable loss wherever evident. Reduced yield due to complete loss of productivity of infected plants is the most significant effect of the disease. Bedi (1957) estimated that the incidence of this disease is as high as upto 75% in some parts of Punjab, Himachal Pradesh, and Haryana.
Symptoms of Flag Smut Disease:
The leaves and leaf-sheaths are most commonly affected, but stems, culms and ears are also attacked occasionally. Grey to greyish-black linear sori occurs on the leaf blade and sheath. Primarily the sori develop under the epidermis but, later on, the epidermis ruptures exposing a black powdery mass of spores and, finally, the leaf tissue disintegrates along the linear sori.
All this adds to twisting, drooping, and shedding of the infected leaves resulting in the death of the whole plant. If the infection takes places during the seedling stage, the plant becomes severely stunted and ears do not develop. But, if infection occurs late, the grains are formed but they are shrivelled and do not germinate or germinate very poorly.
Causal Organism of Flag Smut Disease:
The teleutospores (generally called spores) are borne in tiny balls. Each ball contains 1-6 spores, mostly 3, which are surrounded by a layer of sterile peripheral cells. Each spore is globose to subglobose, reddish to olivaceous in colour, smooth-walled. They germinate in situ forming a short promycelium (or basidium) without or with 1 -2 septa.
At the distal end of the pro-mycelium (or basidium) are formed primary sporidia which Ainsworth (1973) are 1 -4, mostly 3, in number. The primary sporidia are cylindrical, aseptate or 1-2 septate, and measure 12-15 x 3 µm. They usually remain attached to the pro-mycelium and germinate by a slender germ tube, which may be of considerable length and represents infection thread or infection hypha.
Flag Smut Disease Cycle:
(i) Perennation:
The disease is soil as well as seed-borne. The teleutospore-balls produced on leaf-blades, leaf- sheaths and other parts get adhered to the seeds during threshing and remain dormant thereafter. Moreover, the spore-balls reach the soil via plant debris at the time of harvesting and they may persist in soil for many years.
(ii) Primary Infection:
At the return of favourable conditions in the next season the teleutospores germinate, produce basidia, which give rise to primary sporidia. The latter remain attached to the basidia, produce germ tubes (or infection hyphae), which infect seedlings in the field.
The point of entry during infection is restricted to the young coleoptile less than four mm long. After the seedling is infected the mycelium grows systemically both inter- as well as intracellularly until it begins to sporulate generally first on the leaf blades and then on the leaf sheaths and other parts.
Teleutospores are produced, near the end of the growing season, on infected parts. They are resting structures and serve as source of primary infection during the next growing season.
(iii) Secondary Infection:
No secondary infection during the same growing season has been reported in case of this disease.
Predisposing Factors:
There are various environmental factors such as soil moisture, soil temperature, soil pH, and cultural practices such as the date of sowing, depth of sowing, and variety of host, which predispose the seedling infection. Temperature ranging between 20-22°C is considered optimum for infection, soil containing around 40% moisture increases disease incidence, and the deep sowing of seeds helps successful infection and disease occurrence.
Management of Flag Smut Disease:
(i) Use of resistant varieties in conjunction with seed-treatment is most effective control measure. Pusa-4 variety is resistant to this smut in our country. However, the variety WG 377 has been found most resistant particularly in Punjab.
(ii) Seed treatment with copper sulphate or copper carbonate dust (2 oz/80 lbs seeds) is effective. Steeping the seeds in formaldehyde solution (1 lb/40 gallons of water) is also recommended. Tetrachloronitroanisol (TCNA) has been recommended for our country.
(iii) Crop rotation, early sowing, burning of straw, and hot weather cultivation reduce the incidence of the disease.
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