Drain pineapple and reserve juice. Place juice in saucepan; add butter, sugar and lemon juice. Bring to a boil. Combine cornstarch and water until smooth. Add to the saucepan, stirring constantly. Bring to a boil again; cook and stir for two minutes. Chill.
Stir in mayonnaise. In a large bowl, combine pineapple, apples, grapes, poppy seeds and dressing. Fold in pecans just before serving. Yield: 10 to 12 servings.