Effect of Sowing Times and Rice Varietal Resistance on the Severity of Narrow Brown Leaf Spot Disease in Relation to Environmental Conditions

The objective of this study was to investigate the effects of sowing times and genetic resistance of 10 commercially important rice varieties on the percent disease severity (PDS) of narrow brown leaf spot (NBLS) caused by Cercospora oryzae. Two sowing time treatments (mid-May and mid-June) each year were evaluated for this 2-year study. Rice varieties were drill seeded and arranged in randomized complete block designed plots with four replications for each variety. The effects of environmental factors (high and low temperature, relative humidity and precipitation) on the NBLS severity of the rice varieties at both sowing times were also evaluated. Disease data was analyzed using computer software GenStat 10th edition. Results: By changing cultivation timing of rice varieties, B-Chenab, KSK-434, PK-386, B-Kissan, and KSK-133 from mid-May to mid-June, their resistance levels were also changed from a higher to a lower. Weather conditions during the mid-June cultivation of 2019 induced a higher level of disease severity than did the mid-May weather conditions of 2018. The variation in temperatures affected the percent disease severity more as compared to relative humidity and precipitation. High temperature of 31-33˚C and low temperature of 18-21˚C were found to be most effective in enhancing the NBLS development. Minimum NBLS severity 4.75% and 8.25% was noted on variety B-515 in mid-May and mid-June sowings respectively. Thus, its rating changed from resistant to mildly susceptible by changing sowing timing. Rice variety PK-1121 depicted maximum NBLS severity 60.25 % in mid-May sowings, rated as very highly susceptible, while severity increased to 79.25% in mid-June sowings, and rated as completely susceptible. It is concluded that the severity of NBLS was affected by genetic resistance level of rice varieties, sowing times and weather conditions thus consideration of these factors is the key to the integrated management of the NBLS in rice.