The pilot was spraying a crop with liquid fungicide. As he pulled up to clear trees during the third swath run, the fungicide evidently surged in the hopper, and a quantity escaped past the hopper-lid seal. It then splashed over the windscreen, severely restricting the pilot's forward vision. He elected to return to the strip to clean the screen and check the hopper seal. The strip length was about 1000 metres and the pilot did not consider it necessary to dump the remainder of the load. During the landing, which was made in light downwind conditions, the pilot experienced difficulty in seeing the strip. During the latter stages of the ground roll the aircraft began to veer to the right. Corrective action failed to redress the situation and the right wing struck the fence bordering the strip. The aircraft came to rest after sliding sideways into a ditch alongside the fence, some 315 metres from the point of touchdown. Investigation confirmed that the hopper-lid seal was defective, and had apparently been so for some time. Directional control had been lost during the landing roll shortly after the free-castoring tailwheel had settled onto the ground. The combination of restricted forward visibility, a downwind/crosswind component and the relatively heavy all up weight, had prevented the pilot from making a safe landing.