Mayor Bloomberg is marshaling forces for his next public-health crusade: less salt. Late last month, he quietly gathered health experts and food-industry reps at Gracie Mansion to lay out his plan to cut sodium levels in processed foods by 20 percent over the next five years. At the meeting, city health czar Thomas Frieden called high blood pressure, which is linked to excessive sodium intake, “the greatest public-health threat facing the city” and pressured the industry groups to sign on by the end of November, according to a memo written by one attendee, Scott Vinson of the National Council of Chain Restaurants, and obtained by New York. There won’t be any new regulation, the memo says, but restaurateurs will be encouraged to join a “voluntary” initiative. Frieden is also considering “whether it would be better to publicize lower-sodium efforts via a campaign, or whether that approach could turn off consumers who might be wary of the taste impact,” Vinson wrote. Officially, the city still isn’t talking. “We don’t have an initiative that’s well enough developed to talk about yet,” says a spokesman.