Loose smut disease of wheat commonly occurs in almost all wheat growing regions of the world, it is most severe in humid and sub-humid regions, in India, the disease is relatively more common and severe in northern part of the country; heavy losses are recorded in Punjab and western districts of U.P. The nature of this disease was first studies by Maddox (1895) and later confirmed by Brefeld (1903).
Loose smut causes damage by destroying the kernels of the infected plants and by smearing and thus reducing the quality of the grain of the non-infected plants upon harvest. Losses from this disease, however, have been recorded upto 10-40% in most severely infected areas in a given year.
Symptoms of Loose Smut Disease:
Symptoms become evident only when the plants are headed. Infected heads consist of sooty, dark, olive brown spores in place of normal grains imparting black powdery mass appearance. This is first covered by a delicate silvery membrane, which soon bursts and sets the powdery spores free.
The spores are then blown off by the wind and leave the rachis a naked stalk. Ordinarily all the heads and all the spikelets and kernels of each head are affected, some of them sometimes may remain unaffected. However, diseased plants sometimes head earlier than healthy ones, and smutted heads are often elevated above those of the non-smutted ones.
Causal Organism of Loose Smut Disease:
The mycelium is hyaline primarily but turns to brown near maturity. It is septate, dikaryotic, and grows systemically inside the plant. The mycelial cells get transformed into brown, spherical, echinulate teleutospores. The latter are formed exactly in the manner the chlamydospores are formed and hence are also called “chlamydospores”.
The teleutospores measure 5-9 µm in diameter, germinate readily producing pro-mycelium or basidium consisting of four uninucleate cells. The basidium, unlike other smut causing fungi, does not produce basidiospores. Its cells germinate and give rise to uninucleate hyphae, the primary hyphae; a pair of sexually compatible uninucleate primary hyphae fuses and results in dikaryotic hypha often called ‘infection hypha’. It is this infection hypha which causes infection.
Loose Smut Disease Cycle:
(i) Perennation:
The pathogen perennates as dormant mycelium lying inside the kernel (seed), which has been infected by the infection hypha. This sort of mode of perennation is characteristic to this smut disease of wheat as the pathogens causing other smut diseases of wheat plants perennate as spores carried on the surface of the seed.
(ii) Primary Infection:
The mycelium lies dormant within the seed until next season when they are sown. On seed germination and growth of the seedling, the mycelium becomes active, starts developing, and moves systemically along the growing seedling. The mycelium is hyaline during its growth through the plant, but it turns to brown at maturity. When the head emerges, the hyphae accumulate in the floral parts.
Their cells are transformed into teleutospores, which completely fill in the spikelets. They first remain covered by a delicate silvery membrane, which soon bursts spreading almost all teleutospores to atmosphere. They are now wind-blown leaving a bare rachis behind. This is usually the time when the healthy heads of adjacent plants are in flowering stage.
The disseminated teleutospores lodge between the glumes and reach the feathery stigma, germinate thereupon in moist stigmatic fluid giving out “pro-mycelium” or “basidium” consisting of four cells. The basidium produces no basidiospores, but its cells germinate and produce short, uninucleate, sexually compatible primary hyphae that fuse in pairs and give rise to dikaryotic mycelium, the ‘infection hyphae’.
The infection hypha penetrates the flower through the stigma or the young ovary walls and become established in the pericarp, integuments, and in the tissues of the embryo before the kernels become mature. The mycelium then becomes inactive and remains dormant, primarily in the scutellum of the kernel. Fungal presence in-no-way hampers the grain formation. When such infected kernels (grains or seeds) are sown in the next growing season the hyphae become active and show further course of their action.
(iii) Secondary Infection:
Although the teleutospores fall on flowers of healthy plants and germinate thereupon producing/infection hyphae, they do not result in disease-development and symptom-manifestation during the same growing season as the mycelium so developed becomes inactive and remains dormant inside the seeds. This brings complete absence of any secondary infection cycle in this disease during the same growing season.
Predisposing Factors of Loose Smut Disease:
Moisture and temperature are the most important predisposing factors for disease development. 60-85% relative humidity and temperature between 18-20°C at the time of flowering help heavy infection of healthy flowers because the teleutospores germinate best if such meteorological conditions are prevailing in the field.
Moreover, the temperature during the seedling growth to spike emergence stage determines the severity of the disease. If plants grow at 29°C there is no disease even if all the seeds sown contained dormant mycelia. Moderate disease-incidence has been recorded at 24°C, and if the temperature remains at 19°C during the seedling growth and spike emergence stage the disease incidence is very heavy.
Management of Loose Smut Disease:
(i) Since the disease is internally seed-borne, eradication of the dormant mycelium from the seed employing physical (hot water) and chemical (systemic fungicides) treatments is quite useful. In hot water treatment the seeds are soaked in water for 4-6 hours at 68-86°F are then placed in water for 2 minutes at 120°F, and finally taken out, dried and sown.
A modification to hot water treatment, solar energy treatment is also quite useful. T1 seeds are soaked in water for four hours (8 am- 12 noon), then taken out and dried in sun for four hours (12 noon-4 pm), and then sown. Use of systemic fungicides to control, loose smut of wheat proves excellent. Vitavax (DCMO) and Benlate (Benomyl) are used for seed dressing at the rate of 2.5g/kg seeds and this renders almost complete control of the disease.
(ii) Use of resistant varieties has also been recommended. Varieties like NP710, 718, 761, 770 Bansipali 808, Bansi 224, and P 9D have been recommended for U.P.
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