Helicopters for dusting worked really well, but, very expensive. The two operators I knew who tried them said as long as they didn't have to fly too far to get to the farms they worked. When they hit the first big maintenance mark they both got rid of them.
The bonus was the downwash. The spray would even get up under the plant leaves. Of course, one of the fixed wing pilots said he could get low enough so the downwash from the wing would do the same thing. Don't know how long he lived since he liked coming back with plants in the landing gear.
Drones are cheap enough that an individual farmer might be able to buy one and use it.
