I can't be certain from the picture that the marking is caused by a flashover. You will need to get someone more experienced to actually look at the board. From the picture I can't even follow the tracks in that area of the board. In particular I can't see where the track from the + output pin of the rectifier goes. IF the marking is caused by a flashover the surface of the board will be charred so it will now be conductive. IF so that part of the board will have to be repaired to remove conductive material. If a flashover is not the cause of the problem then something in the switch mode regulator part of the board is probably the cause of the fault so that needs to be identified before replacing the rectifier. As you are only a beginner I think identifying a fault in the switch mode power supply would be too difficult and dangerous for you. It is possible that more pictures of that area of the board with the lighting from different angles and one with it illuminated from below MIGHT help. Have you actually tested the rectifier and fuse with a DMM ?
Les.
Les.