you could go under the tread and install some nailing blocks to the risers and then screw the nailing board to the underside of the tread. Make very sure that the screws do not go through the tread. Use glue also with this process.
Also use zinc or galv treated heavy screws.
Screw the nailing board to the risers right below the tread placement and even with the riser. you could even attach this nailing board to both sides of the middle riser for additional security. Make sure the nailing board is not pine or a soft wood. Pre drill holes in Nailing board also.