Herbert Diess is stepping down as chairman of the board of management of the Volkswagen Group, to be succeeded by Porsche boss Oliver Blume, the company has said. Diess resigns effective September 1, the statement said in an unexpected announcement on Friday evening following an agreement with the supervisory board. The decision was made “by mutual agreement,” according to the statement. Blume will continue in his role leading Porsche in addition to his new post, the group said in a statement to the financial markets. The supervisory board also decided that VW finance chief Arno Antlitz will s…