Dave Lewis steps down on Wednesday after six years as Tesco chief executive, during which he got Britain’s biggest retailer back on track after an accounting crisis, leaving new challenges for his successor Ken Murphy. Murphy, 53, who was formerly at healthcare group Walgreens Boots Alliance , faces the long-term impact of the coronavirus crisis, a recession and possible disruption when Britain’s Brexit transition period finishes at the end of 2020. Tesco was on its knees shortly after former Unilever executive Lewis, 55, joined in 2014 when an accounting scandal knocked millions off its profits and billions off its share price.