That's hype. So what if the efficiency stinks. If you inhale a small particle of infected slime, how much more agent will be in a particle a thousand times larger? These particles will infect the same as the little ones. The little ones may end up deeper in the lungs for a quicker kill, but...
Adjust the orifice size and delivery pressure...
The droplets would fall to the ground right away. They dont jump up into your nose after they have hit the ground. If a cropduster flew over then you would have to be standing in the spray and subsequently smell it to try and figure out what it was before you would get enough to cause infection.
Could it be something as simple as this old guy just likes to "wrestle" with sheep?
You won't get any cases if the droplets just fall to the ground like rain. They have to float around for a while so people can breathe them in. That's why they have to be small, and the cropduster isn't designed for it.
A few modifications, though, and all bets are off.