Agricultural Export Promotion Act of 2018 This bill establishes the United States Agricultural Export Promotion Fund and requires the Department of the Treasury to transfer to the fund an amount equivalent to certain duties imposed on imported goods under the Trade Expansion Act of 1962 or the Trade Act of 1974. The amounts in the fund must be available to the Department of Agriculture (USDA), without further appropriation, for carrying out activities of the Foreign Market Development Program, the Market Access Program, and the Technical Assistance for Specialty Crops program to promote exports of U.S. agricultural commodities and products. In carrying out this bill, USDA must, to the maximum extent practicable, give priority to promoting exports of agricultural commodities and products of U.S. businesses that are adversely affected by increased duties imposed on U.S. exports by foreign countries in response to increased duties imposed by the United States on imports from the countries.