The avocados on the right are Hass, America's favorite variety of the green fruit. At left are GEM avocados, the great-granddaughter of the Hass. GEM avocados grow well in California's Central Valley and, in taste tests, they scored better than the Hass in terms of eating quality. President Trump's tough talk on renegotiating the North American Free Trade Agreement with Mexico and Canada has raised many questions — including how such a move might negatively affect the flow of beloved produce to the U.S. Like the avocado. Americans ate 2 billion pounds of avocados last year, two-thirds of which were...