Fog had forced the pilot to abandon an earlier attempt to reach a property to conduct spraying operations. After a delay of about one hour, the pilot departed for a second attempt. He later reported that during descent through a hole in the lifting fog, the right spray boom became detached. He decided to return to the DEPARTURE aerodrome, but as the aircraft was clearing some mist, it collided with the top of a gum tree. The pilot shut down the engine because of severe vibrations and carried out a forced landing on a ridge. It was considered that the pilot had not checked that the quick-disconnect fitting on the spray boom was securely fastened during the pre-flight inspection. He had been distracted by the displaced boom and had not seen the tree until it was too late to take avoiding action.