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In this article we will discuss about:- 1. Introduction to Sphacelotheca 2. Symptoms of the Smut Caused by Sphacelotheca 3. Mycelium 4. Reproduction.
Introduction to Sphacelotheca:
Sphacelotheca causes smut of Sorghum (Jowar). In India, three species of the genus attack Jowar plants causing serious diseases.
The grain smut or covered smut of Jowar is caused by S. sorghi, the loose smut of Jowar is caused by S. cruenta and S. reiliana causes the head Smut of Jowar.
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These diseases are very common in the states of Andhra Pradesh Madhya Pradesh, Karnataka, Tamilnadu and Uttar Pradesh. Of the three smut diseases of Jowar, covered smut or gram smut causes serious losses to the crop and is the most destructive.
Symptoms of the Smut Caused by Sphacelotheca:
The fungus attacks the ovaries and all the ovaries in the head are turned into sori of the smut. The normal grain is thus replaced by an oval or cylindrical dirty grey sac with a slightly conical tip.
In most cases stamens are absent but the stigma is not transformed. However other floral parts of the host are not affected.
Mycelium of Sphacelotheca:
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The mycelium is mostly intercellular without haustoria, sparsely branched septate and hyaline. The primary mycelium formed by the germination of basidiospores enters the host through seedlings.
When the primary mycelium of the opposite strain also happens to enter the same seedling, dikaryotic mycelium is formed through anastomosis and fusion. This dikaryotic mycelium grows with the seedling and produces the smut sori in the ovaries.
Reproduction of Sphacelotheca:
During threshing, the teleutospores are released from the infected ovaries and get lodged on the surface of seeds. These remain viable for a maximum of period of five years.
Generally in the next season, in presence of water the teleutospores germinate to produce a three
celled promycelium which through budding produces terminal as well as lateral sporidia.
The promycelium may also act as infection hyphae which may enter the seedlings of the host.
Generally sporidia fuse in pairs and form dikaryotic mycelium. The dikaryotic mycelium enters the seedlings, following mainly the apical meristem of the seedlings.
This mycelium ultimately reaches the ovaries of the flowers which ultimately gets filled with dikaryotic mycelium. Individual cells of the hyphae round off, nuclei of the cell fuse and the cells develop a thick wall around them.
These thick walled, uninucleate diploid cells are teleutospores. A large number of these spores are formed which fill the grain which is transformed into a sporesac or sorus.
Each sorus is dark grey in colour, enclosed by a wall made up of fungal tissue, oval or cylindrical and conical at the tip and measure 125 mm in length and 40 mm in width.
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At the base the sorus is surrounded by unaltered glumes. At the centre of the sorus is present Columella which is a slender column of hard tissue made up of fungal mycelium and host tissue.
The sorus is filled by teleutospores but hyaline empty cells are also present in the sorus. Individiual teleutospores are smooth, globose to oval and brownish olive in colour which appears dark brown in mass.
After release from the sorus, the teleutospores come to lie on the surface of seeds and empty sorus with protruding columella is left.
In the next season, the teleutospores germinate in presence of water. The outer exospore ruptures and the inner thin endospore comes out in the form of a germ tube.
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The diploid nucleus undergoes meiosis to produce four nuclei. The germ tube becomes a three celled promycelium which buds off haploid sporidia.
