The Patriots issued 14 walks, hit four batters and threw three wild pitches.
A costly error in the seventh inning by left fielder Troy Schreffler helped the Rams pull away.
The Terps were behind 2-0 entering the ninth inning.
Dean struck out six and allowed two earned runs over 4 2/3 innings.
Sean Fisher struck out five in 5.1 innings in his first career start.
The Terps open the season Friday against Charleston Southern.
Right-hander Elliot Zoellner pitched 3 1/3 scoreless innings of relief, but the Terps faltered late.
Weather forced Thursday's game to continue on Friday morning.
Right-hander Hunter Parsons allowed just one run in 7 1/3 innings.
Lee started 4-for-27, but is now hitting a team-best .301.