Combine sugar, syrup, water, cream of tartar, and salt in a medium-size heavy saucepan. Cook over medium heat, stirring frequently, until mixture comes to a boil and sugar dissolves. Remove from heat.
Beat egg whites (at room temperature) in a medium glass or metal mixing bowl until foamy. While beating at medium speed of an electric mixer, slowly pour hot syrup mixture in a thin stream over egg whites. Turn mixer to high speed; beat until stiff peaks form and meringue is thick enough to spread.
Spread meringue immediately over pie filling, carefully sealing to edge of pastry.