Sheath blight disease of rice caused by Rhizoctonia solani AG-1 IA, has become one of the major diseases in some rice- growing areas in recent years. Primary inoculum density seems to be a major factor in disease outbreak. The aim of the current study was to determine the relationship between the primary inoculum density and type and the disease intensity, grain yield and yield loss. Field experiments were conducted in both years of 2017 and 2018 in Guilan province, Iran. Disease incidence and severity were significantly higher when the highest inoculum densities (mycelial and sclerotial) were tested. When sclerotia were applied as the primary inoculum, disease developed more quickly. Based on the results of the current study, in a temperate lowland rice system in Guilan province, sclerotia floating on the water surface after puddling can be the primary source of inoculum and play a major role in sheath blight epidemics whereas mycelia in plant debris probably lose their viability in winter. These results suggested that control of sheath blight disease in order to prevent sclerotia production and reduce the main disease inoculum can be a promising strategy for suppressing this disease in the rice fields of Guilan province.

Keywords:  rice, sheath blight, primary inoculum, viable sclerotia, Guilan province

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