Witherspoon ran for 1-yard score for a 7-0 lead before Skyler Davis made a career-long, 52-yard field goal late in the first quarter. Davis added a 28-yarder with 10:29 before halftime for a 13-0 lead.
Vincent Amendola got Towson on the board with a 5-yard scoring run at the end of a nine-play, 73-yard drive five minutes later.
But the Phoenix (5-5, 4-3 Colonial Athletic Association) countered with a 10-play, 75-yard drive that ended with Witherspoon carrying it in from 1 and a 20-7 margin with 2:07 before halftime.
Jerry Howard Jr. ran for 126 yards on 17 carries for the Tigers. Towson (4-6, 3-4) has lost back-to-back games and three of its last four.
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