Loose smut of Wheat | Disease Cycle with Diagram, Symptoms & Control
Loose smut of Wheat:
A. OCCURRENCE AND IMPORTANCE
Loose smut of wheat is fairly common in most of the wheat-growing areas of the world, but the incidence of the disease is more abundant and serious in humid and sub-humid regions.
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Loose smut causes damage by destroying the kernels of the infected plants, also by lowering the quality of the seed of the non-infected plants upon harvest.
B. SYMPTOMS
Loose smut generally does not produce any symptoms until the plants are in ear i.e. diseased plants look normal till the ears appear.
Diseased plants usually ear earlier than healthy ones, and smutted ears are rapidly elevated above those of the healthy plants.
In infected ears, each spikelet is entirely transformed into a smut mass consisting of black or olive-green powdery spores. The spores in young spikelets are covered by a delicate silvery membrane which soon bursts and sets the spores free.
when the spores are blown off by the wind, the central axis i.e., rachis of the spikelet is left behind as a naked stalk.
C. THE CAUSAL ORGANISM (The pathogen)
Loose smut of wheat is caused by Ustilago tritici (Pers.) Rostr. [Syn. U. nuda (Jens.) Rostr.]. This pathogen is a basidiomycete.
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E. DISEASE CYCLE WITH DIAGRAM
The pathogen overwinters as dormant mycelium in the cotyledon i.e. scutellum. Such internally seed-borne pathogen becomes active during the germination of the seed.
The mycelium resumes its activity and grows intercellularly through the tissues of the embryos and the young seedling until it reaches the growing point of the plant. The mycelium then follows the growth of the plant and grows best just behind the growing point.
The dikaryotic mycelium, at the time of ear formation is soon transformed into teliospores which after dissemination by wind infects the embryo of the new host plants.
Teliospores fall and terminate on the stigma of the wheat flowers and form infection threads on a septate promycelium (basidium). Infection thread produces monokaryotic mycelium.
After fusion of the sexually compatible monokaryotic mycelium, the resulting dikaryotic mycelium penetrates the flowers through stigma or through the young ovary walls and becomes established in the tissues of the embryo before the seeds become mature.
The mycelium then becomes inactive and remains dormant in the seed until the next growing season.
F. DISEASE CYCLE IN WORD DIAGRAM
G. CONTROL
(1) The best method of controlling loose smut is through the use of certified smut-free seed. When seed is known to be infected with loose smut mycelium, the best way of disinfecting it is by treating seed with hot water.
In this practice, seeds are soaked in water at a temperature of 25°C to 30°C for 4-5 hours.
Next seeds are quickly transferred to warmer water at a temperature of 55.5°C for 10 minutes (which immediately kill the infection threads produced by the pathogen), and immediately afterward in cold water for the seeds to cool off. Then seeds are allowed to dry in sun so that it can be sown.
In the Punjab plains, where temperatures are very high, solar energy treatment of infected seeds has been recommended.
In this practice, seeds are placed in shallow flat bottom tubs filled with water. The tubs containing seeds and water are left under the sun for about 5 hours.
Then water is thrown off and the soaked seeds are spread over in the sun to dry and at the same time to kill the germinating dormant mycelium.
(2) Use of resistant varieties – the varieties C13, PbC 591, NP4, NP 12, NP 710, NP 718, NP 761, NP 770 are susceptible to disease.
