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Lower food and gasoline prices helped push wholesale prices down 0.2 percentage points in September, the Labor Department reported Friday. The drop was the sharpest since January 1983 and marked the first time in eight years that prices fell for two straight months.

The decline in producer prices meant that, for the first nine months of the year, prices at the wholesale level rose a minuscule 1.9 percent, surprising economists who had forecast a 5 percent inflation rate for all of 1984.

Food prices fell 0.4 percent, the fifth reduction in six months, while gasoline prices dropped 0.5 percent, the fourth monthly decline in a row.