Azolla: A new host of Sclerotium oryzae, the incitant of rice stem rot disease in India

摘要

Azolla (Azolla pinnatd) has been found in fresh water ponds, cannals, rice fields etc. in Manipur. The application of azolla to rice field is considered as an alternative to nitrogenous chemical fertilizers and becoming popular among the organic farmers. However, azolla suffers seriously from a blight disease in the valley rice fields since 2012 Kharif. The symptoms were characterized by the appearance of brownish colour blight which led to complete rotting of azolla (Plate 1) and submerged into water. The azolla plants adjoining to the diseased once, were also infected. When isolated, the diseased tissues of azolla on the Potato Dextrose Agar (PDA) medium aseptically, a sclerotium forming fungus was found consistently. Pathogenicity was performed by placing two-5 days old sclerotia of the fungus on healthy azollaand on excised healthy rice stem under laboratory conditions. The fungus thus produced a blight disease of azolla and stem rot of rice, respectively within seven days after inoculation and was identified as Sclerotium oryzae (Plate 2). Azolla which is being infected by Sclerotium oryzae constitutes its first report in India. Sahajahan et al. (1980) reported that Rhizoctinia sp. and Sclerotium sp. also damage the azolla plants in Bangladesh. Rhizoctonia solani Kuhn, the causal agent of rice sheath blight was reported to infect azolla (Singh and Singh, 1985) in India.