Effect of sowing dates, varieties and weather factors on the occurrence and severity of Alternaria leaf blight and yield of Indian mustard

Field experiments were undertaken to determine the effect of sowing dates and weather parameters on the disease progression or severity of Alternaria blight of rapeseed mustard and their impact on yield for the two consecutive years 2008-2009 and 2009-2010. Disease severity gradually increased with delay in sowing irrespective of three different varieties (Binoy, Seeta and Bhagirathi). The crop sowing from 20th October to 5th November recorded significantly less disease severity with the highest seed yield irrespective of varieties and enhanced in subsequent sowing dates. Bhagirathi showed significantly less disease severity with maximum yield followed by Binoy and Seeta on leaf and siliqua infection on all sowing dates. 90 days old plants showed highest disease severity. From 20th October to 5th November, sowing only minimum temperature (9.69 to 12.04°C) had significant negative correlation, while minimum relative humidity (44.9 to 60.2% RH), wind velocity (during morning) (0.55 km to 1.04 km/h.) and total rainfall (5.74 mm) had positive significant correlation with disease progression upto 59%. Whereas in 20th November sowing minimum temperature (9.69 to 15.74°C, minimum(39 to 88%) and maximum (96 to 99%) relative humidity, wind velocity (during morning)(0.4 to 1.29 km/h), bright sunshine hours (3.8 to 9.0 h) and total rainfall (69.6 mm) had positive significant correlation and vapour pressure (noon, morning)(9.7-29.2 mb and 9.2-14.2 mb respectively), wind velocity (evening) (0.4 to 1.3 km/h) had negative significant correlation on disease progression upto 62%. In 5th December sowing, vapour pressure (noon)(9.7 to 17.5 mb), bright sunshine hours (3.8 to 9.2 h), total rainfall (73.7 mm) and minimum temperature(9.7-18.2°C) had positive significant correlation on disease progression upto 73%. In the West Bengal condition the suitable sowing date of mustard will be last week of October with tolerant variety Bagirathi could be cultivated to avoid loss from Alterania blight and increase the seed yield. Key words: Alternaria blight, mustard, sowing date, variety, weather, yield.