Not criticizing either method, but I'm not sure of any reason a helicopter could fly closer to the crop than airplanes. If spraying corn, planes have often touched the tassels with their tires. Helicopters could do the same with skids.

Unless it's hovering, which I assume a helicopter wouldn't do while spraying, the "downwash" from a helicopter rotor should be about equivalent per weight of the craft to that of an airplane. The helicopter merely has a rotating wing, vs a fixed wing. But, in forward flight it's doing the same job. They both exert the same downward force per weight. Only thing is, most spray planes are 2-3x+ heavier than spray helicopters therefore producing more wing downfoce, aka "lift".

I would think the biggest advantage to helicopters is not for the actual application, it's the fact they can refill off a parked truck vs returning to a landing strip required by airplanes. Therefore easier to ferry to the next spray site for helicopters.