Cecilia Davenport’s article today  mentions that the Maine potato harvest is harvested largely by white Americans.  They’ve also been known to use white migrant labor from north of the border, from French Canada. (Maine itself is formerly  French territory, which is why it’s called that,  and why it has cities with names like Presque Isle.) 
A while back, I blogged  about John Steinbeck’s  reported encounter with some French-Canadian potato pickers. I should say that since I blogged about it, it’s come to light that Steinbeck’s non-fiction book Travels with Charley | In Search Of America about his year wandering around the United States, was heavily fictionalized. A writer backtracked his journey, and it’s now less amazing that everyone in the book talks like a character in a John Steinbeck novel. (See my post John Steinbeck's "Travels With Charley", Fact-Checked  for details.) With that caveat, here’s what Steinbeck reported about the 1959 Maine potato harvest.
One important thing about these legal French Canadian farmworkers—they really were  temporary immigrants, since they had their own farms, and a First World country, to go back to.
"In due course these people told me quite a bit about themselves. They came over the border every year for the potato harvest. With everyone working, it made a nice little pool against the winter. Did they have any trouble with immigration people at the border? Well, no. The rules seemed to relax during the harvest season, and besides, the way was smoothed by a contractor to whom they paid a small percentage of their pay.
But they didn't really pay him. He collected directly from the farmers. I've known quite a few migrant people over the years-Okies and Mexican wetbacks , and the Negroes who move into New Jersey and Long Island. And wherever I've seen them there has always been a contractor in the background to smooth the way for them for a consideration. Years ago the farmers tried to draw more labor than they needed so that they could lower wages. This seems to be no longer true, for government agencies channel only as many laborers as are needed, and some kind of minimum wage is maintained. [Emphasis added—this was in the Eisenhower administration.] In other cases the migrants have been driven to movement and seasonal work by poverty and terrible need.
Surely my guests for the evening were neither mistreated nor driven. This clan, having put their own small farm to bed for the winter in the Province of Quebec, came over the line to make a small nest egg. They even carried a little feeling of holiday with them almost like the hops- and strawberry-pickers from London and the Midland cities of England. These were a hardy and self-sufficient people, quite capable of taking care of themselves."
By the way, Steinbeck first made his reputation with the 1938 The Grapes of Wrath , about white American migrant labor—see Steinbeck's myth of the Okies,  by Keith Windschuttle, New Criterion, June 2002.