Alternaria leaf blight of rapeseed-mustard caused by the fungus Alternaria brassicae (Berk.) Sacc. is an economically important disease and occurs regularly during the rabi (dry) crop season in West Bengal with an average percent loss in yield of about 47%. Six new fungicides viz. Azoxystrobin (Amistar), Trifloxystrobin + Tebuconazole ready-mix compound (Nativo), Difenoconazole (Score), Penconazole (Topas), Mancozeb (Dithane M-45) and Azadirachtin (Neemarin) were tested against the disease. Both in vitro and in vivo studies reveal the expected similitude between them, both of which show almost the same order of efficacy of the six different fungicides used. All the fungicides are significantly effective in reducing disease severity and increasing seed yield. However, the lowest disease severity (%) and highest seed yield and significant increase in test weight were recorded from two sprays of Nativo (1 g 1-1).