Combine rhubarb, cut in ¾" pieces, and strawberries in bowl. Sprinkle on sugar, flour, salt and pumpkin pie spice; mix gently. Turn into pastry-lined 9" pie pan; dot with butter. Adjust lattice pastry top, flute edges.
Bake in hot oven (400°F.)40 minutes, or until crust is browned and juices bubble. Cool on rack.