In a large bowl, toss diced apple with lemon juice to prevent browning. Add pineapple, mandarin oranges, grapes and marshmallows.
Drizzle with yogurt and toss gently until fruit and marshmallows are evenly coated. Refrigerate for at least 1 hour or for up to 8 hours to blend flavors. Just before serving, slice bananas and stir into fruit mixture