Temporal and spatial progression of sheath blight in mega rice varieties of South Asia

The temporal and spatial progress of sheath blight was assessed on a set of mega rice varieties of South Asia and local varieties of North India in field experiments for two consecutive years. With artificially created disease focus disease severity was recorded on five un-inoculated plants from the focus in four directions after two, four and six weeks of inoculation. The mega varieties differed in their level of susceptibility to sheath blight. The temporal progression of the disease was significantly increased in Swarna and Swarna Sub-1 followed by PR 114 and PR 121 varieties. Area under disease progress curve (AUDPC) ranged 184.92 – 683.67 in tested varieties. The disease severity decreased as the distance from the disease focus increased in all varieties, the significance of the spatial spread depended on the susceptibility level of the variety. Swarna and Swarna Sub-1 showed higher level of susceptibility. The exponential regression model explained the increased severity with time which get decreased with increase in distance from the disease focus. Relationship of week after inoculation with disease severity indicated that model explained 96.0% variability of the response data in Swarna. The morphological traits like plant height, number of tillers per hill and tiller angle of the varieties had a significant correlation with the disease severity and incidence. In the absence of genetic resistance, this study provides evidence of requirement of a six-week period for differentiating the susceptible or resistant response of rice varieties to sheath blight under field conditions. Plant morphological traits are the key factor which can be consider in disease resistant breeding programme.