Shares in WPP PLC dropped Tuesday as the ad industry’s upheaval took a heavy toll on the world’s largest advertising company, underscoring the challenges facing new chief executive Mark Read.
The advertising giant’s second-quarter results showed a weak performance in North America as the company’s stable of creative agencies—from J. Walter Thompson, Ogilvy & Mather and Young & Rubicam—struggled. Like-for-like net sales—a figure closely watched by analysts to measure the company’s underlying performance—dropped 3.3%...