Before the game begins the umpire shall—
(a) Require strict observance of all rules governing implements of play and equipment of players;
(b) Be sure that all playing lines (heavy lines on Diagrams No. 1 and No. 2) are marked with lime, chalk or other white material easily distinguishable from the ground or grass;
(c) Receive from the home club a supply of regulation baseballs, the number and make to be certified to the home club by the league president. The umpire shall inspect the baseballs and ensure they are regulation baseballs and that they are properly rubbed so that the gloss is removed. The umpire shall be the sole judge of the fitness of the balls to be used in the game;
(d) Be assured by the home club that at least one dozen regulation reserve balls are immediately available for use if required;
(e) Have in his possession at least two alternate balls and shall require replenishment of such supply of alternate balls as needed throughout the game. Such alternate balls shall be put in play when—
(1) A ball has been batted out of the playing field or into the spectator area;
(2) A ball has become discolored or unfit for further use;
(3) The pitcher requests such alternate ball.
Rule 3.01(e) Comment: The umpire shall not give an alternate ball to the pitcher until play has ended and the previously used ball is dead. After a thrown or batted ball goes out of the playing field, play shall not be resumed with an alternate ball until the runners have reached the bases to which they are entitled. After a home run is hit out of the playing grounds, the umpire shall not deliver a new ball to the pitcher or the catcher until the batter hitting the home run has crossed the plate.
(f) Ensure that an official rosin bag is placed on the ground behind the pitcher’s plate prior to the start of each game.
Rule 3.01 contains mandatory language regarding replacing baseballs. E.g., the umpire shall put an alternate ball in play when the pitcher requests an alternate ball.