Cook macaroni according to package directions; drain. In saucepan, melt 6 tb butter or margarine over low heat. Blend in flour and 3/4 ts salt. Add milk all at once. Cook quickly, stirring constantly, until mixture thickens and bubbles. Add cheese; stir until cheese is melted. Stir in tuna, diced apples and drained macaroni; turn into 12x7x2 inch baking dish. Combine melted butter and crumbs. Sprinkle on top of casserole. Bake in 350~ oven for 30 minutes, or until apples are tender.