Mold does not like air circulation.
The air in closets may easily become stagnant and humid enough that mold may become a problem.
This is especially true in the case of a close that it against an outside wall
Air conditioners usually keep indoor air dry enough that mold from high humidity is not an issue.
But the conditioned air does not always get into closets, even if they have louvered doors and even if they may have an AC supply vent.
A powerful closet mold prevention step is to force room air directly into the closet at all times.
In minutes, a small fan system may be installed through a closet wall to move air from an adjacent area directly into the closet.
Small, low flow and quiet fans are available that can be installed by a do-it=yourself homeowner.
A nice option is an Installation performed with a 4″ hole saw directly through an interior closet wall preferably located above a 120 vac outlet.
For installation details, see
Through-Wall Air Transfer Fan
120 vac fan will move 60 cfm into a closet to provide air circulation and pressurization. with dry indoor air.