The pilot reported that while spraying a crop in a scrubby paddock, the engine lost power during a pull-up manoeuvre over a scrub line. There was no suitable landing area ahead, so the pilot turned left through 100 deg in an attempt to reach a suitable clear area. He dumped the load to clear a fence but the angle of approach into the clearing was encroaching on another fence line running in approximately the same direction as the intended landing direction. This required a shallow right turn which was still being completed at touchdown. This resulted in the right-wing spray boom catching in the crop which in turn resulted in a ground loop and left main landing gear collapse.
The maintenance engineer who recovered the aircraft, advised that the engine operated normally when test run. The aircraft is fitted with two fuel tanks, one in each wing. The tanks feed into a common sump and the fuel selector only has two positions - ON or OFF. One tank contained 28 lts of fuel and the other one was empty. The tank vent line for the tank containing fuel was found to be blocked by a wasp's nest. Normally the two tanks would feed evenly to the common sump, but the blocked vent line probably prevented this from occurring and in turn starved the engine of fuel.