Mold Growing on Top of Exterior Door
Background Information

There are really only four things necessary for mold growth:


Mold Spores
Mold Food (Organic Substance)
Appropriate Temperatures
Considerable Moisture


Resources

The best way to control mold growth, is to control moisture. One good resource available to you is the Brief Guide to Mold, Moisture and Your Home from the US EPA. The US CDC has the following recommendations for decreasing mold exposure:


Never mix bleach with ammonia or other household cleaners. Mixing
bleach with ammonia or other cleaning products will produce
dangerous, toxic fumes.
Open windows and doors to provide fresh air.
Wear non-porous gloves and protective eye wear.
If the area to be cleaned is more than 10 square feet, consult the
U.S. Environmental Protection Agency (EPA) guide titled Mold
Remediation in Schools and Commercial Buildings. Although focused on
schools and commercial buildings, this document also applies to other
building types. You can get it by going to the EPA web site at
http://www.epa.gov/mold/mold_remediation.html.
Always follow the manufacturer’s instructions when using bleach or
any other cleaning product.


Advice

As a Certified Indoor Air Quality Technician, I can tell you that bleach will in fact make mold worse. Bleach is very effective in removing the discoloration but leaves the microflora that will enable the mold to return in exactly the same spot when conditions are right. The most effective means of destroying molds, are biocides and anti-microbial solutions.

Recommendation

As far as your door is concerned, specifically:


Check the weather stripping and insure a good seal.
Check the serviceability of your HVAC system. Air Conditioners are
also dehumidifiers.
Keep the doors clean of organic substances - mold can even grow on
dirt.
Ask an IAQT to test the mold spores and determine the type of mold.
Clean the area with rubbing alcohol, or a solution with 60%-90%
alcohol.