Alternaria leaf spot and flower blight of marigold caused by Alternaria tagetica Shome and Mustafee is appearing in mild to severe form in Haryana. Field experiments were conducted to see the effect of weather parameters on development of Alternaria blight of marigold transplanted on different date. Alternaria leaf spot intensity decreased from 77.6 to 71.7 and 86.0 to78.1 during 2013 and 2014, respectively with delay in transplanting. Similarly, flower blight intensity also reduced from 85.4 to 80.4 and 88.7to 82.3% during both the years with delay in transplanting. Disease progression revealed that Alternaria leaf spot and flower blight intensity had significant negative correlation with weekly maximum and minimum temperatures and sunshine hrs whereas; morning and evening relative humidity had significant positive correlation.Maximum and minimum temperature 27.5±1.0°C and 7.5±1.0°C respectively, were found to be conducive for the development of Alternaria blight of marigold. Regression analysis indicated that the 83 per cent variation in Alternaria leaf spot and 87 per cent in flower blight intensity could be explained by weather parameter which was statistically highly significant.