Can anyone please tell me what is this disease...and how to cure this disease

Hi Gunasekaran K . Compare symptoms with Fusarium Wilt by clicking on green link for checking symptoms and taking control measures from Plantix library. Also I think, this lodging of onion crop happened due to heavy rains. Lets check whether forecast available here on home page of Plantix. Open trenches immediately in and around the field to drain out excessive water from the field. Use fertilizer calculater, crop Advisory, see all tasks and weather report sections available here on home page of Plantix to increase your crop productivity.