Dahlia bulbs are often dug up and stored over the winter to be replanted when the weather warms. If storage conditions are too damp, bulbs will develop mold. You should always check your bulbs at least once a week to make sure they aren't getting moldy. If mold is left on a bulb too long, it will spread to other bulbs. Removing the mold is typically quite easy and takes little time.
Put on your garden gloves. If you are sensitive to molds, you may wish to wear a dust mask to do this as well. Take the affected dahlia bulbs outdoors.
Brush mold off of one bulb at a time, using a soft-bristled brush, as recommended by the Portland Dahlia Society. Next, spray the bulb with a household disinfectant spray to kill remaining spores that you can't see, as recommended by DahliaDivas.com.
Place each now-clean bulb in a clean box. Store the bulbs in an area with less humidity than previously, if possible. If it is not possible to move the bulbs to a dry area that is still cool enough for the bulbs, then you'll need to check on them more frequently than once a week to catch new mold growth. DahliaDivas.com recommends storing the bulbs where it is between 40 and 50 degrees Fahrenheit.