Fifty percent more Americans will use a grocery store app to buy groceries this year than did so last year, according to a new report.

About 18 million consumers will use grocery apps in 2018, and by 2019, more than one in five adults who buy things via smartphones will use an app to buy food, according to the latest app usage forecast from New York-based research firm eMarketer.

“Shoppers are becoming more comfortable with ordering online in general, and grocery is a part of that,” says eMarketer senior analyst Patricia Orsini. “A key hurdle, traditionally, for ordering fresh produce and other perishable items online has been delivery time, and the desire to hand-select produce and meat.  Retailers have been able to transcend these barriers with click-and-collect models of delivery—order online, pick up in-store.”

And, Orsini adds, if consumers are ordering from their regular grocery store, familiarity helps them trust that the products will be of the quality they expect. “A bad experience, however, could turn consumers off for good,” she warns, “so retailers need to ensure they provide a good experience from day one.”

Robust growth for grocery apps, Orsini says, is being fueled by the Amazon/Whole Foods merger and by Walmart, which is expanding its grocery delivery from six cities to 100 by the end of the year.

“When Amazon acquired Whole Foods last year, Kroger and Walmart, along with other regional chains, stepped up their online grocery efforts,” Orsini says. “Kroger is investing in a number of initiatives around delivery, including partnering with British company Ocado to build high-tech warehouses where grocery orders will be selected and packed by machines. As retailers figure out how to be more efficient with fulfillment, costs for consumers will come down and another barrier to entry will fall.”