In skillet, cook bacon. Remove and drain. Pour off all but 3 T bacon fat, add onion and celery and stir fry 1-2 minutes. Blend in flour. Add juice and vinegar, stirring until mixture thickens. Add sugar. Preheat oven to 350 deg F. Place potatoes, apples, and crumbled bacon in greases 1.5 qt casserole. Add skillet mixture, mixing lightly. Cover and bake 30 minutes.