Influence of rainfall, temperature and humidity on appearance and development of fungal diseases in Brassica juncea

Alternaria blight (Alternaria brassicae) and white rust (Albugo Candida) are the most threatening fungal diseases of Brassica juncea causing losses up to 50% of seed yield. Field studies were conducted on varieties Laxmi and Varuna at two locations (Sriganganagar and Delhi) during 2006 and 2007 to determine the influence of weather parameters on the appearance and development of Alternaria blight (AB) and white rust (WR) under natural epiphytotic conditions. Significant difference in appearance and progress of diseases was not observed between the varieties, but at two locations, significant differences were observed due to variation in weather parameters. At Sriganganagar, AB and WR first appeared in the 48^th and 49^th standard meteorological week (SMW), respectively on leaves, whereas, at Delhi both appeared in 52^nd SMW of 2006. It was found that AB appearance was sensitive to rainfall and evening relative humidity, whereas, that of WR was sensitive to maximum temperature. Rapid development of AB and WR on leaves occurred at both the locations during 6^th to 8^th SMW of 2007, when rainfall and evening relative humidity increased and weekly mean temperature (maximum and minimum) decreased.