Commemorating the accidental abundance of zucchinis in 1993 that spawned the creation of WQED’s hit series QED Cooks with Chris Fennimore, the Pittsburgh pubcaster will honor the prolific late-summer veggie for the program’s 20th anniversary. Back then, Fennimore, WQED-TV p.d. and an enthusiastic cook, was helping tend a community garden plot that exploded with the green squash. “So I asked Nancy Polinsky, director of continuity at the time, if she’d make a promo asking people for zucchini recipes,” Fennimore said. He had no idea that first recipe for zucchini cheesecake would be the beginning of more than 50 live cooking marathons ranging from “A is for Appetizers” to “S is for Seafood,” a growing line of cookbooks that proved to be popular pledge premiums and a hit show that passed its 100th-episode milestone in March 2012. Polinsky also became his co-host.