A gauge of U.S. economic activity fell in April as the Japanese earthquake hurt auto production.

The Federal Reserve Bank of Chicago says its index was minus 0.45 in April in contrast to a positive reading of 0.32 in March. It was the weakest reading since August.

A reading below zero indicates below-trend growth in the national economy. Economists say normal growth is around a 3 percent pace.

Factories produced less in April, a main reason for the decline. A parts shortage stemming from the March 11 Japanese earthquake forced automakers to cut back output.

Economists say the supply disruptions will most likely be temporary. They also say U.S. companies are likely to be benefit once the Japanese rebuilding effort is under way. Economists say U.S. manufacturers are likely to fill orders related to those efforts.