Philip W. K. Sweet Jr., a banker whose time at the top of Northern Trust was undone by a notorious bank failure that did even more damage to rival Continental Bank, died March 2 at Balmoral Care Center in Lake Forest, according to his son, David. Sweet was 91 and had been treated for an infection. Sweet became CEO in 1981 as the bank accelerated beyond its trust-company roots. The so-called Gray Lady of LaSalle Street opened locations in the Chicago suburbs, Florida and Arizona and chased business overseas. Sweet retired at age 56 in 1984. The bank, controlled by the Smith family since its founding in 1889, for the first time turned to an outsider, former Jewel CEO Wes Christopherson, to take over. Philip Sweet was born in Mount Vernon, N.Y., and joined Northern’s bond department after graduating from Harvard and serving during the Korean War in the U.S. Navy.