MSc

Sympotmatology and etiology of sheath rot disease of rice was investigated and attempts were made in the biological control of the disease. The sheath rot disease of rice initiated as brown oblong lesions on the flag leaf sheath were surrounded by light yellow halo initially. On maturity, these lesions turned dark brown with grey centres. Slight brown discolouration was also seen on the second and third leaf sheaths. Panicles were found choked inside the sheath or partially emerged with discoloured grains.

The pathogen was isolated from diseased rice varieties collected from seven different locations and from ten different varieties. Out of these six isolates of Sarocladium oryzae and four isolates of associated fungi were selected based on the symptomatolgical variations for further study. Slide cultures of the above isolates were prepared and the morphological studies on the nature of mycelium, conidiophore formation, conidial ontogeny and their measurements were recorded. All the isolates were similar in many characters except slight variation in conidiophore length and the length of conidia.

Pathogenicity tests done on eight varieties showed that S. oryzae and the associated fungi inoculated could produce disease symptoms. The nature of symptoms varied with virulence of the pathogen and rice varieties. Combined inoculation of S. orysae and the associated fungi in variety Jaya gave typical symptoms, than their individual action on symptom development. 15 showed the maximum virulence in all the varieties.

Inoculation of the pathogen showed that all the varieties were susceptible to the different isolates of the pathogen tested. Variety Jaya showed the maximum disease intensity.

Studies on the viability of the pathogen in infected grains showed that the pathogen could remain viable for six months in paddy grains. But the frequency of occurrence was found to be reduced from the 3rd month onwards of the storage. Pathogen was viable in the soil up to three months steadily and after that their frequency of occurrence was reduced both at 2cm and 6cm depths.

Cultural characters of the fungus was studied by growing the fungus both in liquid and solid media. Studies on the growth of the pathogen in different solid media showed that different isolates respond varyingly under different media conditions. Czapek’s agar, Oat meal agar and carrot agar were found to be the best media which supported maximum radial growth of the fungus.

Among the different liquid media tested Czapek’s medium was found to be the best followed by Richard’s medium and Coon’s medium. Mannitol was found to be the best carbon source followed by starch and sucrose. Ammonium nitrate and sodium nitrate ranked first in the nitrogen sources.

Effect of different spectra of light was studied on growth and sporulation of S. oryzae. All the light spectra viz., red, blue and green showed a reduction in the growth. Blue light induced maximum sporulation while red light showed least effect.

Studies on the inhibition of spore germination using plant extracts showed that all the plant extracts tested viz., Neem, Carlic, Phyllanthus and Ocimum had inhibitory effect on the spore germination of the pathogen.

Studies on Biological control showed that fungi namely, Chaetomium sp., Pestalotia sp. etc. and bacterial species namely Pseudomonas fluorescens strain 2 and 87 were antagonistic against Sarocladium oryzae under in vitro conditions. Under in vivo conditions Chaetomium sp. was found to minimise the disease maximum among the fungal isolates followed by the bacterial strains P. fluorescens strain 2 and 87.