There is not a credit for the actress who played Aleck's mother

Her name is Julia S. Marshbanks:

http://www.imdb.com/name/nm0551254/

Looks like "Intruder in the Dust" was her only movie.

Have not seen the movie in years. Though I know many of the extras and actors were local people who lived in Oxford MS.

Alberta Dishmon played Paralee; Aleck's mother.
Julia S. Marshbanks. played Molly Beauchamp; Lucas' wife.