The pilot commenced spraying operations before the ground markers had taken up their positions. As he approached the starting point for the first run he glanced towards the markers to check their progress towards their assigned places. The pilot then looked forward and suddenly realised he had temporarily forgotten the presence of a power line which crossed the paddock. Evasive action was taken, however the top of the rudder struck the wire and was severed. Control of the aircraft was maintained and a safe landing was subsequently made.