Mark Leslie joined William Blair as co-portfolio manager of its $326 million Value Discovery small-cap value fund, according to spokesman Mark Zimmer. It is a new position. Mr. Leslie will join portfolio manager David Mitchell, who had been managing the fund by himself. Mr. Leslie was a portfolio manager with US Bancorp Asset Management, where he managed small-cap and midcap value equities. Teri Charest, U.S. Bancorp Asset spokeswoman, did not return calls for comment by press time.