The weak south west monsoon in Tamil Nadu created a drastic reduction in the area of cultivation during the Kharif season. Eighteen districts reported more than 50 per cent deficit of rainfall, including the delta districts of Nagapattinam, Tiruvarur and Thanjavur, an official press release said on Saturday.Chief Minister M Karunanidhi has ordered a 50 per cent subsidy to farmers who used pump sets for irrigation.