Development of weather based forewarning models for Downy mildew and Alternaria blight diseases of Rapeseed (Brassica campestris) in north bank plain zone of Assam

Downy mildew and Alternaria blight are two economically important fungal diseases of Rapeseed and Mustard affecting the crop to a great extent. Data were generated from the field experiment conducted on sandy loam soil of the farm of B. N. College of Agriculture, Assam Agricultural University, Biswanath Chariali, Assam (26 degrees 42' N and 93 degrees 15' E) with two rapeseed cultivars TS-36 and M-27, which were grown in two consecutive rabi (post rainy) seasons (2009-10 and 2010-11). The cultivars were grown on six different dates starting from 15 October at seven-day intervals to enable the crop to get exposed to varying weather conditions. Incidence. of downy mildew and Alternaria blight were recorded on 10 pre-labeled plants from each plot at 4-day intervals and percent disease index (PDI) were calculated based on the field observations. Correlation studies were done betwegn PDI of diseases and average weather conditions (maximum and minimum temperature, morning and afternoon relative humidity, bright sunshine hours and rainfall) and accumulated growing-degree-days (ODD) in preceding five days of the date of record of diseases for both cultivars and seasons. Development of diseases expressed in terms of PDI in both the cultivars was significantly and negatively correlated with average daily minimum temperatures (r = 0.54 to 0.90) and accumulated GDD (r = 0.51 to 0.74) in preceding five days of the date of record of diseases. Hence, these two parameters were used to develop model equations to predict PDI of the diseases on real time basis which could explain up to 51 to 60 and 53 to 58 per cent of the variability in development of Alternaria blight and downy mildew, respectively.