The pilot reported that after take off he made a standard turn to line up with the field to be sprayed and began the descent under two sets of powerlines. He then observed a telephone pole to the right and thought he had plenty of clearance to avoid it, but then he heard and felt the impact of a main rotor blade strike the pole. The helicopter yawed to the right before it impacted the ground, rolled forward, and came to rest on its left side. The pilot reported no mechanical malfunction or failure that would have precluded normal operation.