Field experiments were carried out with two Brassica varieties Pusa Jaikisan and Bio-169-96 (a genotype at the final stages of release) at the Indian Agricultural Research Institute farm during two rabi seasons of 2004-05 and 2005-06. The two varieties were sown on 15th and 30th October. Observations on incidence and spread of white rust were recorded to develop a suitable forewarning model. Incidentally, the 30th October sown plants were found to be infested more by white rust to the tune of 30 and 35 per cent in Bio-169-96 and Pusa Jaikisan respectively, as compared to about 16 and 17 per cent in the 15th October sown plants. A thumb rule was developed to forewarn the incidence of white rust and model equations were developed using hourly weather data of the past to assess the disease severity quantitatively.