In this article we will discuss about:- 1. Occurrence of Sclerospora 2. Symptoms of the Disease Caused by Sclerospora 3. Thallus 4. Control of the Disease.
Occurrence of Sclerospora:
The genus is represented by 15 species of which 13 are known to occur in India. All the species of Scierospora are obligate parasitise causing diseases in members of Family Graminae.
The serious and common species Scierospora graminicola parasitises Bajra (Pennisetum typhoides) causing the disease known as Green ear disease or Downy mildew disease of Bajra. S. graminicola is also parasitic on Setaria italica—the foxtail grass. Other common Indian species of Scierospora are S.sorghi on Sorghum vulgare and S. maydis on maize.
Symptoms of the Disease Caused by Sclerospora:
Since Butler (1907) studied and reported the Green ear disease caused by S. graminicola, the disease has been reported from almost all the Bajra growing areas of the country. Although Butler (1907) considered the disease to be of sporadic in nature, the loss to Bajra crop may be upto 27 or 30 per cent.
The symptoms of the disease appears in two stages viz.:
(a) Downy mildew stage in which the lower surface of the leaf gets covered with downy growth of the fungus.
(b) Green ear stage in which ears or cobs get affected.
According to Singh (1990), Indian strains of Sclerospora produce more of oospores and less of sporangiophores. As a result of this, the downy mildew stage on the leaves is not so conspicuous as the green ear stage.
The affected plants show stunted and dwarfed growth and become pale yellow in colour giving a sick appearance. Sooner or later the leaves start showing chlorosis in streaks on the upper surface. Just below the streaks on the lower surface fine downy growth of the fungus may be observed.
The chlorotic areas turn brown and in advanced stages shredding of the leaves along the streaks/veins may take place. Usually the lateral nodal buds are stimulated to grow and develop into lateral shoots or branching. This abnormality is attributed to the disease because otherwise the plant is unbranched. (Fig. 6.45).
The main symptoms, however are produced in the inflorescence or ears. Abrin formalities set in at the heading stage and the floral organs of the inflorescence get deformed and transformed into twisted green leafy structures. This gives the ear an appearance of green leafy mass and it is because of this symptom that the name of the disease ‘Green ear’ has been adopted.
Transformed ears may be of three types:
(a) The entire ear is transformed into a leafy mass and the cob length is normal;
(b) The lower part of the ear is transformed into a leafy mass and the upper part bears grains, the cob length is normal
(c) The development of cob is totally suppressed and in its place a bunch of leafy structures develop. (Fig. 6.45)
All the floral parts become hypertrophied and malformed. The bristles of the spikelets become hypertrophied and contorted. The stamens and pistil become sterile and leaf like or may be suppressed altogether. In severely affected plants stamens and pistil rarely develop and are replaced by small, leafy shoots or horn like outgrowths.
In advanced stages, the affected plants bear green leafy ears along with brown twisting and shredding leaves. Side branches on the affected plants may bear leafy masses instead of ears.
Anatomically also the symptoms are apparent. In green ear diseased leaves, the vascular bundles lack the sclerified of epidermal cells and the stomata is missing.
Thallus of Sclerospora:
The thallus in Sclerospora is eucarpic and is made up of well-developed filamentous coenocytic endoparasitic mycelium consisting of branched aseptate hyphae. The hyphae are intercellular and freely branched.
The fungus draws its nourishment from the host cells by means of haustoria, which are digitate button shaped in the stem cells but are simple branched finger shaped occupying a major portion of the cell cavity of the leaf. The fungus is an obligate endoparasite and therefore never kills the host tissue.(fig 6.46)
Control of the Disease Caused by Scelerospora:
The disease can be checked by adoption of the control measures listed below:
(i) Rotation of crops avoiding the bajra and related crops for long seasons may considerably check the spread of the disease.
(ii) Removal of diseased plants from the fields may check the secondary infection.
(iii) Spraying of plants with Dithane M-45 also helps in controlling the disease.
(iv) Seed treatment with Metalaxyl 25 or Metalaxyl-35 kills the pathogen present in the seeds and helps in controlling the disease. Seed treatment followed with one spray of metalaxyl effectively controls the disease.
(v) Studies on seed treatment with certain fungicides carried out under All India Coordinated Millet Improvement programme suggested that 50% control of the disease could be achieved through treatment of seeds. The fungicides tested were a mixture of 0.1 percent Agrosan GN and 0.4 percent Thiram.
(vi) Use of diseases resistant varsities is the only effective measures to control and check the spread of the diseases. The resistant varieties currently used are:HB-5 PHB-10and PHB- 14.