South Africa’s largest labour union group, with more than a million and a half members, said on Wednesday it would stage a nationwide strike on Feb. 13 over large-scale layoffs at ailing state-owned firms and at private companies. The decision by the Congress of South African Trade Unions (Cosatu) poses a major threat to President Cyril Ramaphosa’s plan to revive the economy by cutting the government’s wage bill and reforming state firms that are drowning in debt and plagued by corruption.