Peggy Seeger's version of "The Grape Pickers Tragedy" is quite different from the lyrics Jack Warshaw posted above. Her recording is on From Where I Stand: Topical Songs from America and England. I think her version tightens the song up a bit, so it makes better sense. Still, it's quite possible for pickers to be working in the fields when cropdusters are active - more likely when the pickers were dressing vines, not picking. Fresno is the center of California agriculture, and I worked in the Fresno area back in the late 70's, and I'd occasionally see people working in the fields when there were cropdusters above - they may have been in the next field over, but that's still too close. You still occasionally hear of farmworkers getting sprayed by cropdusters.

Here are the lyrics from the liner notes:

THE GRAPE PICKER'S TRAGEDY
(Jack Warshaw)

The night-time is hot and the city is sleeping,
Ramon Sanchez takes Maria, his wife, by the hand;
The American border's a mile down the highway,
The border you've got to cross over to work on the land.
     Cross over to work on the land.

Ramon and Maria just follow the foot-steps
To a place called "The Hole" a little ways over the line;
Ten-thousand are begging for work in the vineyards,
Where fruitflies and hoppers are thicker than grapes on the vine.
     Thicker than grapes on the vine.

At four the bus starts on another day's journey.
At six see the sun in the sky and the hills glowing red.
At eight they arrive, and by nine they are weary.
At eleven the drone of the duster is heard overhead.
     The duster is heard overhead.

The dusterplane's loaded with tanks of spray poison
To kill off the fruitflies and hoppers that damage the crops,
But it's killed off the insects that feed on the fruitflies,
They're breeding unchecked and so fast that the dusting can't stop.
     So fast that the dusting can't stop.

The dusterplane circled low over the vineyard;
Ramon knew the sound that means 'run for the trucks or the shed';
The poison rained down where Maria was working;
Maria worked on, and a few hours later was dead.
     And a few hours later was dead.

"Ramon," said the grower, "We're all mighty sorry.
She must have been sick long before she come up from the south.
That spray don't hurt people, you know we don't want trouble,
So here's your day's pay, go away, and don't open your mouth.
     Go away, and don't open your mouth."

He turned on his heel and walked out of the office.
Maria was taken to town and they gave him her things.
There's nowhere to go but back over the border.
Adios, mi Maria, it's their blood the next harvest brings.
     It's their blood the next harvest brings.