Preheat oven to 350° F. Combine first 2 ingredients in a bowl. Stir until smooth. Add the next 6 ingredients. Blend until moist. Fold in rhubarb and nuts. Pour batter into greased 8 x 4 inches loaf pan. In a separate bowl, combine butter and sugar. Mix until crumbly. Spread on top of batter. Bake at 350° F for 50-55 minutes, or until a tester comes out clean when inserted in center. Place on wire rack and let cool before slicing.