Japan’s Nikkei stock index suffered its worst plunge since its 1987 Black Monday crash, triggered by fears about a weakening American economy and a strong Japanese yen. It fell 12.4% Monday in a sales panic, which was followed by drops in indexes in South Korea and Taiwan, which were both down more than 8%. European shares were down 2.6% in near six-month lows, Reuters reported. And U.S. were already down Monday morning, Barron’s reported. The loss of 4,451.28 points on the Nikkei was its largest single-day drop ever. The plunge wiped out all stock gains this year. The Nikkei has fallen 25% ov…