WEST GROVE, Pa.—Nine-plus hours into his shift, Marcos Pantoja was still plucking white button mushrooms off peat-moss beds, trimming stems with the flick of a knife. He had another hour of work left in the cool, windowless growing house.

The 39-year-old is toiling longer than usual these days amid a labor crunch that is straining the more than 50 growers in Chester County, Pennsylvania’s mushroom heartland, where almost half of America’s mushrooms are produced.

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