Last month was no picnic for Wal-Mart ( WMT).The world's largest retailer estimated Saturday that sales at its stores open at least a year rose just 1.2% in June, coming in at the low end of the company's own expectations for 1% to 3% growth. In a statement posted on its Web site, Wal-Mart offered no explanation for the results. A year earlier, the Bentonville, Ark.-based company reported more robust same-store sales growth of 4.5%. Saturday's announcement may raise fears that other large retailers will have disappointing news Thursday, when most are scheduled to report June sales. Wal-Mart shares ended the week at $48.17.