“….after Chance had held two sessions with his infield around the pitcher’s mound. The conference must have helped, for Davis fanned for the third out after Lord’s two-bagger.” Indianapolis Star, October 18, 1910, p1
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1939. “The umpires [in the Southern Association]…are urged to cut down on the number of conferences between managers and pitchers” (The Sporting News, June 8; Peter Morris).