Rhizoctonia solani is the causative agent of rice sheath blight, which has become a major problem in rice production. Seaweed provides a rich source of structurally diverse and biologically active secondary metabolite and is proved to be better in decreasing foliar fungal diseases which ultimately increase its fertility and help in the growth of plants. The use of natural products is the ultimate way of combating this disease. In this context, five different seaweeds such as Sargassum wightii, Dictyota bartyrensiana, Ulva reticulate, Gelidiella acerosa and Odonus niger were used together with the fish powder extract in the control of sheath blight disease in rice were studied. Evaluation of marine products against R. solani was carried out by paper disc assay; agar well method and mycelial dry weight. Among the five marine extracts tested, extracts of S. wightii [brown seaweed algae] at a high concentration (20%) was found to be the best in the reduction of spore germination (19.60%). The leaf extracts of S. wightii [brown seaweed algae] at highest concentration of 20% showed a maximum reduction in both paper disc method and agar well method with 44.65 and 45.90% zone of inhibition, respectively. The antifungal compounds were identified through gas chromatography mass spectroscopy. The results revealed that, 18 compounds were present in S. wightii and among that n-hexadecanoic acid which was closely related to 9, 12-octadecadienoic acid may be responsible for the inhibition of the growth of R. solani. The present study revealed that the efficacy of seaweed extracts against fungal pathogens may be due to higher levels and early accumulation of phenolics and phytoalexins, and the pot study proved that R. solani can be controlled by the application of marine products which may be further used for field study.
Key words: Seaweeds, Rhizoctonia solani, antifungal compounds, rice.
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