During the week of June 24, the president tweeted recently, Immigration and Customs Enforcement “will begin the process of removing the millions of illegal aliens who have illicitly found their way into the United States.”

Three days before he made that promise, I drove uphill between the concrete and wood structures of a cluster of Chester County mushroom farms, pungent from composted manure and compressed sawdust, in an intensely farmed southern corner of the county, “America’s Mushroom Capital.” It’s a place where workers are already scarce, foreigners take the hard jobs, and growers wonder why their government seems to be pushing them away from their family businesses of filling the produce aisles.

In one farm office, the manager showed me a new letter from the Department of Homeland Security, which runs ICE. The assistant special agent in charge wrote that ICE agents had stopped by recently to copy employment records. Over the next four weeks, they had checked those documents, and found that some of the workers "appear, at the present time, not to be authorized to work in the United States.”

Why? Some work documents appeared to “pertain to other individuals.” Others didn’t match official records, provide employment authorization, or had expired. For all those reasons and more, they failed to “satisfy the Form I-9 employment eligibility verification requirements of the INA,” the Immigration and Nationality Act of 1952. As amended.

The letter listed the familiar Spanish names: Carlos, Jesus, Maria, Miguel. Gonzalez, Martinez, Zavala. In combinations and hyphenations. With hire dates, Social Security numbers, and the status of those numbers, marked “Invalid.” More than two dozen.

Unless they can come up with new, valid documents, “they are considered by [Homeland investigators] to be unauthorized to work in the United States.” If he keeps them on, the farmer may be subject to “civil penalties ranging from $548 to $4,384 per unauthorized alien for a first violation.” And more if ICE finds this has happened here before.

“This is a very serious matter that requires your immediate attention.”

Within 10 business days, the assistant special agent in charge concluded, the employer must either verify the workers’ status “or take other appropriate actions.” The penalties could include “civil” fines. Or worse: “Criminal charges may be brought against any person or entity that engages in a pattern or practice of knowingly hiring or continuing to employ unauthorized aliens.”

Most of the men and women on the list are already gone, the farm manager tells me. If the last seven from the list can’t find valid papers and also end up leaving the farm, some of their fully documented family members will go with them, reducing his workforce even more.

The owner had invited me so he could vent about the damage. The manager asked me not to name the farm. He says ICE people have been polite and professional. But why risk annoying the assistant special agent in charge?

The owner and the manager don’t want to lose these workers. They don’t know how they are going to replace them.

An experienced picker in a busy week can earn more than $1,000. But Chester County is the richest of Pennsylvania’s 67 counties, and it’s easy enough for people who speak standard English to get a job in an air-conditioned store, or a warehouse, without the stooping, the clinging dirt, the repetitive cutting, the careful rapid handling, the cyclical sweating and chilling, the variable hours, and the predawn start times that get our favorite fungi to ShopRite or Wegmans, fresh, for a few bucks a pound.

Immigration agents have periodically busted Chester County mushroom farms since year-round Mexican workers began replacing U.S.-citizen winter harvesters from Puerto Rico, as the industry went from seasonal canning to fresh production, in the 1970s.

Thousands of mushroom pickers won legal status with the Reagan-era amnesty. But those workers are now in their 50s.

Growers face other perennial problems -- foreign competition, pests, energy. But labor is key. The owner, who voted for the president, says the government “is taking my already-depleted labor force away while my mushrooms rot on the beds.”

The largest mushroom farmers have diversified -- trucking, cold storage, real estate development. With labor so scarce, production will move overseas, or to growers who invest in automation or who can offer better-than-industrial wages for higher-priced specialty fungi.

The manager hasn’t heard of a recent mass enforcement campaign. He thinks “someone called Homeland Security on us.”

Who would do that? A competitor? A neighbor with a beef? The kind of person who can’t stand the idea of someone who wants to work but doesn’t have papers up here earning several hundred dollars a week in a sweaty job that Americans won’t do? He won’t guess.

“These people are good at this,” says the manager. To be sure, some workers with papers, who have married and settled here, aren’t raising their kids to do this work: “The Mexicans are moving up, like the Italians moved up."

But there are sons and cousins in rural Mexico who would like to work the harvests here in the North, and some hungry Central Americans, who have, as the president says, “found their way,” outside the limited guest-worker and citizenship programs, without the right papers.

The owner and the manager say they don’t understand why the workers have to go home, or who benefits from their departure. “We need them to harvest our mushrooms,” the manager says. “There should be a program in place.”