Heat oven to 350°F and grease a 9x5-inch loaf pan.
Beat the sugar, sour cream, eggs and vanilla until smooth, about 2 minutes.
Whisk the flour, baking powder, baking soda and salt; stir into the batter just until combined.
Fold in the apple and 1/2 cup pecans. Spread in the pan and sprinkle with the remaining pecans, pressing lightly.
Bake 55 to 60 minutes, until a toothpick comes out clean. Tent with foil if browning too fast.
Meanwhile, bring the butter and brown sugar to a boil in a small saucepan, stirring, and boil 1 minute.
Spoon the glaze over the hot loaf in the pan. Cool 20 minutes, then remove from the pan.