It wasn't Doc Petticord, but it looked a bit like him.
Pat (Patrick) Crenshaw played Doc Petticord in 'Double Sting' (season 1), but his character was only named "Doctor" in 'And in This Corner, Luke Duke' (season 3) .
Parley Baer started playing Doc Appleby in 'Coltrane vs. Duke' near the start of season 4, but then didn't appear again until 'Too Many Roscos' near the start of season 6.
In 'Dear Diary' (near the end of season 4), Doc Appleby was played by a different actor, Elmore Vincent. This was his only appearance in Dukes and one of only 29 acting credits between 1956 and 1983.