Starting Monday, online retail giant Amazon will cut prices on certain items at Whole Foods stores. The announcement, made Thursday, has competitor grocers worried.

Organic avocados, bananas, and yes, organic kale, are some of the items that will have price cuts. Amazon is also expected to formally close on its $13.7 billion purchase of Whole Foods after receiving Federal Trade Commission approval Wednesday. Shares of both Amazon and Whole Foods rose after the price-cutting announcement.

Prices at competitor grocers including Kroger, Costco and Phoenix-based Sprouts fell after Thursday's announcement, as grocers scramble to compete. Sprouts now delivers some products through Amazon. Wal-Mart, the nation’s largest grocer, uses Uber for deliveries and said it will partner with Google to let shoppers order by voice.