The fleshy root of the plant Ipomoea batatas. The sweet potato was mentioned as being grown in Virginia as early as 1648. In 1930 the selection of outstanding strains of the Porto Rico variety, which was introduced into Florida in 1908, was begun in Louisiana, and the best strain, Unit I Porto Rico, was released in 1934. The Unit I Porto Rico is now being replaced by the Centennial, a yam type which has three times its vitamin A content. More than 70% of the commercial crop in the United States is of the Centennial variety. Another variety, Nemagold, is used largely in the Virginia area because of its resistance to nematodes. Most varieties released now have multiple disease resistance, particularly resistance to wilt and soil rot.