In a twice-yearly report, the German institutes forecast the German economy -- Europe's largest -- will contract by 6 percent this year and expect a "painful" time ahead.  "Given the depth of the economic slump and low inflation in the euro area, the European Central Bank should lower its main interest rate to 0.5 percent," said Kai Carstensen of the Ifo economic institute, presenting the report at a news conference.  Germany is Europe's biggest economy and the world's biggest exporter.  The institutes forecast consumer prices in the 16-nation euro area would decline