Business confidence in South Africa slumped to its lowest in 13 years in April, a survey showed on Wednesday, reflecting concerns about stubbornly sluggish economic growth.The Business Confidence Index (BCI) fell to 90.4 in March from 93 in February, the weakest it has been since April 2000, the South African Chamber of Commerce and Industry (SACCI) said.The main sub-indices used in the survey, including inflation, the rand exchange rate, borrowing costs, share prices and precious metal prices all turned negative compared with the previous month.