The agricultural pilot had completed a spray run and pulled up to clear trees at the end of the field. The pilot reported that several trees and windmills were located at the southern end of the field and as the airplane made a climbing left turn, the right wing struck a windmill blade. The collision tore the spray boom from the airplane, which then impacted the airplane’s right side stabilizer and fuselage, causing substantial damage. The pilot was able to perform a successful precautionary landing on a nearby gravel road.