The worst sell-off in emerging-market currencies in five years is starting to show ill effects from the Federal Reserve???s tapering of monetary stimulus, amid political and financial instability. The Turkish lira and South Africa???s rand fell yesterday to a level weaker than 11 per dollar for the first time since 2008. Argentine policy makers devalued the peso by reducing support in the foreign-exchange market to let the currency drop its furthest in 12 years to an unprecedented low. Investors are losing confidence in some of the biggest developing nations, extending the currency-market rout triggered last year when the Fed first signaled it would reduce stimulus. While Brazil, Russia, India, China and South Africa were the engines of global growth following the financial crisis in 2008, emerging markets now pose a threat to world financial stability. ???The current environment is potentially very toxic for emerging markets,??? said Eamon Aghdasi, a strategist at Societe Generale SA in New York.