a common problem lots of low temp washes 40' or even less with a powder that contain s NO BLEACH this allow BACTERIA and BLACK MOLD to thrive, once the mold has eaten into rubber no cure, only replacement
do a service wash monthly where no clothes are added a boil wash selected, along with washing powder ,cup neat bleach and some white vinegar when finished wipe around the door seal leave the door open and pull the powder drawer open this will aid VENTILATION problem mold breeds in damp conditions
wirecutterman, August 2010