Heat oven to 350°F. Grease bottom only of 9x5 or 8x4-inch loaf pan. In large bowl, combine 3/4 cup sugar and margarine; beat until light and fluffy. Add eggs; beat well. Add buttermilk; blend well.
Lightly spoon flour into measuring cup; level off. In small bowl, combine flour, pecans, baking powder, baking soda and salt; mix well. Add to buttermilk mixture; stir just until dry ingredients are moistened. Fold in rhubarb. Pour batter into greased pan. Sprinkle 1 tablespoon sugar over top.
Bake at 350°F. for 50 to 60 minutes or until toothpick inserted in center comes out clean. Cool 15 minutes. Remove from pan. Cool 1 hour or until completely cooled. Store in refrigerator.