Food prices in the Washington area last month experienced their largest drop in 15 months because of a bumper fruit crop and a "milk war."

The Consumer Price Index for grocery-store food prices went down 1.8 percent in October.

It was the steepest decline this year and the biggest since the major supermarkets in the Washington metropolitan area took each other on in a general price contest in mid-1981.

October's decline, following a decrease of 0.7 in September, has helped hold the increase in food prices here to 3.8 percent for the first 10 months of 1982.

At this rate, food prices would rise by 4.6 percent for all of 1982. Chiefly because of price wars, food prices dropped 0.2 percent in 1981.

The most significant change between September and October came in the price of fresh fruits, particularly oranges and apples. Their 7.4 percent price slide was caused largely by a 21 percent increase in the orange crop this year and a 9 percent gain in the apple harvest, according to the United Fresh Fruits and Vegetables Association in Alexandria.

Dairy products prices fell 3.2 percent, largely as the result of a price war over milk at the beginning of October. Giant Food started it by cutting 20 cents off a gallon of Vitamin D milk.

The move was soon matched and followed by 10- and 20-cent reductions per gallon on other grades of milk at Safeway and Magruder's.

The Labor Department statistics reported price declines as well for beef, fresh vegetables, ice cream and roasted coffee.

On the other hand, prices rose moderately for fresh fish, colas, other carbonated drinks, and canned and packages soups. Cereal and bakery products showed the biggest price gain, 0.7 percent.