Pelwatte Sugar is a listed company.
Pelwatte Sugar was trading at 16.50 rupees, up 50 cents.
The tractor was being used to inspect electrified fences of a Pelwatte Sugar Company cane plantation, military spokesman Udaya Nanayakkara said.
The bomb went off around 10.30 am in the Konketiya area of Buttala.
The Buttala area has seen a series of terror attacks on civilians by Tamil Tigers where several farmers had been killed.