haven't had that problem, not even in my basement. of course my tools don't sit long enough either.
But, for mold and mildew problems, simply take a spray bottle and put one part bleach and 4 parts water. Mist a spray over the effected items and let air dry. Just like your shower curtains and such. Within minutes, the mildew vanishes and remains gone for a while anyway. In the heat of the humid summer months, i do have a dehumidifier in my basement that collects water and i dump it a couple of times a week--but that's only around july and august each year. If i keep my a/c on, i usually don't have to worry about that.
You could have a window a/c unit in your garage if it really bothers you and set it for a high temp to keep humidity out. But, a simple spray once a month as mentioned will also do the trick.
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